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Biology 30
Module 2
Lesson 8
Nervous and Sensory Systems
Copyright: Ministry of Education, Saskatchewan
May be reproduced for educational purposes
Biology 30
189
Lesson 8
Biology 30
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Lesson 8
Lesson 8
Nervous and Sensory
Systems
Directions for completing the lesson:
Text References for suggested reading:

Read BSCS Biology 8th edition
Nervous System Pages 361-362, 437-493
OR
Nelson Biology
Nervous System Pages 241-265

Study the instructional portion of the lesson.

Review the vocabulary list.

Do Assignment 8.
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Lesson 8
Vocabulary
astigmatism
axon
brain
cephalisation
cerebellum
cerebrum
cones
corpus callosun
dendrite
depressants
dorsal
fore brain
ganglia
hallucinogens
hind brain
hyperopia
hypothalamus
interneuron or association heurons
Jacobson’s organs
medulla oblongata
meninges
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meningitis
midbrain
motor neuron
myelin sheath
myopia
nerve
nerve net
neuron
neurotransmitter
ommatidia
parasympathetic system
perception
pons
rods
sensory neuron
stimulants
sympathetic system
synapse
tetanus
thalamus
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Lesson 8
Lesson 8 – Nervous and Sensory
Systems
Introduction
A great deal of regulation and co-ordination of activities in the bodies of organisms is
carried out by chemicals or hormones. These can stimulate or inhibit various cell
and body processes so that appropriate responses can be made to external or
internal stimuli. For most organisms which are sessile or stationary in a particular
location, such as larger plants, auxins or hormones and possibly other chemical
messengers are sufficient in maintaining satisfactory levels of responses for species
survival.
Heterotrophic organisms with complex, multicellular bodies are usually adapted for
moving about or changing locations. This increases their chances of obtaining
organic nutrients and satisfying other needs. Organisms which move about,
ourselves included, have many of their internal conditions regulated by endocrine
glands and hormones. Even though hormones in most warm-blooded animals can
reach target cells and begin a response in a minute or less, the overall time for
completion of a response may still be too long to be effective or to be successful.
Coordinating hundreds of muscles, usually as opposing pairs, in even the "simple"
action of walking requires stimulations and responses which take place in fractions
of a second. Hormones alone can not accomplish this. Species which have a greater
dependence on movements have evolved a complementary system based on nerves,
nerve networks and nervous control.
The degree of nervous development and functioning varies among taxonomic groups.
Descriptions of other cell or body activities have generally been traced through a wide
range of life forms, including plants and animals. Studies of unicellular organisms
and of plants have not yet identified anything that resembles a nerve cell or an
impulse that travels along a nerve network. For this reason, this section on sensory
reception and nervous control focuses on animal groups. The range of different
animal life looked at is further narrowed as the basic structures and actions of nerves
are fairly similar. Only in the general organization of nerve networks and in the
degree of development of some parts of the nervous system and sensory receptors are
there differences.
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Lesson 8
After completing this lesson you should be able to:
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•
explain the need for the presence of nerves or nerve
networks in multicellular, motile organisms.
•
describe the basic structure of a nerve cell or neuron.
•
list the three general types of neurons.
•
describe how a nerve impulse occurs.
•
compare the organizations of nerve networks in some
taxonomic groups.
•
describe the major divisions and functions of the central
nervous system in a vertebrate.
•
describe the major divisions and functions of the central
nervous system in a vertebrate.
•
describe the nature and role of the peripheral nervous
system, with special emphasis on the autonomic system.
•
indicate some effects of some drugs on the nervous
system.
•
describe some nervous disorders and how they occur.
•
discuss the general role(s) of receptors and sensations on
organisms’ lives.
•
name the general types of receptors and sensations which
organisms may have.
•
describe how unicellular organisms, such as protists,
may detect and react to various stimuli.
•
trace the developments of some of the major types of
receptors and sensations in some of the different
taxonomic groups.
•
explain the meaning of nerve fatigue and indicate its
possible effects on organisms.
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Lesson 8
We will start this lesson studying about neurons, impulses, nervous
systems in representative groups and synapses. After this ground work is
laid we will look at each nervous system a little more closely.
The Neuron
A cell is a basic unit of both structure and function of any organism. In multicellular
organisms, cells are generally adapted for different functions. This often means that
cells of different systems have features or characteristics making them distinct from
other body cells. The basic cell unit of any nervous system or nerve network is called
a neuron. Nerves are composed of many individual neurons bundled together.
A neuron is made up of a cell body, dendrites and axons. Projections or extensions
that carry impulses toward the cell body are called dendrites. These are usually
highly branched. A single, very thin projection from the cell body, is called an axon.
The axon carries impulses away from the cell body towards either other neurons or
towards effectors. The axon ends in a series of branches with slight enlargements on
their ends called axon terminals.
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Many nerve cells or neurons are microscopic in size, just as other cells are; however,
this does not mean that neurons cannot be long. Some human nerve cells (to and
from the legs) can be between a meter and 1.5 meters in length. In other animals,
some neurons can be much longer. In a nervous system, non-nerve or glial cells are
sometimes associated with certain kinds of neurons. One kind of glial cell, called a
Schwann cell in the peripheral nervous system, forms a shiny white, fatty protein
(fat does not conduct electricity well) wrap around axons called the myelin sheath.
These neurons are said to be myelinated. The myelin sheath has three main
functions. It functions in the protection of the nerve fibre. It also serves as a good
insulator because of the fatty tissue it is made of. The myelin sheath increases the
rate of transmission of the nerve impulse along the axon. There are gaps between the
sections of the myelin sheath. The gaps are called nodes of Ranvier (see diagram).
The impulses jump from node to node at a very fast rate. Nerve impulses travel
much faster along myelinated nerves than nonmyelinated ones.
Types of Neurons
The description and illustration of a neuron represents a generalized form that is
similar to most types of neurons. Some differences do exist within organisms and
these have enabled neurons to be divided into three types, according to form and
function.
Sensory Neurons (See #1 in the following diagram)
Sensory neurons carry impulses from specialized nerve endings, called receptors
(from where the action is in the environment) to the spinal cord or brain. These
receptors can be specialized for heat, light, pressure, etc. The cell body of the sensory
neuron is located in clusters called ganglia, next to the spinal cord. The axons
usually terminate at the interneurons.
Note: You should know which is the sensory neuron, which is the motor neuron and
where the interneuron is.
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Interneurons or Association Neurons (See #3 in the diagram on the previous
page)
Impulses from sensory neurons often do not directly cause an impulse in a motor
neuron. Some "interpretation" of the sensory information may have to occur in the
spinal cord or brain before an appropriate response is decided upon as to what
muscles or glands will be made to react - if there is to be a reaction. Sensory neurons
and other interneurons stimulate interneurons. Impulses picked up by these
interneurons can be directed to and from the brain and to motor neurons. In some
instances, an impulse can be routed by an interneuron directly from a sensory
neuron to a motor neuron, without first "notifying" the brain. This is a reflex action,
such as occurs when we blink if something comes close to an eye.
Motor Neurons (See # 2 in the diagram on the previous page)
Muscles or glands make reactions or responses to stimuli. These are sometimes
called effector organs. Motor neurons carry the impulses from the interneuron in the
brain or the spinal cord to an effector that causes a reaction in either a gland or a
muscle. The cell body of a motor neuron is located within the spinal cord and brain.
Since the impulses travel away from the cell bodies, motor neurons have long axons.
The Reflex Arc
The most basic of nervous responses by organisms having a nervous system is a
reflex arc. The reflex has five parts that work together. They are:





the
the
the
the
the
receptor,
sensory neuron,
interneuron in the spinal cord,
motor neuron and
effector(muscles)
The receptor detects the stimulus (ie cold). This information is carried to the spinal
cord. Interneurons relay the information to the motor neuron. The motor neuron in
turn, carries the message to activate the response (ie. Move hand away from the cold
item) in the muscle.
Many reflex arcs are withdrawal or protective actions. The brain does not have to
participate in reflex arcs. However, some interneurons do carry impulses to the brain
so that it does become aware of an action – even though the action has been
completed.
Other common reflex arcs include the startle response, blinking, knee jerk, sneezing,
coughing, and the changing of pupil size with different light intensities or distances of
objects viewed.
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Activity 1
Papillary Reflex - Have a friend close one eye for one minute.
Then tell them to open their eye. Compare the size of their pupils.(You have to watch very closely as
the reflex occurs quickly) Which is larger?
Activity 2
Babinski Reflex - Have your friend remove a shoe and sock. Have him/her sit on a chair, stretching
out their leg on another chair for support. Using a blunt, smooth object (smooth end of a pen would
work), quickly slide it across the sole of the subject’s foot, beginning at the heel and moving towards
the toes. Describe the movement of the toes.
Arrangements of Neurons
Individual neurons do not make direct connections with one another at their ends.
Instead, there is a short space or gap between a set of axon terminals and dendrites
called a synapse. The significance of a synapse will be examined shortly under the
section “Impulse Transmission”.
Individual fibres very frequently come together to lie side-by-side in collective bundles
more commonly known as nerves. Thousands of individual neurons make up the
nerves leading to and from body areas.
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Some nerves are:



Sensory, in that they consist entirely of sensory neurons.
Motor nerves, in that motor neurons join as they leave the spinal cord.
Mixed nerves that are bundles of both sensory and motor neurons. These kinds of
nerves are probably the most numerous. Such mixed nerves do separate at the
ends. The sensory neurons of a nerve enter at the dorsal root or posterior area of
the spinal cord. Motor neurons leave the spinal cord at the ventral root.
The different neuron types are not arranged in a simple one to one relationship. The
axon terminals of one sensory neuron may form synapses with many interneurons
and one interneuron may have axons from many different sensory neurons and
interneurons converging upon it. Some motor neurons could have synapses with
over 5000 different interneurons and sensory neurons.
All the different possible routes impulses may take account for great variations in
responses that are made. It also shows why certain skills or actions can be improved
upon through practice or repetitions. By doing so, certain impulses can become
established on shorter and on more specific routes.
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Comparison of Nervous Systems
The next section will briefly compare the progressing complexity of Nervous Systems
beginning with the systems of simple unicellular organisms to the more complex
systems of vertebrates.
It is important that you are able to:


compare the complexity of nervous systems in the planaria, earthworm, and
human.
know the value of cephalization
Nervous Systems of Representative Groups
1. Unicellular organisms
Unicellular organisms such as Amoeba and Paramecia make responses to external
stimuli even though they lack any kind of obvious nerve network.
2. Cnidarians
The first appearance of any apparent network is seen in Cnidarians, such as Hydra.
Cnidarians have nerves and sensory cells arranged in what is called a nerve net.
There are a few clusters of ganglia joined by interneurons. There is no brain. Nerve
impulses can run both ways. If a certain area of the body is stimulated, impulses
originate in that area and spread uniformly outward to other body parts. Such a
reaction generally brings on an overall response. For instance, a Hydra touched in
one part of the body will contract all over, rather than in just that particular area.
Nerve nets do exist in other groups of organisms. Vertebrates have them in the walls
of parts of the digestive tract.
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3. Flatworms
Flatworms show an advancement with the development of two ventral nerve cords.
The two cords run the length of a body. Ganglia occur along the entire lengths of the
cords with larger ganglia present in the anterior or head end. Concentrating larger
numbers of nerves or ganglia in a "head" area is cephalization.
Cephalization and the development of larger nerve cords running in specific
directions, enables organisms to begin to better control or regulate their
movements. As well, reaction times are faster.
4. Annelids
Annelids show greater developments of what had first appeared in the flatworms.




Has a centralized system with a ventral nerve cord that has distinct ganglia along
its length.
Cephalization has also advanced to the point where the various ganglia at the
head end can collectively be called a brain – even though a very simple one.
Cephalization and bilateral symmetry are closely associated. These two
characteristics enable organisms to receive many stimuli in a concentrated area
where they can be more quickly interpreted and responded to.
The development of major nerve cords go along with cephalization in allowing
organisms to better regulate and co-ordinate movements in different parts of their
bodies.
5. Arthropods
Only minor differences exist between the nervous systems of annelids and the next
major group, the arthropods. Additions of more nerve cells and greater
concentrations of these in specific body areas allow arthropods to become aware of
and to respond more quickly to stimuli.
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6. Vertebrates
Vertebrates show some distinct differences from the other groups in the general
organization of their nervous systems.
Two main differences are:


A single nerve cord takes the place of the two which are characteristic of the other
groups previously described. The nerve (spinal) cord is enlarged on the anterior
end to form a brain. The vertebrate nervous system is divided into two parts: the
central nervous system (CNS) made up of the brain and the spine and the
peripheral nervous system (PNS) which consists of sensory and motor neurons.
This nerve or spinal cord is also located dorsally in the vertebrate body.
The Nerve Impulse
A.
What the nerve membrane is like before the impulse
Nerve impulses are transmitted along neurons. There is an electric potential
between the outside surface and the inner area of a neuron. This polarization
develops when an uneven distribution of ions builds up. (See diagram #1) On
the outside surface there is a high concentration of sodium (Na+) ions, while the
inner area has potassium (K+) and chlorine (Cl-) ions, as well as protein
molecules. The much greater concentration of positive sodium ions on the
outside to the potassium ions inside creates a potential difference (of about 70
millivolts) between the positive outside and the negative inside.
Diagram 1
B.
What happens to the nerve membrane during the impulse:
In a resting condition (not transmitting an impulse), the nerve cell membrane is
highly impermeable to the sodium ions and only slightly permeable to the
potassium ions. A variety of stimulations, such as light, temperature changes or
pressure, initiate an impulse in a nerve cell. This stimulation causes a
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Lesson 8
membrane to become more permeable to the sodium ions (Na+) and then to the
potassium ions (K+). A rapid inward movement of sodium ions, with a slower
potassium ion movement to the outside, causes a charge reversal or
depolarizes the membrane and cancels out the potential difference. (See
diagram 2)
Diagram 2
When an impulse occurs, the inward movement of sodium ions "overshoots",
causing a momentary reversal with the inner part becoming positive and the
outer part negative. As the impulse moves forward, almost immediately
(approximately 1/1000th of a second) the original potential is being reestablished behind it. (Like a ‘wave’ motion)
Sodium ions are actively transported out of the cell by a special protein in the
plasma membrane. This protein sometimes called a sodium-potassium pump,
uses energy from ATP to move the sodium ions and to restore the
impermeability of the membrane. Potassium ions may be moved by the "pump"
or may move back in by simple diffusion. (See diagram 3)
Diagram 3
In a warm-blooded vertebrate, the electrochemical reaction or impulse may
travel up to 100 meters per second (or better than 300 km/h) in myelinated
nerve fibres. After an impulse passes, it takes about .001 to .002 seconds to
regenerate the original potential. This means that a nerve fibre can conduct
500 to 1000 impulses in one second.
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Lesson 8
Impulses in cold-blooded organisms take much longer times to travel. Impulse
speeds in amphibians, reptiles and fishes can be one-half to one-third those of
mammals.
Electricity, or the flow of electrons, occurs at approximately 300 000 km/s and
is not affected by temperatures. This is why it should be kept in mind that a
nerve impulse is an electrochemical or (chemical) ion movement, rather than a
straight electron movement.
C.
Movement of the impulse along the neuron
Once a nerve impulse starts, it moves along a fibre (like a wave at a football
game) without any further stimulation. The impulse stays strong as it moves
down the neuron because of this wave motion. A change in membrane
permeability triggers a change in the adjoining section as the impulse moves
along.
In myelinated fibres, depolarization occurs only at the nodes of Ranvier and a
depolarization of one node almost immediately triggers the next node. So an
impulse actually jumps from node to node in a myelinated fibre rather than
moving continuously along as in non-myelinated fibres. Movements in
myelinated fibres are as much as four times faster.
Other facts to know about nerve impulses:
 A neuron has an "all-or-none" reaction to a stimulus.
 A stimulus must exceed a certain threshold value in strength before an
impulse starts. Any stimulation below the threshold will not cause or
produce any impulses. Stimulation above the threshold value does not
change the strength of an impulse. However, stronger stimulation could
cause more impulses to be sent along a neuron or fibre, or cause
impulses to occur in more individual neurons. This is how organisms
become aware of higher or lower sensations of smell, taste, pressure
(pain) and other stimuli.
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Lesson 8
Synapses and Their Significance
Neurons are not directly connected to other neurons at their ends. Instead, a space
exists between them. This space is called a synapse.
So, how do the impulses get across the synaptic space?
When an impulse reaches the end of an axon, vesicles release a chemical, known as a
neurotransmitter, into the space. The neurotransmitter diffuses across the synapse
to the membrane of the dendrite of the next neuron.
What does the released chemical (the neurotransmitter) do?
The chemical alters the permeability of the dendrite's membrane and a new impulse
is initiated.
Where does the neurotransmitter go when its job is finished?
Almost as soon as an axon terminal has released the transmitter chemical, some of it
begins to be reabsorbed. Enzymes released by the axon terminal break down the rest.
This very fast action prevents the transmitter from continually "firing" or "triggering"
impulses at the dendrite.
If the enzymes did not break down the transmitter chemical, muscles would be in an
almost steady state of contraction or paralysis and very likely opposing each other’s
actions, resulting in a loss in co-ordination.
Nerve gases developed for warfare purposes block the breaking down actions of the
enzymes and bring about these very conditions. In larger amounts or over a period
of time, these enzyme-blocking chemicals will cause death.
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Lesson 8
Insecticides make use of this knowledge! Chemicals are used to block the work of
the enzyme so the transmitter chemical does not break down. The insect heart is
completely controlled by nerves – so the insect heart continues to contract but
never relaxes.
Nerve Adaptation (Nerve Fatigue)
A very common feature about receptors and impulses is what is variously called
nerve adaptation or nerve fatigue. Constant stimulation of particular nerves will
cause those nerves to stop carrying impulses. Sensation of a particular type will be
reduced or decreased below the conscious level. Odor receptors are one of the faster
groups to experience this. An individual may walk into a room and notice, and
perhaps even be bothered by, a strong odor. Yet after a short while, awareness of it
is lost. The same can be noticed about people working in livestock operations and
the "barnyard" smell. Similarly, an individual can lose awareness of his or her own
body odor and may not even notice if it becomes offensive to other people. Nerve
fatigue may develop as continual stimulation results in a situation where, as
described previously, transmitter chemicals cannot be formed and broken down fast
enough between axons and dendrites. This stops the movement of impulses from
neuron to neuron.
One benefit of nerve adaptation and the dulling of certain sensations is that
individuals are not continually "bothered" by a host of ongoing stimulations. These
could include certain odors, certain repeating sounds or light pressures (such as a
wristband or necklace) on some body part. Removal of a particular stimulus for a
period of time will restore an individual to a condition where awareness of the
stimulus is made apparent if the stimulus returns.
Facts about Chemical Transmitters:
1.
At least 50 neurotransmitters have been identified. All are either amino acids
(the building blocks of proteins) or are derived from amino acids.
Some examples of neurotransmitters are:
 Acetylcholine is a common neurotransmitter of many nerve cells.
 Noradrenaline (also known as norepinephrine) is another. Noradrenaline is
a hormone that is secreted by the adrenal medulla as well as being released
as a neurotransmitter
 Dopamine, serotonin and gamma-aminobutyric acid (GABA) are
transmitters in the central nervous system. (In the brain and spinal cord.)
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Disorders involving neurotransmitters are:
Disorder
2.
Thought to be caused
by
Effects
Alzheimer’s Disease
has been associated with
the decreased production
of acetylcholine
symptoms are decreased
mental capacity and
memory loss
Parkinson’s disease
an inadequate
production of the
neurotransmitter
dopamine
symptoms are involuntary
muscle contractions and
tremors
Another important characteristic about the chemical transmitters is that while
some are "stimulatory" or "excitatory", others are "inhibitory".
Excitatory transmitters cause membranes to be more permeable so that the
thresholds at which impulses start are lower. Some axons produce "inhibitory"
chemicals that actually cause adjacent membranes to be less permeable. This
makes it harder for impulses to start by raising the thresholds. Inhibitors are
just as important as the stimulators.
An example of this is how voluntary muscles function as
opposing pairs to enable you to throw a ball. Muscles on
the back of your upper arm receive ‘excitatory’
neurotransmitters that tell the muscle to contract while
the muscles on the front of your arm receive ‘inhibitory’
neurotransmitters that tell the muscle to relax. This
occurs so that the muscles don’t waste energy pulling
against each other.
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Lesson 8
A single motor neuron or interneuron often has many axon terminals near its
dendrites. These could include varying combinations of both excitatory and
inhibitory types. Whether an impulse begins or not in a dendrite depends on the
cumulative effect of all of these.
A certain area of the cell body, near the point where an
axon emerges, seems to be able to combine all the
chemical signals along the dendrites' membranes and
either "fire" or "not fire". It may take the combined
impulses from a number of excitatory synapses to trigger
another impulse. The actions of some inhibitory synapses,
at the same time, could prevent an impulse from
beginning.
3.
Certain features of neurons and synapses result in impulses travelling
from neuron to neuron in only one direction. Only axon terminals
produce the chemical transmitters and only dendrites are sensitive
to them. Therefore, impulses only move from axon terminals to
dendrites and not the other way.
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Lesson 8
The Organization of the Nervous System
The nervous system of a vertebrate organism consists of two major divisions. These
are designated as the central nervous system and the peripheral nervous system.
The central nervous system is made up of the brain and spinal cord and their nerves
carrying information in and out. The peripheral nervous system is made up of nerves
that carry information between the organs in the body and the central nervous
system.
The two main divisions of the nervous system are shown in the diagram below. We
will look at each part briefly.
A.
The Central Nervous System
The central nervous system (CNS) is made up of clusters or concentrations of
ganglia that are connected by many interneurons. The CNS is almost like a
control panel that receives input signals, analyses them and then sends out
appropriate "instructions". At the same time, it stores information for future
use. The two major areas of ganglion concentrations are the brain and the
spinal cord.
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Lesson 8
1.
The Brain
Determinations of the actual activities or functions
in a brain have been difficult. There is still much to
be learned.
From the top or dorsal view, the human brain is
seen to consist of two large hemispheres. Three
protective membranes or meninges cover the entire
organ. These membranes provide a cushioning
effect that is further helped by a cerebrospinal fluid
between the second and third layers. The
cerebrospinal fluid also functions in:




The average adult
human brain has
an approximate
mass of 1.5 to 1.75
kg. Estimates
place the number
of neurons in its
ganglia at
anywhere from 10
billion to 5000
billion.
nutrient distribution,
waste removal,
movements of hormones
movements of white blood cells.
The fluid extends into four spaces or ventricles within the brain and also
continues into and around the spinal cord.
The brain is divided into three regions:



forebrain,
midbrain
hindbrain.
In adult humans and other vertebrates, these areas are not that distinct.
The hindbrain is made up of the cerebellum, the
pons and the medulla oblongata.
Nerve impulses controlling some of the vital body
processes that are involuntary such as breathing
and heartbeat originate in the medulla. A heavy
blow or injury to this area can result in quick
death.
The pons (means bridge) acts as a relay station
passing information between the cerebellum and
the medulla. The pons is a pathway connecting
the various parts of the brain with each other.
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Lesson 8
The cerebellum coordinates muscle actions, balance and posture.
Muscle "instructions" from the cerebrum and nerve impulses from body
parts enter the cerebellum, where they are coordinated so that an
organism can maintain its balance. This part is especially vital in
carrying out such actions as jumping or spinning or other intricate
maneuvers. Skaters, dancers or gymnasts would not be able to perform
if this part were not functioning.
The midbrain also assists in maintaining balance.
Its major function seems to be that of relaying
nerve impulses between the forebrain and
hindbrain. It also relays impulses between the
forebrain and the eyes and ears, so it is important
in some eye and ear reflexes.
The brain stem is made up of the medulla
oblongata, the pons and the midbrain. Extending
through the middle of the medulla, pons and
midbrain are special nerve cells (reticular formation) which seem to
awaken or activate the forebrain to various sensations. Their selective
nature frequently keeps the group from arousing the forebrain to familiar
stimuli – such as the ticking of a clock, humming of fans or machines
and subdued voices. Injury or some sort of destruction to this group of
nerves would lead to a coma.
The forebrain is the largest part of the organ and the cerebrum makes
up most of it. The other parts of the forebrain are the thalamus and
hypothalamus.
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Lesson 8
Cerebrum
The most interesting part of the
forebrain is its largest part. This
is the cerebrum, which is divided
into left and right hemispheres.
From the outside, each
hemisphere is seen to consist of
many folds and creases. The
deepest crease or fissure creates
the two hemispheres. This
extends down to the corpus
callosum, where nerve bundles
cross over from one hemisphere to
the other, allowing communication
between the right and left cerebral
hemisphere. Interestingly, nerve
tracts originating on the right side
of the body cross over either in the
spinal cord or brain and end up in
the left hemisphere. Nerves from the left side are connected to the right
hemisphere. (Therefore, the actions of a right-handed individual are
controlled by the left side of the cerebrum.)
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Lesson 8
Other deep creases or fissures divide each hemisphere into four
segments or lobes. Extensive animal and human studies utilizing
dissections, observations of injuries or disorders, recordings of electrical
activities (EEG's or electroencephalograms) and other methods (CAT, PET
and BEAM scans), have identified certain brain areas as being associated
with certain processes.
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Lesson 8
In general, the cerebrum controls many body actions and is also involved
in learning, interpreting or analysing, memory retention, emotional
developments, speech, reasoning and other mental activities.
The greatest challenge facing researchers today is in finding out how
these occur. Neural actions, or the way impulses travel in the brain, may
be genetically predetermined for many behavior patterns or instincts,
reflexes and involuntary actions occurring internally. Many other
actions, especially in humans, are probably developed through learning
processes. In these, the way impulses travel or the pathways chosen,
could be developed, modified or completely changed by establishing new
pathways.
Researches have discovered that in a person who has
experienced a stroke, other cells can be trained or ‘taught’ to
take on the function of the damaged cells. Over a period of time
some or all speech, movement, and hearing can be potentially
restored. This would apply to some brain injuries as well.
Below the cerebrum is the
thalamus. It is an area through
which sensory messages to the
cerebrum pass and these enable
individuals to become aware of
such things as pressure, pain or
temperature extremes.
Below the thalamus is the
hypothalamus. Briefly, this part
monitors many internal conditions
and produces responses either
through nerve connections (as
part of the autonomic nervous
system) or by its influence on the
pituitary gland and therefore,
many of the other glands and
internal body organs.
Concentrations of various substances such as water and salts are
maintained in blood and lymph, rates of many body processes are
regulated and body developments and emotional behaviors are all
affected by the hypothalamus.
The role of the hypothalmus is examined more fully under hormonal
controls.
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Brain Comparisons
Following is a discussion of brain comparisons progressing from the
worms through to the more complex systems of the mammals.
It is important that you are able to understand:

The progression in development of the brain
Beginning with the worms and continuing to arthropods, a major feature
in the development of nervous systems was the beginning of ganglion
concentrations at the anterior ends. As the number of ganglia that
cluster together increases, it seems more suitable to use the word brain.
This term becomes even more appropriate when groups of organisms
begin to show obviously distinct parts or divisions to these ganglion
concentrations. Such concentrations could include the optic and
olfactory lobes, cerebrum and others. These are areas that control the
senses, behavior, and coordination.
Vertebrates have similar brain parts. Variations among groups occur
largely in the proportional sizes of these parts to each other and in the
overall size. The relative sizes of the different brain parts in each kind of
organism does give some indication of the possible uses made of certain
kinds of senses.
For instance, in amphibians and reptiles there are no brain parts which
are really outstanding in development compared to the other vertebrates.
Their eyes are probably one of the more important sense receptors.
In birds and mammals, the cerebrum and cerebellum come into
prominence. These developments indicate the greater muscular
coordination in these groups and also the appearance of more complex
behaviors. The illustrations are not drawn to scale, so do not give a true
indication of relative brain sizes between the groups. Brain sizes
increase from amphibians to mammals and there is an especially large
increase between birds and mammals.
2.
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The Spinal Cord (The second part of the central nervous system)
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Lesson 8
The other major coordinating center and part of the central nervous
system is the spinal cord. In cross-section, a human spinal cord has a
butterfly-shaped area of gray matter surrounded by a lighter outer
portion.
The gray matter is made up of nonmyelinated interneurons. The
interneurons are organized so that they connect the spinal cord with the
brain. The surrounding white matter is made up of myelinated nerve
fibres of the sensory and motor neurons.
Sensory axons enter a dorsal root in the spinal cord while motor axons
leave by a ventral root. A ganglion is a common feature in dorsal roots.
Cell bodies of sensory neurons are concentrated here. Situations have
shown that cutting or tearing of a dorsal root causes a loss in sensation
or feelings to a particular part, but that it can still be muscularly
controlled. Separation or cutting of the ventral root enables sensations
to be received from that particular part but it is paralyzed or incapable of
being moved.
The spinal cord carries out two major functions with respect to nervous
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Lesson 8
control:


Many reflex arcs or actions occur through the spinal cord. These
actions occur rapidly, without first "notifying" the brain, and largely
serve in protective or survival roles for bodies.
The other major function of the spinal cord is to serve as a link
between all the peripheral nerves and the brain, where interpretation
occurs. Appropriate responses are then sent back along the cord and
out through the motor neurons. Inside the spinal cord, sensory and
motor neurons tend to be organized into groups or tracts of similar
nerves. Again, tracts from the left side of the body make a crossover
in the spinal cord or brain into the right cerebral hemisphere; tracts
from the right side of the body cross into the left cerebral hemisphere.
Brain or spinal cord injuries, where cells are destroyed or major nerves
are severed, have largely been untreatable up to this time. Permanent
paralysis in body areas normally served by nerves going through an
affected area has had to be an acceptable condition for many individuals.
Nerves of the peripheral nervous system have been able to regenerate if
injured (but with the cell bodies left intact) and this is presenting possible
treatments of spinal cord injuries. Researchers have been attempting to
use nerves from the peripheral system as bypasses around injured areas
in the spinal cord. Results, so far, have been mixed.
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Lesson 8
B.
The Peripheral Nervous System
All nerves and nerve networks outside of the brain and spinal cord are part of
this system.
The Somatic Nerves
Many of the cranial (brain) and spinal nerves are voluntary nerves, sometimes
considered as a somatic system. This system carries messages for voluntary
actions back and forth between the body parts and brain. The body has
conscious control over these. The somatic system controls skeletal muscle,
bones and skin.
The Autonomic Nerves
Another group of peripheral nerves belongs to an involuntary or autonomic
nervous system. In most instances, and for most individuals, the actions or
responses carried out by this system cannot be consciously controlled. The
autonomic system largely regulates conditions in the internal environment or
inside the body by controlling the actions of organs or glands. Two sets of
peripheral nerves connect organs or glands to the central nervous system.
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Lesson 8
They are:
Sympathetic
Parasympathetic
Nerves of the sympathetic system
come from the thoracic vertebrae
(ribs) and lumbar (small of the back).
The sympathetic system is involved
in all internal adjustments that
prepares the body for action or
increased levels of stress.
Nerves of the parasympathetic
system leave the brain directly or
from either the cervical (the neck
area) or caudal (tail bone) sections
of the spinal cord. The
parasympathetic nerves carry
impulses that return a body to
"normal" functioning after a period
of stress is over.
The opposing actions of the sympathetic and parasympathetic systems are
excellent examples of homeostasis occurring within the body. Fine
adjustments are continually being made so that glands and organs are
functioning at levels appropriate to a body at a particular time.
Sympathetic
Dilates pupils
Inhibits saliva
Constricts blood vessels
Accelerates heart
Bronchi dilate
Glycogen converts to glucose
Bladder relaxes
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Parasympathetic
Constricts pupil
Stimulates salivation
Dilates blood vessels
Reduces heart rate
Constricts bronchi
Stimulates bile release
Contracts bladder
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Lesson 8
Nervous Disorders
Nervous disorders can be caused by:
1.
Injuries
Physical juries resulting in swellings of nerve cells (such as from blows)
could interfere with transmission of impulses. Temporary numbness or
loss of sensations, along with partial paralysis, could result. Swellings or
pressure (as of bones) on some nerves could bring about continual pain
or discomfort (as with sciatica). Severing of a spinal cord higher up, near
the head, produces a quadriplegic with no feelings or control in any
limbs.
2.
Diseases caused by:

Bacteria
Some nervous disorders are brought about by pathogens or
disease-causing organisms. Bacterial infection can cause swellings of
the brain's membranes or meninges. The resulting condition of
meningitis can result in fevers and headaches and, in severe form,
coma and death. Another type of bacteria can
enter open wounds and produce tetanus.
This bacteria blocks inhibitor transmitters
from being formed in synapses. Impulses
move in a continuous fashion across synapses
resulting in sustained contraction of voluntary
muscles – particularly those in the jaw and
throat. Such muscle contractions lead to
spasms, convulsions and possible death.

Viruses
On the prairies there are two diseases caused by viruses that get more
public attention. They are:
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Lesson 8
Rabies
Transmission
Rabies virus is commonly transmitted in the saliva of warm-blooded
animals. It moves into the spinal cord and brain of an infected organism
and, in the active phase, begins to destroy nerve cells. This active phase
could begin after an incubation period of anywhere from ten days to
seven months after actual infection.
Symptoms



In the active phase the virus to destroy brain and spinal cord cells.
Muscle spasms and convulsions appear. These often occur first in
jaw and throat muscles so that affected animals cannot swallow.
Paralysis of other muscles eventually leads to unconsciousness and
death. The seriousness of the disease is emphasized by the fact that
there are few (less than three) who have survived the an active phase.
It is therefore important to treat pets with anti-rabies vaccine as a
preventative measure or to begin immediate anti-rabies vaccinations
of humans infected by confirmed rabid animals.
Encephalitis
Western Canada is occasionally subject to outbreaks of virus-caused
sleeping sickness or encephalitis. This seems more common to horses
and humans, although it does occur in other mammals as well as birds.
Transmission
The virus is transmitted through the salivary glands of such insects as
mosquitoes, which are intermediate hosts.
Symptoms
As with rabies, the virus begins destroying nerve cells in the brain.
Chills, fevers and inflammation of the brain progress to the point where
muscle paralysis begins. Unless treated, this can lead to death. Even if
treated, some permanent damage could remain.
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Lesson 8
3.
Abnormal growths or tumors could create pressures on certain areas of
the brain. These pressures could result in losses of certain functions
and could also bring about emotional and personality changes. Blood
vessels to and from the brain play important roles in supplying nutrients
and removing wastes.
4.
Strokes, due to partial blockages of vessels because of blood clots or
swellings, or because of the bursting of vessels, could deprive nerve cells
of food and oxygen. These situations can lead to deaths of brain cells
and various degrees in loss of functioning.
5.
Nervous disorders that often tend to create uncomfortable feelings in the
general public are those related to abnormal emotional or behavioral
development. While some can be traced to genetic and other specific
causes, others are not so easy to explain. Sudden swings in moods or
emotions can have unsettling effects as well as dangerous implications
for some individuals. Paranoia and schizophrenia fit into this category.
Chemicals and the Nervous System
A human body ingests or takes in a wide range of matter. Much of this is nutrient
material and is essential for supplying the body with energy or building parts. Along
with the nutrients, humans take in other materials – either knowingly or unknowingly,
which have wide ranging effects on the nervous systems.
The term "drugs" may be applied to some of these, although the term can unfairly
give the idea of something which is either prescribed by doctors or taken illegally in
order to get a "high" or some euphoric feeling.
Drugs or chemicals which affect nervous systems are often part of many natural
nutrients, especially of the plant varieties. Seeds, leaves, stems and roots of certain
plants can yield substances which can be ingested directly, ground into powders for
mixing with other substances, boiled, fermented or smoked. The chemicals taken
into bodies can have wide-ranging effects on nervous systems. Nerves can be easily
affected.
Quite often, chemicals have their greatest effects at the synapses,
where they either increase the movements of impulses from neuron
to neuron or slow them down.
The effects which chemicals have on nerves and synapses can divide them into three
general classes.
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Lesson 8
1.
Stimulants are chemicals which have the effects of imitating the stimulatory
transmitters produced at synapses. They enable impulses to travel more easily
and quickly from nerve to nerve. . Stimulants initially heighten or sharpen an
individual's awareness. They also stimulate the sympathetic nervous system,
which ordinarily prepares a body for increased activity.
Coffee, cola or chocolates could supply varying amounts of the stimulant
caffeine. Smokers take nicotine into their systems. Dieters, athletes or
workers wanting to stay awake may take amphetamines
2.
Depressants work in an opposite fashion to stimulants. They slow or stop the
movements of impulses across synapses. Individuals have slower reaction
times and a "dulling" of the senses as the depressants block impulses.
Depressants reduce awareness or sharpness so that stress and tension are
reduced and there is a decreased feeling of responsibility as well. Depressants
also affect parts of the brain in different ways. Some individuals undergo
emotional and behavioral changes. Centers of the brain used for muscular
co-ordination and balance, vision and memory are affected. Large amounts of
depressants can affect the areas of the brain stem responsible for vital
functions, such as breathing, so that coma and death can result.
Pain killers and tranquilizers are often prescribed medicinally to ease physical
pain or emotional upsets. Alcohol, opiates, barbiturates, marijuana and
anesthetics from glues or petroleum products are other depressants.
A danger of both stimulants and depressants being taken over longer periods of
time or in larger amounts is that they can become both psychologically and
physically addictive. Psychologically, a coffee or a cigarette may be used to fill
some time or to occupy one's hands or actions in some way. Over time,
physical dependence means that a body has reached a state where removing a
chemical produces unpleasant withdrawal symptoms so that an individual
often cannot break a "habit" alone. To make matters worse, individuals take
more and more of a substance to achieve certain effects. In larger amounts,
some chemicals bring on memory failures, severe personality changes and
hallucinations. Suicidal tendencies frequently develop from these conditions.
Alcoholics can develop alcohol psychosis, with the symptoms just mentioned.
Hospitalization and psychotherapy are the treatments for such individuals.
3.
Stimulants and depressants, in larger amounts, can bring about hallucinations
– sights, sounds or other sensations that are not really there. These chemicals
are normally classified as hallucinogens. Drugs such as LSD, and PCP ("Angel
Dust") are often placed in this category. As a group, hallucinogens are
probably the most unpredictable and dangerous in the kinds of effects they
could cause. Extreme behavioral changes are possible and a recurrence of
symptoms or flashback at a future date is possible with some of these
substances.
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Lesson 8
The ingestion of chemicals or drugs cannot be avoided. They enter bodies in food,
water or air. Their use, or amount of it, should probably be viewed in much the same
way as many other things: in controlled amounts or in situations that warrant them,
they could be harmless or even helpful; in large amounts they could be fatal.
This would be a good time to take a break before continuing
with the rest of the lesson.
The Senses
The first part of this lesson looked at possible ways in which messages or impulses
could be transmitted within bodies. Ways messages can be transmitted within
bodies are:


hormones
by electrochemical impulses.
The kinds of reactions which occur in organisms vary greatly. These variations could
be due to the different types of stimuli received or to the manner in which they are
received.
A stimulus can be regarded as any kind of change in the external or
internal environment that somehow captures the awareness of an
organism.
Externally, stimuli can include changes in temperature, sound levels or lighting
conditions. Internal stimuli can also include temperature; in addition, these stimuli
can include pressure within organs or vessels, glucose or carbon-dioxide levels in the
blood, different hormone levels and salt concentrations.
An environmental change or stimulus is not necessarily perceived by an organism. A
weakness humans often have in the studies of other organisms is in wrongly
assuming that most species detect stimuli in similar ways.
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Lesson 8
Different species have different abilities in detecting stimuli.


A dog or a cat is much more sensitive to a greater range of sounds or smells than
humans are.
On the other hand, cats and dogs and other mammals do not seem to have the
same abilities to detect colors or visual details under light conditions that humans
do. Many mammals are completely or partially colorblind – seeing the features
around them in shades of grey or in "washed-out" pastel colors. These animals
can quickly notice objects when movements occur; however, as hunters, livestock
producers and various other people know, a person can often escape detection by
other animals only a short distance away by remaining as motionless as possible.
The kinds of stimuli that affect an organism at a certain time and the manners, in
which they are received and noticed, make up that organism's sensation or
perception. Much depends on both the kinds and numbers of receptors present and
the amount of ganglia or brain development to interpret the impulses received.
A receptor is an individual sensory cell that responds to stimuli either inside
or outside the body by sending impulses along a neuron. Receptors in the
skin respond to pain, to touch. In a sense organ like the eye receptors like
rods or cones occur.
Some indication of the differences in perception or sensation between species was
referred to earlier. These differences can result in a great range of responses or
behaviors to what seemingly is the same stimulus. Conscious sensation to a
stimulus in a more complex organism really takes place in the brain. Pain in a leg is
only recognized as pain by a brain. Even if receptors are present to pick up light,
sound or other environmental conditions, sensations will not become evident if
impulses cannot be transmitted to the brain or if neural centers there are not
effective in interpreting them.
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Lesson 8
Some type of "awareness" of what is taking place externally or internally is one of the
necessary conditions of homeostasis. This enables organisms to regulate and
co-ordinate those activities necessary to sustain their own lives and, on a larger
scale, that of their species.

Sensations could be of a conscious type with organisms recognizing some stimuli
and reacting to them.Such stimuli generally originate externally and the most
common behaviors of life forms are to move towards or away from them. Sound,
light (or the sight of something) and smell are a few stimuli which organisms may
become conscious of and react to.

There are also sensations, often below the conscious level of an organism, which
mostly have internal origins and are just as important as any of the external ones.
Internal receptors pick up body temperatures, fluid and salt concentrations and
amounts of gases. Such sensations could be passed on to neural centers or the
autonomic nervous system to be acted upon automatically by the body. Some
awareness may take place, as with knowing that our breathing rates have gone
up, that we may be sweating, or that we may be hungry or thirsty or have a
craving for something.
General Roles of Receptors and Senses
Reactions to stimuli received by receptors and the resulting sensations could be
summarized into a number of general roles.
1.
Internally, there are constant adjustments made to stimuli in keeping cells and
bodies operating as efficiently as possible.
2.
Locating and ingesting the proper kinds of food with the help of smell, sight,
sound and even taste are especially important for heterotrophic organisms
which must locate organic matter.
3.
Many heterotrophs, predators included, must survive situations which could
be harmful to them. These situations range widely from being aware of other
potentially harmful organisms to carrying out certain actions without causing
injury to self. (Consider trying to walk a distance without the benefit of sight or
sound!) Even sensations of pain could be protective in that organisms will
attempt to avoid or to prevent more pain (at least until healing is complete).
4.
On a wider scale, senses play an important role in social behavior and species
survival. Sight, smell, sound and contact all play roles in such actions as
establishing territories and finding mates, courtship, bonding between
offspring and parents and setting up "social" systems among groups of
organisms.
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Lesson 8
Reception of Stimuli
The types of receptors which receive stimuli can be divided into two very general
groupings.
1.
The less specialized of the two is a neurosensory receptor. Bringing to mind
that neurons can be sensory, associative (interneuron) or motor, a
neurosensory receptor is only a modified sensory neuron. This receptor cell
may have a slightly modified dendrite for receiving a stimulus and passing an
impulse on to the cell body and onward along the axon. All less complex
organisms up to the invertebrates have this type of receptor. More advanced or
complex organisms have these as well.
2.
Receptors have also appeared in the advanced groups that can be regarded as
specialized, non-nerve cells. A sensory neuron is closely associated with each
receptor, but the dendrite of the sensory neuron merely picks up and passes
along the impulse that had originally been received by the special receptor.
When some of the sensations are examined in more detail, it will be seen that there
are special receptor cells adapted for specific stimuli. The following illustrations
indicate some of the different receptor cells in the skins of some animals.
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Lesson 8
Most receptor cells tend to be scattered throughout the body or over a body surface,
although concentrations could vary from area to area. The human hand has more
touch receptors at the fingertips and palm than on the back. A cat or a mouse has a
high number of touch receptors at the base of each whisker, which helps to guide
movements in dark areas. In some instances, certain sense receptors are clustered
together in specific regions to form sense organs, such as eyes and ears.
Five senses are usually referred to:
sight,
smell,
taste,
touch
hearing.
There are many others present in a human body.

There are separate receptors for heat and cold in the skin. More important
receptors for heat and cold are located in the hypothalamus of the brain. It is
these receptors which, by detecting body temperature from the blood, set
involuntary actions into motion to cool or warm up a body.

Receptors to pressure (a slightly different sensation than touch) are located deeper
in the skin. Other pressure receptors are located in the walls of blood vessels
such as carotid arteries. Stimulations of these can affect heart rate.

Special pressure or balance receptors are present in ears, muscles, joints and
tendons which make an organism aware of the location of body parts, movements
and balance
Close your eyes. Extend your arms out in front
of you. Now bring your index fingers together.
Open your eyes. How close were you?

Receptors sensitive to concentrations of C02, water, salt and other substances are
located in other body areas.

Nonhuman organisms possess receptors for stimuli that we cannot detect. Some
insects can receive impulses from ultraviolet light while certain snakes can locate
warm-blooded animals from infrared or heat radiation that is given off by the
animals.
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Lesson 8
Thermoreceptors – are the receptors capable of picking up stimuli of heat and cold,
whether on body surfaces or internally.
Photoreceptors – include those cells which are sensitive to light intensities as well as
the different wavelengths of light or the colors.
Chemoreceptors – are the cells in which impulses can be stimulated by chemicals
dissolved in the air or water. Chemoreceptors would include those sensations
associated with taste and smell.
Mechanoreceptors – contain a varied group of different receiver cells that are
somehow stimulated by pressure or tension changes. Sound waves in air, liquids or
in solids; actual physical contacts; and, different degrees of tension in muscles and
other connective tissues, can create impulses in mechanoreceptors. These impulses
lead to such sensations as sound, touch, pressure and balance.
Sensations in Unicellular Organisms
Many unicellular organisms make some responses or movements to certain
conditions in their environments. Responses may simply be of the types showing an
avoidance to or an attraction for, certain stimuli. Since receptors, transmitters and
neural areas are not obvious, one can only assume that the movements may occur as
reactions to sensations created by the stimuli.
Some unicellular life forms do begin to show distinct sensory developments.
Eyespots make their first appearances in organisms like Euglena. They cannot
make out movements or form images. Eyespots have pigment molecules that react
chemically with light. The reactions enable individuals to sense light intensities or to
determine the direction of its source. Eyespots are also prominent in some of the
simple, multicellular organisms such as copepods (crustacean group) and planaria
(flatworms). In simple "animal" organisms, these tropism-like movements (common
to plants) are called taxes. The movement of Euglena towards a light source would
be positive phototaxis.
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Lesson 8
Receptors and Sensations in Multicellular Groups
Receptors in invertebrates are usually neurosensory cells or sensory neurons that
have only slight modifications in their dendrites. The lack of specialized receptors for
picking out only certain kinds of stimuli, means that the neurosensory cells can be
affected by more than one stimulus. Reactions or body movements can therefore
occur when there are changes in light, temperature, humidity or chemicals. Some
regulation of responses is controlled by larger masses of ganglia or "brains" at
anterior ends. It has been shown that removing "brain" ganglia from such
invertebrates as worms and some insects still enabled them to live; however, the
individuals made many uncoordinated muscular reactions to any stimuli which
seemed strong enough to arouse an impulse in a neurosensory cell. The role of the
larger numbers of "brain" ganglia therefore seems to be that of regulating which body
parts make responses or in what order they occur. It does not seem that
invertebrates are able to form sensations which are as distinct, or movements which
are as precise, as those of vertebrates. Reactions that take place are probably quick
responses to general stimuli – such as an indistinct blur or movement causing an
insect to fly or to make a leap.
Chemoreceptors – Taste and Smell
Smell
An ability to smell is an important benefit to many organisms in locating the right
foods. In addition, smell can be used to detect and avoid predators, to establish
territories (with scent or pheromone markings), to find mates, to return to spawning
waters and many other uses.
For smells to be received, there must first be sources of chemicals dissolved in vapor
or liquid (water). Dissolved molecules can move in air or liquid so that smell is
common to both
terrestrial and aquatic or marine organisms. We may think it unlikely that
organisms can "smell" underwater, however some fish can detect small amounts of
chemicals from great distances.
Receptors for smell can be in a variety of locations on or in bodies. Simple
invertebrates, such as earthworms, are able to receive chemical stimulations through
skin receptors all over the body. Some fish also have receptors scattered over the
body, in addition to clusters located in nasal cavities. Some arthropod groups have
smell and taste receptors concentrated on antennae, which can receive molecules
from the water or air.
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Lesson 8
For many groups, there is little to distinguish between smell and taste. Both
sensations originate with dissolved molecules adhering to receptors somewhere on or
in the bodies. The only distinction which could be made is that smell is generally
associated with molecules being received from farther away.
Taste
Taste is associated with the process of eating or being in contact with food. In
vertebrate groups, the distinction is more apparent as receptors for the sensations
tend to be clustered in slightly different areas – smell in nasal cavities or passages
and taste in and around the mouth.
Researchers feel that nasal structures were adaptations initially intended for smell.
In most fish and in some amphibians, during parts or all of their life cycles, this may
be evident in that the nasal structures are cavities without any openings to the
mouth. Water circulates in and out through external openings, carrying chemical
molecules that may stimulate olfactory cells in these cavities. Terrestrial life forms
have an internal passage to the mouth that assists in the function of breathing.
Whether nasal developments are cavities or are part of passages to the mouth,
certain areas have concentrations of olfactory receptors. Unlike many other sensory
receptors, olfactory cells pick up impulses through their dendrites and transport
them directly along their own axons to the olfactory lobes of the brain. (Sensory
receptors for other stimuli are usually associated with other nerve cells which
transmit the impulses to neural centers.) Olfactory cells have hairlike endings in
their dendrites extending to the surface area of surrounding epithelial cells. These
surrounding cells produce a mucus layer in which the dendrites lie. Molecules in the
nasal structures must diffuse through the mucus to reach the olfactory dendrites
and stimulate them. In our own situation, this movement may slow down when we
have excessive mucus buildup during colds.
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Lesson 8
It is not known with certainty just how the smell receptors are stimulated and why
differences in smells can be noticed. Speculation is that chemical molecules may be
shaped to fit onto only certain receptors. Each receptor type may be sensitive to a
particular kind of molecule, which may include those which could cause musty,
pungent, "flowery" or other smell sensations. Stimulations of the different olfactory
receptors at the same time could result in varying smells.
In some groups of organisms, most noticeably in snakes and lizards, there is a
separate and additional development at the anterior portion of the nasal structure.
Jacobson's organs are pouchlike areas with their own clusters of olfactory cells.
Narrow passages connect these with the roof of the mouth. Snakes and lizards may
flick their forked tongues out of their mouths and draw them back in. Inside the
mouth, the two forks of the tongue are pushed into twin slits leading into the
Jacobson's organs. Molecules picked up on a tongue can then stimulate the olfactory
nerves there. In this way, these animals are able to increase their smell sensations
by using their tongues.
A vertebrate group in which olfactory cells seem to be scarce in the
nasal passages of many species is that of birds. Sensations of smell
apparently are not that important for most bird species. In fact, a
poor smelling ability may be an advantage to an owl eating a skunk or
to crows or vultures eating long-dead carcasses.
Taste receptors are usually activated during the process of eating or being in close
contact with food. In many invertebrate organisms, the chemoreceptors scattered
throughout the skin or located on antennae can be regarded as producing both smell
and taste sensations (combined). Besides antennae, some insects have chemical or
taste receptors on bristles on parts of their bodies, including the legs. This helps
some flies to taste their food just by walking on it. Some fish have taste receptors
scattered over their bodies just as some invertebrates do. Catfish, suckers, sturgeon
and other bottom-foraging fish are especially benefited by this arrangement in murky
waters. In these fish and others, taste receptors are also clustered on the outside
surfaces near their mouths and inside as well.
Most vertebrates have the greatest
concentrations of taste receptors or taste buds
on their tongues. Smaller numbers are
scattered around the sides and backs of
mouths. Taste buds are located near the upper
surface of a tongue and near the sides. Each
bud is recessed slightly below the surface and
holds a group of taste cells.
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Lesson 8
The tongue must be moist and the food molecules must be dissolved before molecules
can enter the pores to stimulate the cells. Again, as with smells, colds can cause
more mucus secretions which reduce the ability to taste.
Human taste cells give rise
to four main taste
sensations: sweet, sour,
salty and bitter. These are
scattered all over the
tongue, but in most
individuals there are
specific patterns where
more receptors for each
kind of sensation are
located. Sweet sensations
are mostly picked up by
receptors at the tip of the
tongue. Salt sensations
also originate at the tip, but
extend farther back. Sour
tastes originate along the
sides while bitterness is mostly received on the back. Different combinations of the
taste receptors being stimulated account for variability in taste sensations. A great
deal of the final taste sensation is also associated with smell. Individuals holding
their noses closed while eating something, or having their nasal passages blocked by
congestion, may not recognize some foods for some time, if at all.
Mechanoreceptors – Touch, Pressure, Hearing
Many mechanoreceptors are cells whose dendrites are highly branched and extend
into skin, muscle and tendon areas. These receptors are stimulated by physical
movements or stresses which can have both external and internal origins.
Touch receptors are located near skin surfaces and their dendrites frequently form
networks around the bases of hairs. External movement of a hair will usually start a
touch impulse. Firmer or slightly stronger skin contacts will activate impulses in the
deeper pressure receptors. In many fish, pressure receptors are concentrated along a
lateral line running along both sides from head to tail. These receptors, or lateral line
organ, can be used to detect water movements and pressure changes which could be
due to objects or other creatures in the water.
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Vertebrates also have receptors to muscle fibers and tendons which are continually
stimulated by contractions. These stimulations provide a "muscle sense" to the
nervous system. A body is aware of the state of contraction of the various muscles
and also the position of various body parts, which aid in balance. "Muscle sense" can
give some amputees the impression that they still have certain limbs, which they
don't. Sensation in a "ghost" limb could develop at the point where the amputation
was performed. Stimulation of nerves cut at this point and still remaining alive could
make it seem as though the sensation is coming from the missing limb.
Mechanoreceptors in muscles and tendons help to provide for a sense of balance. In
vertebrates, another very important area that contributes to balance is the inner ear.
As with the nose, which we may relate more to breathing than to smell, we may think
of ears more in terms of hearing than balance. Yet once more some biologists feel
that ear chambers developed from structures originally adapted for balance. Some
invertebrates, such as crayfish, have fluid-filled cavities lined with hairlike dendrites.
Inside the cavities are calcium particles which are free to move about. As they do so,
they stimulate the hairlike nerve endings so that individuals are aware of their body
position and balance.
Some vertebrates have enclosed cavities without any external openings, just as some
arthropods do. These are evident in fish. Movements of fluid in the cavities lined
with sensitive nerve "hairs" enable balance to be maintained. Modifications of such
cavities may have led to semicircular canals found in other vertebrates. These
consist of three fluid-filled canals, called semicircular canals, arranged at right
angles or perpendicular to each other. Each canal has the hairlike structures of a
nerve positioned at one of its ends. Movements of a head or body cause fluids in
these canals to move as well and, depending on the direction of movements, the
nerve ends have impulses activated which produce a sense of balance. (Spinning
movements can "confuse" a brain with almost simultaneous nerve impulses from
these canals, resulting in dizziness and a loss in balance.)
INSERT PICTURE OF EAR HERE
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Regardless of what its primary function may have been originally, an ear has become
an organ for forming another important sensation – sound. Sound is originally
produced by vibrating objects. The vibrations can then be transmitted through air,
liquids or solids.
In a vertebrate ear, sound vibrations pass along the auditory canal until they reach
the tympanic membrane or eardrum. This thin membrane separates the outer and
middle ear. An Eustachian tube connects the middle ear to the throat (See diagram
above). This serves to keep air pressures the same on both sides of the eardrum.
Swollen membranes due to colds, or sudden altitude changes in planes or even in
vehicles going up and down hills, can result in unequal pressures and slight hearing
loss. (Yawning or chewing and swallowing can lead to a "popping", where air
pressures become balanced.)
Sound vibrations start the eardrum vibrating. Vibrations are passed along three,
small connecting bones with the common names of hammer, anvil and stirrup.
INSERT PICTURE OF HAMMER/ANVIL/STIRRUP HERE
The last of these bones is connected by an oval window to a snail-shaped structure
called a cochlea. Vibrations are transmitted through a fluid in the cochlea. A
membrane and sensitive hairlike nerve endings line the cochlea. The shapes and
lengths of these nerve ends vary so that only certain ones are affected by particular
pitches of sound. Microscopic examinations have shown that nerve deafness to
certain pitches or tones develops when high intensity levels cause the "hairs" to
actually disintegrate. High intensity levels of varying pitches cause disintegration of
nerve endings throughout the cochlea and lead to general deafness. Many prairie
farmers suffer general losses of hearing, which become more evident in their later
years, after operating high noise level machinery. Motorcycles, all-terrain vehicles,
airports and even loud music are some other causes of deafness.
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Different species have different sensitivity levels to sounds. Many nonhuman
vertebrates have come to rely quite heavily on sound in their lives. Consequently,
many have the ability to pick out high or low frequencies which we cannot. "Silent"
dog whistles have high frequencies that dogs can detect but which humans cannot.
Some creatures such as bats and dolphins emit high frequency squeaks or "clicks" as
a form of echo location in guiding them around obstacles or to prey. High freguency
show a variability in the development of hearing structures. Invertebrates show a
variability in the development of hearing structures. Insects commonly have a
tympanum or ear membrane at the side, either at the back of the thorax or near the
beginning of the abdominal segments. Many invertebrates have no hearing
structures. Sensitivity to vibrations, which can be picked up through legs and other
body parts, keeps up their awareness in their surroundings.
Photoreceptors – Eyespots and Eyes
The beginning of distinguishable receptors to light are the eyespots of some
unicellular and simple, multicellular groups. Euglena and the "cross-eyed worm",
planarian, are examples having eyespots. Eyespots contain pigment molecules which
undergo chemical breakdown when exposed to certain light intensities. In a
unicellular organism this seems to affect the cell's cytoplasm. In multicellular
organisms, the pigment reactions start impulses in neurosensory cells. These
impulses travel along nerve axons to ganglia. Eyespots are not able to form images.
Their sensitivity to different light intensities permits a choice in movements either
towards or away from light. This is positive/negative phototaxis.
Many arthropods have compound eyes with the ability to form images. A compound
eye is made up of many repeating units or ommatidia (See diagram).
Each unit is shaped like a long tube with a number of
visual cells inside. Pigment layers around the sides
prevent the passage of light to adjacent units. The
presence of the pigment layers thus allows light to
enter each unit only from directly in front. Light from
the sides or angles is blocked off.
Each unit usually forms an image of only a
small part of an object close to an arthropod.
This is similar to looking microscopically at the
pages of many magazines or newspapers. This
would show print and pictures to be made up of
many small dots which, combined together, form
an overall image.
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It is a misconception to think that each unit of a compound eye forms a complete and
separate image. A fly, grasshopper or crayfish does not see hundreds of complete
images when looking at one object. All the units together form a very blurred
(because of unit overlapping), indistinct image. Compound eyes are very poor for
forming distinct images. Objects moving across the fields of view of the units of a
compound eye create a "flickering" effect and therefore movement is very easily
detected. Some species with compound eyes also possess simple eyes sensitive to
light and movement. A grasshopper, for instance, has three simple eyes in addition
to the compound pair. Some groups have only simple eyes. A spider may have up to
eight or nine simple eyes scattered over its head.
The visual receptor cells in arthropods are varied in the kinds of light-absorbing
pigments which they have. The different pigments are stimulated by different
wavelengths of light and this produces different color sensations. Most arthropods
are believed to have some degree of color vision. In addition, the eyes of some
(insects) can be stimulated by short wavelength, ultraviolet light (which is normally
screened from us by many substances such as clouds, glass and our own eyes).
Ultraviolet stimulations enable insects to know where the sun is even on cloudy days.
It also creates image sensations of objects different from our own. Insects see
patterns or lines that we cannot.
The basic structure of vertebrate eyes is essentially similar among all groups.
Adaptations, however, have resulted in many minor differences. The human eye will
serve as our illustration.
INSERT ILLUSTRATION OF EYE
The Human Eye
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All eyes are roughly spherical in shape. A tough, white scleroid coat forms the outer
part of the eye. A small area called the cornea is clear to allow the passage of light.
In terrestrial vertebrates the curved cornea also helps to deflect light inward. The
next layer is the choroid coat. This has many blood vessels running through it to
maintain the cells of the eye. It also contains pigments. In animals active mainly in
light conditions, the pigment is of a type that absorbs light and prevents its reflection
back onto the retina through which it has passed. This prevents secondary
stimulations and distortions. Nocturnal organisms need as much light as possible
and therefore, reflecting light back to the retina is an advantage to them. Their
choroid coats contain pigment types which reflect, rather than absorb, light. This
mirror-like effect results in the "glowing" effect of some eyes if a strong light is shone
on them. The inner layer contains light sensitive receptor cells which could be of two
types – rods or cones. Portions of the choroid and retinal layers are modified to form
the iris or covering in front of the lens. An opening in the iris, the pupil, is controlled
by surrounding muscle layers which also change the shape of the lens. The cavities
in front and behind the lens are filled with watery, jellylike fluids.
Images are formed when light stimulates the receptor cells in the retina. Rods
contain a combination of two pigments, one of which is derived from Vitamin A. The
combination is sometimes called visual purple, since the two pigments together are
reddish-purple. Light striking visual purple causes it to break down. This sets off an
impulse in a rod which is transmitted along a series of nerves to the brain. Impulses
from the rods result in the formation of images which are in shades of grey (like black
and white television). Visual purple is very sensitive to light and breaks down
quickly, but reforms very slowly. It could take up to 30 minutes going from a bright
area to a dimly lit one for the combined pigment to reform and be ready for further
stimulation. This is why it takes individuals a while to become adjusted going into
darkness from a bright area. Inability to form visual purple, perhaps due to vitamin
A deficiency, could produce night blindness. Whereas rods contain only visual
purple, cones have a number of other different pigments which are sensitive to
different wavelengths of light and identify colour. Humans have three types of cones
which, if their pigments break down, create impulses to the brain resulting in
sensations of red, green and violet. Stimulations of various cone combinations can
result in many intermediate colors.
All vertebrate eyes have rods. However, the presence or numbers of cones are not
the same among groups. Many mammals, particularly nocturnal ones, have few or
no cones. As a result, many of these animals are colorblind. The image sensations
which their brains form are in different shades of grey. The presence of some cones
results in some color image sensations. If there are few cones, colors are subdued or
somewhat like pastels where the colors are apparent but still have greyish tints to
them. It is not yet known for sure just what color capabilities different groups of
animals have. Indications from research suggest birds and reptiles have very good
color vision. Among the mammals, only the primate groups seem to have good color
vision. Other mammals are colorblind for specific colors or have so few cones that
most of their image sensations are mostly in shades of grey. (It may not make much
difference to animals whether big game hunters wear white or red, although certain
shades could possibly have fluorescent effects.)
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If cones and rods are both present, most of the cones are concentrated in a small
area at the back of the eye called the fovea. This somewhat depressed area is packed
very tightly and almost exclusively with cone cells. Very few rods or other cells,
including blood vessels, are located in this area. The sensitivity of cones to color and
also their characteristic of having more nerve connections to the brain produce the
most accurate images in light conditions. The much more numerous rods are located
in the rest of the retinal area. Their extreme sensitivity to light causes them to be the
major sources of impulses for image formation when it is dark. The fovea with its
cones is largely ineffective in darkness. These conditions result in humans often
noticing movements in darkness out of the "edges" or "corners" of their eyes, rather
than gazing directly at the movements. The images produced from impulses from the
rods are not as sharp or as distinct as those from cones. Many more rods share
nerve circuits to the brain and this does not favor sharp images.
The million or so optic nerve fibers from the rods and cones of each eye pass through
the retina and form the optic nerve (tract). At the point where the nerves pass
through the retina, there are no rods or cones so there is a small blind spot. The
brain or the other eye fills in the image so that a blind spot is not noticeable, unless a
special exercise is carried out.
To find your blind spot, cover your left eye with one hand.
Stare at the following cross with your right eye while moving
the page very slowly away or towards you. You should find a
distance where the circle disappears. When this happens,
the circle has been focused on your blind spot.
+
o
The optic nerves from each eye commonly cross over to different hemispheres of the
brain. For organisms whose eyes are focused laterally or to the sides, the brain
actually produces two images. Primates, animals of prey and some other groups,
have eyes focused ahead. The brain produces separate, but almost identical,
overlapping images. This stereoscopic vision results in a perception of depth and a
better estimate of distance. Another characteristic of mammals whose fields of view
overlap, is that when the two optic nerves cross, nerves from corresponding parts of
each eye join together and go to the same part of the brain. This helps the brain to
join the two images together.
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Vision Defects
Three very common vision defects are:
nearsightedness (known as myopia) – occurs when the eyeball is too long. The lens
is not able to flatten enough to project the image on the retina. The image is brought
into focus in front of the retina. The person who is nearsighted can focus on images
that are close but objects that are at a distance are blurry. To correct
nearsightedness glasses that are concave are used.
farsightedness (known as hyperopia) – occurs when the eyeball is too short. Objects
in the distance are brought into focus behind the retina instead of on it. So the
person who is farsighted can see objects in the distance clearly but cannot see
objects up close clearly. This defect is corrected by glasses with a convex lens.
Astigmatism – The lens or cornea of the eye is irregularly shaped. They should be
symmetrical so the light is refracted on identical angles on all surfaces. This would
form a sharp focal point. Glasses can correct this.
Thermoreceptors – Heat and Cold
Separate receptors for cold and heat are present in the skin of vertebrates, with a
four to five times greater number of the cold receptors. Near the surfaces, such
receptors stop functioning if there is any damage or destruction to the skin. Loss of
cold sensations can be especially bad in freezing conditions as, with feeling gone,
wider and deeper areas of tissue can be permanently damaged. A different group of
thermoreceptors function in the hypothalamus of the brain. These are sensitive to
internal body temperatures (determined from the blood). Slight fluctuations can start
actions of the autonomic nervous system in correcting the situation.
A special group of heat receptors is evident in a group of snakes called pit vipers.
Pits located slightly below the nostrils and in front of the eyes contain nerve receptors
very sensitive to heat radiation. Rattlesnakes can locate warm-blooded animals in
darkness by the heat which those animals give off.
Pain
Some question exists as to the types of nerve cells which carry impulses to produce
pain sensations. All of the previous sensations can actually cause pain. Suddenly
bright or glaring light, intense or certain frequency sounds and even tastes – such as
sweetness producing aches, can cause uncomfortable or painful sensations. Some
pain may be caused by impulses carried by the different types of receptors, if those
impulses are of a higher frequency. That is, if the nerve cells experience a higher
than usual number of stimulations in a short period of time. There are also nerves
which are specifically adapted for pain and exist in all body parts. These pain
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receptors have many-branched, bare (without myelin) dendrites extending out from
cell bodies. Although located in all body areas, concentrations are not the same.
Some body parts or areas will experience pain more readily than others.
Two receptor types maintain some degree of continual awareness as long as nerves are
not damaged.

One of these is that of the pressure receptors to muscles, tendons and organs. This
group's continual impulses to the autonomic nervous system are necessary for the
proper balancing and functioning of a body.

The other receptor type is pain. Pain is an ongoing sensation without medical
treatment, and disappears only as body repair or healing occurs. Pain causes an
organism to favor an injured or affected body part and to avoid situations which can
risk additional injury (or more pain). This avoidance and favoring action generally
results in faster recovery. It is probably important that these types of sensations of
internal pressures and pain be maintained for their survival value to an individual.
Summary
Inability to transmit "messages" internally or to lose sensations could have serious, if
not fatal, effects for living things. To lose the ability to maintain awareness or to make
responses, whether conscious or of the involuntary system, would mean that
adjustments in body conditions could not be carried out to match external or internal
changes. In natural conditions, animals which are born or hatched blind, with no sense
of hearing or without some other sense, rarely survive for very long. A loss of thirst or
hunger, balance, taste or pain, could also be potentially dangerous for any individual.
In plants, the consequences and losses in abilities to control or to make adjustments to
internal conditions are not as obvious. However, with continuing studies, observations
may show these to be just as important as in any other group of living organisms.
Besides survival value, sensations provide organisms with comfortable feelings or
feelings of enjoying particular stimuli. Birds or animals stretching out in warmer,
sunny areas during cool periods or wallowing in cool mud during hot weather; a cat
rubbing itself on catnip; a cow, moose or bear scratching itself on a fence, branch or
log, all show obvious signs of enjoying various sensations. We could add many other
enjoyable sensations which we know from our own experiences. These may include
certain tastes, sounds, sights, colors or touch sensations. It is perhaps unfortunate
that most of us probably take too many sensations for granted. We do not realize
their place in our lives unless we are born without certain senses or lose some after
birth.
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