Download The Living World - Chapter 28 - McGraw Hill Higher Education

Essentials of
The Living World
First Edition
GEORGE B. JOHNSON
23
The Nervous System
PowerPoint® Lectures prepared by Johnny El-Rady
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23.1 Evolution of the
Animal Nervous System
The nervous system links sensory receptors & motor
effectors in all vertebrates (and most invertebrates)
Association neurons (or
interneurons) are located in
the brain and spinal cord
Central Nervous
System (CNS)
Motor (or efferent) neurons
carry impulses away from CNS Peripheral
Nervous System
Sensory (or afferent) neurons (CNS)
carry impulses to CNS
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Fig. 23.1 Organization of the vertebrate nervous system
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Fig. 23.2 Three
types of neurons
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Invertebrate Nervous Systems
Sponges are the only major phylum of
multicellular animals that lack nerves
Cnidarians have simplest nervous system
Neurons are linked to one another
through a nerve net
Fig. 23.3
No associative activity
Just reflexes
First associative activity is seen in
free-living flatworms
Two nerve cords run down bodies
Permit complex control of muscles
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Evolutionary path to vertebrates
1. More sophisticated sensory
mechanisms
2. Differentiation into central and
peripheral nervous systems
Fig. 23.3
3. Differentiation of sensory
and motor nerves
4. Increased complexity of
association
5. Elaboration of the brain
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23.2 Neurons Generate
Nerve Impulses
All neurons have the same basic structure
Cell body – Enlarged part containing the nucleus
Dendrites – Short, slender input channels
extending from end of cell body
Axon – A single, long output channel extending
from other end of cell body
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Most neurons require nutritional support provided by
companion neuroglial cells
Schwann cells (PNS) and oligodendrocytes (CNS)
envelop the axon with fatty material called myelin
Myelin acts as a electrical insulator
During development cells wrap themselves around
each axon several times to form a myelin sheath
Uninsulated gaps are called nodes of Ranvier
Nerve impulses jump from node to node
Multiple sclerosis and Tay-Sachs disease result
from degeneration of the myelin sheath
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Fig. 23.4 Structure of a neuron and formation of the myelin sheath
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The Nerve Impulse
When a neuron is “at rest”, active transport channels
in cell membranes move Na+ out and K+ into cells
Concentration of Na+ builds up outside the cell
K+ may diffuse out through open channels
Thus, neuron’s outside is more positive than inside
Cell membrane is said to be “polarized”
Resting potential is the charge separation between
cell’s interior and exterior
– 70 millivolts
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A nerve impulse results from ion movements of in
and out of voltage-gated channels
A sensory input causes Na+ channels to open
Sudden influx of Na+ into cell causes “depolarization”
Local voltage change opens adjacent Na+ channels
Thus, an action potential is produced
After a slight delay, K+ voltage-gated channels open
K+ flows out of the cell
The negative charge in the cell is restored
Na+ channels snap close again
The resting potential is restored by the action of the
sodium-potassium pump
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Fig. 23.5
How an
action
potential
works
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Fig. 23.5
How an
action
potential
works
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23.3 The Synapse
A synapse is the junction of an axon and another cell
Presynaptic membrane
Located on the near
(axon) side of the
synapse
Postsynaptic membrane
Located on the far
(receiving) side of
the synapse
Fig. 23.6
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Neurotransmitters are chemical messengers that
carry nerve impulses across synapses
Bind to receptors in the postsynaptic cell
Cause chemically-gated channels to open
Fig. 23.7
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Kinds of Synapses
Excitatory synapse
Receptor protein is a chemically-gated sodium channel
On binding the neurotransmitter, the channel opens
Na+ floods inwards
Action potential begins
Inhibitory synapse
Receptor protein is a chemically-gated potassium or
chloride channel
On binding the neurotransmitter, the channel opens
K+ floods outwards or Cl– floods inwards
Action potential is inhibited
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Kinds of Synapses
An individual nerve cell
can possess both kinds
of synapses
Integration
Various excitatory
and inhibitory
electrical effects
cancel or reinforce
one another
Occurs at the
axon hillock
Fig. 23.8a
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Neurotransmitters and Their Functions
Acetylcholine
Released at the neuromuscular junction
Have an excitatory effect on skeletal muscle
and inhibitory effect on cardiac muscle
Glycine and GABA
Inhibitory neurotransmitters
Important for neural control of brain function
Biogenic amines
Dopamine – Control of body movements
Serotonin – Sleep regulation and mood
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23.4 Addictive Drugs Act
on Chemical Synapses
Neuromodulators are chemicals that prolong the
effect of neurotransmitters
Aid their release
Prevent their reabsorption
Example:
Depression may be
caused by a shortage
of serotonin
Prozac, inhibits its
reabsorption
Fig. 23.9
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Drug Addiction
A cell that is exposed to a chemical signal for a
prolonged time, loses its “sensitivity”
It tends to lose its ability to respond to the
stimulus with its original intensity
Nerve cells are particularly prone to this loss of
sensitivity
They respond to high neurotransmitter exposure
by inserting fewer receptor proteins
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Drug Addiction
Cocaine is a neuromodulator
It causes large amounts of neurotransmitter to
remain in synapses for long periods of time
Dopamine transmits pleasure messages in the
body’s limbic system
High levels for long periods of time, cause
nerve cells to lower the number of receptors
Addiction occurs when chronic exposure to a drug
induces the nervous system to act physiologically
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Fig. 23.10 How drug addiction works
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Addiction to Smoking
“Nicotine receptors” normally served to bind
acetylcholine
Brain adjusts to prolonged exposure to nicotine by
1. Making fewer nicotine receptors
2. Altering the pattern of activation of nicotine receptors
Addiction occurs because the brain compensates
for the nicotine-induced changes by making others
There is no easy way out
The only way to quit is to quit!
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23.5 Evolution of the Vertebrate Brain
Brains of primitive fish, while small, already had the
3 divisions found in contemporary vertebrate brains
1. Hindbrain
Rhombencephlon
2. Midbrain
Mesencephlon
3. Forebrain
Prosencephlon
Fig. 23.12
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Hindbrain
Major component of early fishes, as it is today
An extension of the spinal cord devoted primarily to
coordinating muscle reflexes
Most coordination is done by the cerebellum
Midbrain
Composed primarily of optic lobes
Receive and process visual information
Forebrain
Devoted for processing olfactory (smell) information
Note:
Brains of fishes continue growing throughout their lives!
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Starting with the amphibians, sensory information is
increasingly centered in the forebrain
Diencephalon
Thalamus – Relay center between incoming sensory
information and the cerebrum
Hypothalamus – Coordinates nervous and hormonal
responses to many internal stimuli and emotions
Telencephalon
Devoted largely to associative activity
Cerebrum (mammals)
Dominant part of the brain
Receives sensory data and issues motor commands
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Fig. 23.13 The evolution of the vertebrate brain
Cerebrum
dominance
is greatest
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23.6 How the Brain Works
Cerebrum is ~ 85% of the weight of the human brain
Functions in
language, thought,
personality and
other “thinking and
feeling” activities
Much of activity
occurs in the
cerebral cortex
Gray outer layer
Fig. 23.14
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Fig. 23.15 The major functional regions of the human brain
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The cerebrum is divided by a groove into right and
left halves called cerebral hemispheres
Linked by bundles of neurons called tracts
Serve as information highways
In general, the left brain is associated with language,
speech and mathematical abilities
The right brain is associated with intuitive,
musical, and artistic abilities
Stroke
A disorder caused by blood clots blocking blood
vessels in the brain
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Thalamus
Major site of sensory processing in the brain
Controls balance
Fig. 23.16
Hypothalamus
Integrates internal
activities
Body temperature,
blood pressure, etc.
Controls pituitary
gland secretions
Linked to areas of
cerebral cortex via
limbic system
Center for pain,
anger, sex,
hunger, etc.
Memory center
Responsible for deep-seated drives and emotions
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Cerebellum
Extends back from the base of the brain
Coordinates muscle movement
Even better developed in birds
Brain Stem
Made up of midbrain, pons, and medulla oblongata
Connects rest of brain to spinal cord
Controls breathing, swallowing, digestion
As well as heart beat and blood vessel diameter
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Language and other higher functions
Left hemisphere is “dominant” hemisphere for language
It is adept at sequential reasoning
The “nondominant” hemisphere (the right hemisphere in
most people) is involved in
Spatial reasoning (assembling puzzles)
Musical ability
Short-term memory appears to be stored electrically in the
form of a transient neural excitation
Long-term memory appears to involve structural changes
in certain neural connections
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Alzheimer Disease
Memory and thought processes become dysfunctional
Two hypotheses have been proposed for the cause
1. Brain nerve cells are killed from the outside in
Accumulation of plaques of abnormal external
proteins called b-amyloid peptides
2. Brain nerve cells are killed from the inside out
Accumulation of tangles of abnormal internal
proteins called tau (t)
Researchers continue to study whether tangles and
plaques are causes or effects of Alzheimer disease
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23.7 The Spinal Cord
The spinal cord is a cable of neurons extending from
the brain down through the backbone
Neuron cell bodies in the center
Gray matter
Axons and dendrites on the outside
White matter
It is surrounded and protected by the vertebrae
Through them spinal nerves pass out to the body
Motor nerves from spine control most of the muscles
below the head
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Fig. 23.19 The vertebrate nervous system
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23.8 Voluntary and Autonomic
Nervous Systems
Are two
subdivisions
of vertebrate
motor
pathways
Fig. 23.20
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The voluntary nervous system relays commands to skeletal
muscles
Can be controlled by
conscious thought
Reflexes are sudden
involuntary movements
Are rapid because
sensory neuron
passes information
directly to a motor
neuron
Most involve single
connecting interneuron
between sensory and
motor neurons
Fig. 23.21 The
knee-jerk reflex
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The autonomic nervous system stimulates glands and relays
commands to smooth muscles
Cannot be controlled by conscious thought
Composed of elements that act in opposition to each other
Sympathetic nervous system
Dominates in time of stress
Controls the “fight-or-flight” reaction
Increases blood pressure, heart rate, breathing
Parasympathetic system
Conserves energy by slowing down processes
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Fig. 23.22 How
the sympathetic
and
parasympathetic
nervous systems
interact
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23.9 Sensory Perception
The sensory nervous system tells the central
nervous system what’s happenin’!
Sensory receptors
Adapted to
nocturnal life
Specialized sensory
cells that detect
changes inside and
outside the body
Sensory organs
Complex sensory
receptors
Eyes, ears,
taste buds
Fig. 23.23 Kangaroo rats have specialized ears
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The path of sensory information is a simple one
1. Stimulation
Physical stimulus impinges on a sensory receptor
2. Transduction
Stimulus-gated ion channels in sensory neuron are
opened or closed
An action potential is generated
3. Transmission
Nerve impulse is conducted to the CNS
Two main types of sensory receptors
Extroreceptors sense stimuli in external environment
Introreceptors sense stimuli in internal environment
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Sensing the Internal Environment
Vertebrates use many different sensory receptors
to respond to changes in internal environment
Temperature Change
Two nerve endings in the skin
One stimulated by cold, the other by warmth
Blood chemistry
Receptors in arteries sense blood CO2 levels
Pain
Special nerve endings within tissues near the surface
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Muscle contraction
Sensory receptors
embedded within
muscle
Fig. 23.24
Touch
Pressure receptors
buried below skin
Fig. 23.25
Blood pressure
Neurons called
baroreceptors in
major arteries
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23.10 Sensing Gravity and Motion
Receptors in the ear inform the brain where the body
is in three dimensions
Balance
Gravity is detected by shifting of otolith sensory receptors
These are located in a gelatin-like matrix in the utricle
and saccule chambers of the inner ear
Motion
Motion is detected by the deflection of hair cells by fluid in
a direction opposite to that of motion
These hair cells are found in the cupula
Tent-like assemblies in the three semicircular canals
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Fig. 23.26 How the inner ear
senses gravity and motion
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23.11 Sensing Chemicals:
Taste and Smell
Taste
Taste buds are
located in raised
areas called
papillae
Fig. 23.27
Food chemicals
dissolve in saliva
and contact the
taste cells
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23.11 Sensing Chemicals:
Taste and Smell
Smell
Olfactory receptor
cells are embedded
in the epithelium of
the nasal passage
Fig. 23.28
These are far more
sensitive in dogs
than in humans
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23.12 Sensing Sounds: Hearing
When a sound is heard, air vibration is detected
Eardrum membrane is pushed in and out by
waves of air pressure
Three small bones (ossicles) located on other
side of eardrum increase the vibration force
Amplified vibration is transferred to fluid
within the inner ear
Inner ear chamber is shaped like a tightly
coiled snail shell and is called cochlea
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Cochlea sound receptors are hair cells that rest on a
membrane running up and down the chamber
They are covered by another membrane
Sound waves entering the cochlea cause this
membrane “sandwich” to vibrate
Bent hair cells send nerve impulses to brain
Sounds of different frequencies cause different
parts of the membrane to vibrate
Different sensory neurons are fired
Sound intensity is determined by how often the
neurons fire
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Fig. 23.29
Structure and
function of the
human ear
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The Lateral Line System
Supplements the fish’s sense of hearing
Fish are able to sense objects that reflect
pressure waves and low-frequency vibrations
The system consists of canals running the length of
the fish’s body under the skin
Canals have sensory structures containing hair
cells projecting into a gelatinous cupula
Vibrations produce movements of the cupula
Hair cells bend and depolarize associated
sensory neurons
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Fig. 23.30
The lateral
line system
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Sonar
Some mammals perceive distance by sonar
Bats, shrews, whales
They emit sounds
and then determine
the time it takes for
the sound to return
This process is
called echolocation
Fig. 23.31 Using ultrasound to locate a moth
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23.13 Sensing Light: Vision
Vision begins with the capture of light energy by
photoreceptors
Many invertebrates
have simple visual
systems
Photoreceptors are
clustered in eyespot
Perceive light
direction but not
a visual image
Fig. 23.32 Simple eyespots in the flatworm
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23.13 Sensing Light: Vision
Members of four phyla have evolved well-developed,
image-forming eyes
Annelids
Mollusks
Arthropods
Vertebrates
The eyes are strikingly similar in structure
But are believed to have evolved independently
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Fig. 23.33 Eyes in three phyla of animals
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Structure of the Vertebrate Eye
The vertebrate eye works like a lens-focused camera
Fig. 23.34
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Structure of the Vertebrate Eye
Cornea – Transparent covering that focuses light
Lens – Completes the focusing
Ciliary muscles – Change the shape of the lens
Iris – Shutter that controls amount of light
Pupil – Transparent zone
Retina – The back surface of the eye
Contains two types of photoreceptors
Rods and cones
Fovea – Center of retina
Produces the sharpest image
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How Rods and Cones Work
Rods are extremely
sensitive to dim light
Cannot distinguish
colors
Do not detect edges
Produce poorly
defined images
Cones can detect color
Detect edges well
Produce sharp
images
Fig. 23.35
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Pigment in rods and cones are made from carotenoids
cis-retinal is attached to a protein called opsin
This light-gathering complex is called rhodopsin
When light is absorbed by cis-retinal, it changes shape to
trans-retinal
Fig. 23.36
This induces a
change in the shape
of the opsin protein
A signal-transduction
pathway is initiated
leading to generation
of a nerve impulse
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Color Vision
Three kinds of cone cells exist, each with its own opsin type
Differences in opsin shape, affect the flexibility of the
attached cis-retinal
This shifts the
wavelength at
which it absorbs
light
420 nm – Blue
530 nm – Green
560 nm – Red
Fig. 23.37
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Color Vision
Colorblindness is a condition in which a person cannot see
all three colors
Caused by a lack of
one or more types of
cones
It is inherited as a
sex-linked trait
More likely to
affect males
Fig. 23.38
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Conveying the Light Information to the Brain
Rods and cones are at
the rear of the retina,
not front!
Light passes through
four types of cells
before it reaches them
Photoreceptor
activation stimulates
bipolar cells, and then
ganglion cells
Nerve impulse
travels through the
optic nerve to the
cerebral cortex
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Fig. 23.39
Binocular Vision
Primates and most predators
have eyes on front of the head
The two fields of vision overlap
Allows the perception of
3-D images and depth
Fig. 23.40
Prey animals generally have eyes
located on sides of the head
This prevents binocular vision
However, it enlarges the
perceptive field
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23.14 Other Types of
Sensory Reception
Fig. 23.41
Heat
Pit vipers can locate warm
prey, using infrared radiation
Heat-detecting pit organs
Electricity
Used by aquatic
vertebrates to
locate prey and
mates
Magnetism
Eels, sharks and many
birds orient themselves w.r.t
the Earth’s magnetic field
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