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Equine hippocampus
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The Nervous system.
Every living organism must be able to react appropriately to changes in its
environment if it is to survive. The nervous system monitors and controls almost
every organ system through a series of positive and negative feedback loops. The
nervous system consists of brain, nerves and spinal cord. The nervous system pass
messages around the body control the body. The brain also controls the senses.
Vertebrates have complex sense organs and exhibit complex behaviors. These
require a complex nervous system. The vertebrate nervous system is extremely
cephalized.
The nervous system can be subdivided several ways depending on if one is looking at
function or location:
In terms of function,
Somatic NS
voluntary muscles and
reflexes
vs
Autonomic NS
visceral/smooth and cardiac muscle
Sympathetic NS
increases energy
expenditure
prepares for action
Parasympathetic NS
decreases energy
expenditure
gains stored energy
these have the opposite effects on the same organs
--OR—
In terms of location,
Peripheral NS
vs
sensory and motor neurons
Central NS (CNS)
interneurons: brain and spine
covered with three membranes, the meninges
inflammation of these is called meningitis
brain has gray matter on outside and white in center
spine has white matter on outside and gray in center
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The central nervous system (CNS) is the brain and spinal cord.
The peripheral nervous system (PNS) is composed of the nerves and ganglia.
Ganglia are clusters of nerve cell bodies outside the CNS.
The nervous system consists of two types of cells. Nerve cells are called neurons.
The typical neuron is an elongated cell that consists of a cell body, containing the
nucleus. Various support cells are associated with the neurons, most typically,
Schwann cells. The parts of a neuron include the dendrite which receives the
impulse (from another nerve cell or from a sensory organ), the cell body (numbers
of which side-by-side form gray matter) where the nucleus is found, and the axon
which carries the impulse away from the cell. Wrapped around the axon are the
Schwann cells, and the spaces/junctions between Schwann cells are called nodes of
Ranvier. Collectively, the Schwann cells make up the myelin sheath (numbers of
which side-by-side form white matter).
Schwann cells wrap around the axon (like the camp food,
“pigs in a blanket”). Having an intact myelin sheath and nodes of Ranvier are critical
to proper travel of the nerve impulse. Diseases which destroy the myelin sheath
(demyelinating disorders) can cause paralysis or other problems. Schwann cells are
analogous to the insulation on electrical wires, and just as electrical wires short out
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if there’s a problem with the insulation, so also, neurons cannot function properly
without intact myelin sheaths.
Autonomic Nervous System
This part of the nervous system sends signals to the heart, smooth muscle, glands,
and all internal organs.
It is generally without conscious control.
The autonomic nervous system uses two or more motor neurons:
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The cell body of one of the motor neurons is in the CNS. The cell body of the other
one is in a ganglion.
Sympathetic Division-The sympathetic nervous system prepares the body to deal
with emergency situations. This is often called the "fight or flight" response.
Stimulation from sympathetic nerves dilates the pupils, accelerates the heartbeat,
increases the breathing rate, and inhibits the digestive tract.
The neurotransmitter is norepinephrine.
Sympathetic nerves arise from the middle (thoracic-lumbar) portion of the spinal
cord.
Parasympathetic Division- When there is little stress, the parasympathetic
system tends to slow down the overall activity of the body. It causes the pupils to
contract, it promotes digestion, and it slows the rate of heartbeat.
The neurotransmitter is acetylcholine.
The actual rate of stimulus to each organ is determined by the sum of opposing
signals from the sympathetic and parasympathetic systems.
Parasympathetic nerves arise from the brain and sacral (near the legs) portion of
the cord.
Enteric Division- The enteric division contains neurons that control the digestive
tract, pancreas, and gallbladder. Activity of the enteric division is usually regulated
by the sympathetic and parasympathetic divisions.
The nervous system has three basic functions:
1. sensory neurons receive information from the sensory receptors,
2. interneurons transfer and interpret impulses, and
3. Motor neurons send appropriate impulses/instructions to the muscles and
glands.
Meninges
CNS is covered by meninges which is made up of three membranes viz. duramater,
arachnoid and piamater. Outer most being duramater and innermost piamater with
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arachnoid in between. Cranial meninges are closely united with endosteum of cranial
cavity. Epidural space is the space between spinal meninges and the periosteum.
This space is used for epidural anaesthesia.
The brain contains fluid-filled ventricles that are continuous with the central canal
of the cord. Fluid within the ventricles and central canal originates from the blood.
It slowly circulates, carrying nutrients and wastes from cells. The fluid eventually
returns to the circulatory system and is replaced by fresh fluid.
Brain- it is enclosed in cranial part of skull.
During embryonic development, the brain first forms as a tube, the anterior end of
which enlarges into three hollow swellings that form the brain, and the posterior of
which develops into the spinal cord. Brain can de divided: three different ways to
sub-divide the brain.
1) Embryonic division; telen- , dien-, mesen-, meten-, myelin- cephalon.
2) Gross anatomy divisions; cerebrum, cerebellum (part of meten-), brain stem
(dien-, mesen-, pons, myelin- cephalon.).
3) Clinically useful divisions; forebrain (telen-, dien- cephalon), hind brain
(meten- and myelin- cephalon).
But for our better understanding the Brain is divided into forebrain, midbrain and
hindbrain.
1) Forebrain is made up of cerebrum, mammillary body, pineal body, thalamus
and hypothalamus.
2) Midbrain consists of a small area that serves as a connection between
forebrain and hindbrain.
3) Hindbrain consists of medulla oblongata, pons and cerebellum. Convoluted
part of brain is called gyrus and the depression between gyri is called
fissure.
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Parts of the brain as seen from the middle of the brain.
Brain stem
Medulla
Oblongata
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Arachnoid
Longitudinal fissure
Cerebral
hemisphere
Forebrain
1) Thalamus- Like the midbrain of mammals, the thalamus serves as a relay
area to the cerebrum from other parts of the spinal cord and brain. For
example, it receives sensory input (except smell) and sends to appropriate
areas of the cerebral cortex.
The Thalamus contains part of the reticular formation (see below).
Reticular Formation- The reticular formation is a net of nerve cells
extending from the thalamus through the brain stem (midbrain, pons and medulla
oblongata) to the spinal cord.
It acts as a filter to incoming stimuli and discriminates important from
unimportant. Hundreds of millions of sensory receptors flood the brain; the brain
does not have the capacity to deal with even a small fraction of this information, so
much of it must be ignored.
The reticular activating system (RAS) is the part of the reticular formation that
controls wakefulness.
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Sleep centers are located in the reticular formation. Neurons in one sleep center
secrete serotonin, a chemical that inhibits the RAS and thus causes drowsiness and
sleep.
Another sleep center secretes factors that counteract serotonin and bring about
wakefulness.
Damage to these centers can lead to unconsciousness or coma.
2) Hypothalamus- The hypothalamus regulates the endocrine system by
controlling the secretions of the pituitary gland or by producing some of the
hormones that are secreted by the pituitary. These hormones affect the
body or affect other glands in the body. Their overall affect is to maintain
homeostasis. The hypothalamus also contains neurons associated with the
limbic system (below).
Limbic System- The limbic system contains neural pathways that connect
portions of the cortex, thalamus, hypothalamus, and basal nuclei (several areas deep
within the cerebrum).
It causes pleasant or unpleasant feelings about experiences (rage, pain, pleasure,
sorrow). This helps guide the individual into appropriate behavior that is more likely
to be beneficial.
3) Cerebrum- the largest part of the brain is divided into left and right
hemispheres. The hemispheres are covered by a thin layer of gray matter
known as the cerebral cortex and it is divided into four lobes: occipital,
temporal, parietal, and frontal lobes. Figure below show lobes of cerebrum.
The cerebrum became greatly enlarged as evolution progressed from the
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earliest vertebrates to mammals. In reptiles and mammals, it receives
sensory information and coordinates motor responses.
Motor responses to the skeletal muscles originate in the cerebrum but are refined
and coordinated by the cerebellum.
Olfactory Bulbs- The anterior parts of the cerebral hemispheres are called the
olfactory bulbs. It receives input from the olfactory nerves (smell). The olfactory
bulbs of primitive vertebrates comprise a large proportion of the cerebrum.
Cerebral Cortex- Over evolutionary time, gray matter developed over the
cerebrum. This is the cerebral cortex and it is an information-processing center. It
increased in size more rapidly than the skull so that it has become folded
(convoluted) in order to fit in the skull.
Intelligence, emotion, creativity, learning, and memory are localized in the cerebral
cortex.
Lobes of the cerebral cortex
The cerebral cortex is divided into four lobes; each receives information from
particular senses and processes the information into higher levels of consciousness.
Lobe
Frontal
Function
motor functions; permits conscious control of skeletal muscles; contains
the primary motor cortex
conscious thought
Parietal sensory areas from the skin; contains the primary sensory cortex
Occipital The primary visual cortex is located within the occipital lobe.
Temporal hearing and smell
Primary Sensory and Primary Motor Cortex- The primary sensory cortex is a
narrow band of cortex tissue that extends from one side of the cortex near the
ear over the top of the brain to the other side. Information from sensory
receptors in the skin arrives at this area. The motor cortex is a band of cortex
tissue directly anterior (in front) of the primary sensory cortex. Signals that
control the skeletal muscles originate in this area.
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Corpus Callosum- The corpus callosum contains neurons that cross from one side
of the brain to the other, allowing each half to communicate with each other.
Corpus callosum
Hindbrain
1. Medulla oblongata- The medulla controls vital functions such as
breathing, heart rate, and blood pressure.
It also contains reflexes such as vomiting, coughing, sneezing, hiccupping,
swallowing, and digestion.
Information that passes between the spinal cord and the rest of the brain must
pass through the medulla. In the medulla, sensory and motor axons on the right
side cross to the left side and axons on the left side cross to the right side. As a
result, stimuli passing through from the left side of the body are sent to the right
side of the brain and signals passing through from the right side of the brain
stimulate the left side of the body.
2. Cerebellum- The cerebellum coordinates and refines complex muscle
movements. Movement information that is initiated in higher brain centers
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(the cerebral cortex) is compared to the actual position of the limbs. The
cerebellum then adjusts and refines the movement.
It is large in birds because flight requires considerable coordination.
3. Pons- The pons is involved in some of the same activities as the medulla.
For example, it assists the medulla in controlling breathing.
The pons functions as a connection between higher brain regions, the cerebellum,
and the spinal cord.
Midbrain
The midbrain receives some sensory information and sends it to the appropriate
part of the forebrain.
The midbrain originally functioned for reflexes associated with visual input. It is
the most prominent part of the brain in fishes and amphibians and has major
control of the body.
The midbrain of reptiles and mammals controls visual reflexes such as the pupil
response to light intensity but the forebrain of these vertebrates processes the
visual information (see diagram below).
The midbrain also controls some auditory reflexes and helps control posture.
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Brainstem
The medulla oblongata, pons, and midbrain look like the spinal cord and appear to
connect the rest of the brain to the spinal cord. They are collectively referred to
as the brainstem.
Summary of Brain Structure
Brain
Structure
Function
Vital functions such as breathing, heart rate, and blood pressure
Medulla
oblongata
Reflexes such as vomiting, coughing, sneezing, hiccupping,
swallowing, and digestion
Neurons cross
Pons
Breathing, connects spinal cord, cerebellum and higher brain
centers
Cerebellum
Motor coordination
Receives visual, auditory, and tactile information
Midbrain
In mammals, this information is sent to the thalamus and higher
brain centers. In lower vertebrates, the information is further
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processed in the midbrain.
Thalamus
Hypothalamus
Cerebrum
Relays sensory information to the cerebral cortex.
Contains part of the reticular formation (controls arousal).
Maintains homeostasis, regulates the endocrine system
Contains part of the Limbic system (controls emotion)
Processes sensory information and produces signals that move the
skeletal muscles.
This is the outer layer of the cerebrum.
Cerebral
Cortex
Thinking, intelligence, and cognitive functions are located here.
Processing of sensory information and motor responses
Spinal cord
The spinal cord runs along the dorsal side of the body and links the brain to the
rest of the body. Vertebrates have their spinal cords encased in a series of
(usually) bony vertebrae that comprise the vertebral column. The gray matter of
the spinal cord consists mostly of cell bodies and dendrites. The surrounding white
matter is made up of bundles of interneuronal axons (tracts). Some tracts are
ascending (carrying messages to the brain), others are descending (carrying
messages from the brain). The spinal cord is also involved in reflexes that do not
immediately involve the brain. Function of spinal cord is summarized below.
Spinal Cord
the spinal cord receives information from skin, joints,
and muscles
 sends back signals for both voluntary and reflex
movements
 transmits signals from internal organs to the brain and
from the brain to internal organs
 connects the brain to peripheral organs and tissue
 in addition, the spinal cord contains
ascending pathways through which sensory information

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reaches the brain
descending pathways that relay motor commands
from the brain to motor neurons
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The Spinal Cord-The vertebrae surround and protect the spinal cord. Cerebrospinal
fluid within the central canal functions to cushion the spinal cord. Many sensory - motor
reflex connections are in the spinal cord. Interneurons often lie between sensory
and motor neurons.
White matter- White matter contains tracts that connect the brain and the
spinal cord. The white colour is due to the myelin sheaths.
Gray matter- Gray matter looks gray because it is unmyelinated. It contains the
short interneurons that connect many sensory and motor neurons. Sensory neurons
enter the gray matter and the axons of motor neurons leave the gray matter. The
cell bodies of these motor neurons are located in the gray mater.
Piamater
White mater
Gray mater
T/S: spinal cord of mammalian
Peripheral nervous system (PNS)
PNS are made up of cranial nerves, spinal nerves and autonomic nervous system.
Each of this is described on their function as below:
Nerve
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A nerve is an enclosed, cable-like bundle of nerve fibers or axons. Based on its
origin a nerve can be cranial or spinal.
Cranial nerves originate from the brainstem, and mainly control the functions of
the anatomic structures of the head.
There are 12 pairs of cranial nerves that originate from various parts of brain and
supply different sense organs in head.
Number
Name
Function
I
Olfactory Nerve
Smell
II
Optic Nerve
Vision
III
Oculomotor Nerve
Eye movement; pupil dilation
IV
Trochlear Nerve
Eye movement
V
Trigeminal Nerve
Somatosensory information (touch, pain) from the
face and head; muscles for chewing.
VI
Abducens Nerve
Eye movement
VII
Facial Nerve
Taste (anterior 2/3 of tongue); somatosensory
information from ear; controls muscles used in
facial expression.
VIII
Vestibulocochlear
Nerve
Hearing; balance
IX
Glossopharyngeal Nerve
Taste (posterior 1/3 of tongue); Somatosensory
information from tongue, tonsil, pharynx; controls
some muscles used in swallowing.
X
Vagus Nerve
Sensory, motor and autonomic functions of viscera
(glands, digestion, heart rate)
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XI
Spinal Accessory Nerve
Controls muscles used in head movement.
XII
Hypoglossal Nerve
Controls muscles of to tongue
Spinal Nerves
Spinal nerves take their origins from the spinal cord. They control the functions of
the rest of the body. In cattle, there are 37 to 39 pairs of spinal nerves: 8
cervical, 13 thoracic, 6 lumbar, 5 sacral and 5 -7 coccygeal. The naming convention
for spinal nerves is to name it after the vertebra immediately above it. Thus the
fourth thoracic nerve originates just below the fourth thoracic vertebra. This
convention breaks down in the cervical spine. The first spinal nerve originates
above the first cervical vertebra and is called C1. This continues down to the last
cervical spinal nerve, C8. There are only 7 cervical vertabra and 8 cervical spinal
nerves.
Spinal nerve is made up of dorsal and ventral root and its branches. Dorsal root
emerges from dorsal portion of spinal cord and carries afferent (sensory) impulses
to the spinal cord from peripheral parts of body. Ventral root emerges from the
ventral part of spinal cord and carries efferent (motor) impulses from spinal cord
to the effector (muscles). Figure below show components of spinal nerve.
Functional unit
Neuron or a nerve cell is an anatomic and physiologic unit of nervous system. It is
made up of cell body and processes dendrites and axon. A process is called dendrite
if it carries impulse towards cell body and an axon if it carry impulses out of cell
body. Axon is called nerve fiber and membrane covering the axon is called
axolemma. In a myelinated axon axolemma is surrounded by myelin sheath called
neurilemma, at regular interval there is myelin free gaps called nodes of Ranvier
that facilate faster conduction of impulses. A group of nerve cell bodies in CNS
(brain and spinal cord) is called nucleus while group of nerve cell bodies outside CNS
is called ganglion. Bundle of neuron fibers within the CNS is called a tract while
bundle of neuron fibers outside CNS is called nerve. Figure below show the neuron.
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Cell body
Dendrites
Axon
Myelin sheath
Node of Ranvier
Terminal bulb
Figure A
Figure B
Amyelinated (A) and Myelinated (B) neurons
Synapse
Junction between two nerve terminals is called synapse. There is no physical
contact between the neurons instead there is space in between, across which
impulse is conducted through chemicals.
Impulse conduction
For effective conduction of nerve impulse there are three stages viz. resting
potential, depolarization and repolarization.
Membrane Potentials
Membrane potentials were first demonstrated using the giant axons of a squid
(1mm dia). An oscilloscope measured the electrical difference by placing one
electrode outside the neuron and the other inside the neuron.
Resting potential
The sodium-potassium pump pumps out 3 sodium ions (Na+) for each 2 potassium
ions (K+) pumped into the neuron. This results in more potassium ions inside and
more sodium ions on the outside.
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Unequal pumping (3 Na+ out to 2K+ in) results in more positive charge on the outside
compared to the inside. The membrane is polarized.
Some K+ channels are open so K+ tends to leak out. This contributes to the negative
charge inside. The charge difference prevents further leakage.
The charge difference is measured in millivolts.
Gated Channels
The membrane contains channels that open or closes, allowing the polarity of the
membrane to change as ions pass through the channel.
Ligand-gated channels are found in the synapses on postsynaptic cells. They open
when bound to specific ligands (molecules or ions) such as specific
neurotransmitters.
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Voltage-gated channels open when the membrane becomes depolarized. For
example, sodium gates open and then close slowly when the membrane is depolarized
but remain closed when it is polarized. When the sodium channel is open, sodium can
pass through.
In a resting (polarized) neuron, sodium gates are closed. A slight depolarization will
not cause the gates to open but if the depolarization is greater than a threshold
value, the gates will open.
Propagation of an Action Potential
Stimulation of the neuron causes Na+ gates open allowing Na+ to rush in. This results
in depolarization of the membrane in the area where the stimulation occurred.
Depolarization of an area of membrane stimulates more Na+ gates in adjacent areas
to open, thus spreading the depolarization.
Immediately after depolarization, then Na+ ions channels close and K+ channels open
causing K+ to flow out. This process returns positive charge to the area just outside
the membrane, thus restoring the resting polarity.
The depolarization and repolarization events described above are called an action
potential. During an action potential, the depolarization spreads all the way to the
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action terminal where the axon joins another cell. The action potential is "all or
nothing." The intensity of an action potential does not diminish as it spreads along
an axon.
The sodium-potassium pump operates continuously to restore the ionic gradient.
In the diagram below, depolarization caused by the influx of sodium can be seen
spreading to the right.
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Refractory Period
The action potential cannot reverse its direction because membrane that has just
been depolarized cannot be depolarized again until after a brief recovery (called
refractory) period. During this period, the membrane is insensitive to stimulation.
The diagram below shows the voltage difference across the membrane as an action
potential proceeds. Initially, the inside of the membrane is approximately -65 or 70 millivolts compared to the outside. When sodium gates open and sodium ions rush
in, the inside temporarily becomes positively charged. Potassium gates then open
and potassium ions rush out, restoring the negative charge.
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Most action potentials last a few milliseconds and there may be as many as several
hundred action potentials per second.
Saltatory Conduction
The gap between the Schwann cells in the myelin sheath is called a node of Ranvier.
Gated channels are concentrated in this area and not in the area under the myelin
sheath.
The action potentials jump from node to node (saltatory conduction). This increases
the speed at which a neuron can conduct a signal.
Sodium-potassium pumps require a substantial amount of energy to pump the ions,
so the presence of insulation reduces the amount of membrane that requires active
sodium-potassium pumps, thus saving energy.
The diameter of the neuron also is related to the speed of conduction. Larger
diameter axons conduct faster.
Synaptic Potentials
Synapses
A synapse is a junction between a neuron and another cell. It is separated by a
synaptic cleft.
In most synapses, the axon terminal of the presynaptic cell contains numerous
synaptic vesicles with neurotransmitter stored within them.
The action potential causes calcium channels to open in the plasma membrane of the
presynaptic cell. The calcium ions (Ca++) diffuse into the neuron and activate
enzymes, which in turn, promote fusion of the neurotransmitter vesicles with the
plasma membrane. This process releases neurotransmitter into the synaptic cleft.
Neurotransmitter molecules diffuse across the cleft and stimulate the
postsynaptic cell, causing Na+ channels to open. Depolarization of the postsynaptic
cell results.
The depolarization of the postsynaptic cell is referred to as a synaptic potential.
The magnitude of a synaptic potential depends on:
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The amount of neurotransmitter
The electrical state of the postsynaptic cell. If it is already partially depolarized,
an action potential can be produced with less stimulation by neurotransmitters. If it
is hyperpolarized, it will require more stimulation than normal to produce an action
potential.
After the neurotransmitter is released into the synaptic cleft, it must be quickly
removed or inactivated to prevent the postsynaptic cell from being continuously
stimulated and to allow another synaptic potential.
In some cases there may be enzymes present in the synaptic cleft that break down
the neurotransmitter immediately. For example, acetylcholinesterase breaks down
the neurotransmitter acetylcholine.
In other cases, the axon terminal may reabsorb neurotransmitter and repackage it
into vesicles for reuse.
Excitatory and inhibitory postsynaptic potentials - A synaptic
potential can be excitatory (they depolarize) or inhibitory (they polarize). Some
neurotransmitters depolarize and others polarize.
There are more than 50 different neurotransmitters.
In the brain and spinal cord, hundreds of excitatory potentials may be needed
before a postsynaptic cell responds with an action potential.
Synaptic integration- Synaptic integration is the combining of excitatory and
inhibitory signals acting on adjacent membrane regions of a neuron.
In order for an action potential to occur, the sum of excitatory and inhibitory
postsynaptic potentials must be greater than a threshold value.
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Synapses closest to the trigger zone will have the greatest influence.
Temporal and Spatial Summation
The effect of more than one synaptic potential arriving at a neuron is additive if
the time span between the stimuli is short. This is called temporal summation. The
summation effect is greatest when the time interval between stimuli is very short.
The effect of more than one synaptic potential arriving at a given region of a
neuron can also be additive. This is called spatial summation. The summing effect is
greater if multiple stimuli all arrive at nearby areas of a membrane. The effect is
less if they stimulate separate, distant areas.
Direction of
Impulse
1
Figure sowing the direction of impulse and synapse
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Autonomic nervous system (ANS)
The organs (the "viscera") of our body, such as the heart, stomach and intestines,
are regulated by a part of the nervous system called the autonomic nervous system
(ANS). The ANS is part of the peripheral nervous system and it controls many
organs and muscles within the body. In most situations, we are unaware of the
workings of the ANS because it functions in an involuntary, reflexive manner. For
example, we do not notice when blood vessels change size or when our heart beats
faster.
The ANS is most important in two situations:
1. In emergencies that cause stress and require us to
"fight" or take "flight" (run away)
and
2. In nonemergencies that allow us to "rest" and "digest"
The ANS regulates:
 Muscles
o in the skin (around hair follicles; smooth muscle)
o around blood vessels (smooth muscle)
o in the eye (the iris; smooth muscle)
o in the stomach, intestines and bladder (smooth muscle)
o of the heart (cardiac muscle)
 Glands
.
The ANS is divided into two parts:
 The sympathetic nervous system
 The parasympathetic nervous system
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The autonomic
nervous system
is responsible
for controlling
involuntary
muscular and
metabolic
activities
within the body
such as heart
rate,
peristalsis,
vascular
constriction
and dilation,
etc. Different emotional states can have a direct impact on the types of changes
that are exerted by the ANS at any particular time. The sympathetic system,
which originates from ganglia outside the thoracic and lumbar regions of the spinal
cord, is responsible for exciting the various systems in the body in what is often
called the "fight or flight" response. The parasympathetic system, on the other
hand, is responsible for maintaining routine metabolic functions and quieting the
body after being stimulated by the sympathetic system. The two systems work
antagonistically and are functioning at all times
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The sympathetic nervous system
It is a nice, sunny day...you are taking a nice walk to Gomchen’s restaurant.
Suddenly, an angry bear appears in your path. Do you stay and fight OR do you turn
and run away? These are "Fight or Flight" responses. In these types of situations,
your sympathetic nervous system is called into action - it uses energy - your blood
pressure increases, your heart beats faster, and digestion slows down. Notice in
the picture below that the sympathetic nervous system originates in the spinal
cord. Specifically, the cell bodies of the first neuron (the preganglionic neuron) are
located in the thoracic and lumbar spinal cord. Axons from these neurons project to
a chain of ganglia located near the spinal cord. In most cases, this neuron makes a
synapse with another neuron (post-ganglionic neuron) in the ganglion. A few
preganglionic neurons go to other ganglia outside of the sympathetic chain and
synapse there. The post-ganglionic neuron then projects to the "target" - either a
muscle or a gland.
Two more facts about the sympathetic nervous system: the synapse in the
sympathetic ganglion uses acetylcholine as a neurotransmitter; the synapse of the
post-ganglionic neuron with the target organ uses the neurotransmitter called
norepinephrin
The parasympathetic nervous system
It is a nice, sunny day...you decided to listen to your favourite music and lie on your
bed and relax. This calls for "Rest and Digest" responses. Now is the time for the
parasympathetic nervous system to work to save energy - your blood pressure
decreases, your heart beats slower, and digestion can start. Notice in the picture
below, that the cell bodies of the parasympathetic nervous system are located in
the spinal cord (sacral region) and in the medulla. In the medulla, the cranial nerves
III, VII, IX and X form the preganglionic parasympathetic fibers. The
preganglionic fiber from the medulla or spinal cord projects to ganglia very close to
the target organ and makes a synapse. This synapse uses the neurotransmitter
called acetylcholine. From this ganglion, the post-ganglionic neuron projects to the
target organ and uses acetylcholine again at its terminal.
Here is a summary of some of the effects of sympathetic and parasympathetic
stimulation. Notice that effects are generally in opposition to each other.
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The Autonomic Nervous System
Structure
Sympathetic Stimulation
Parasympathetic Stimulation
Iris (eye muscle)
Pupil Dilation
Pupil Constriction
Salivary Glands
Saliva production reduced
Saliva production increased
Oral/Nasal Mucosa
Mucus production reduced
Mucus production increased
Heart
Heart rate and force increased
Heart rate and force decreased
Lung
Bronchial muscle relaxed
Bronchial muscle contracted
Stomach
Peristalsis reduced
Gastric juice secreted; motility increased
Structure
Sympathetic Stimulation
Parasympathetic Stimulation
Small Intestine
Motility reduced
Digestion increased
Large Intestine
Motility reduced
Secretions and motility increased
Liver
Increased conversion of
glycogen to glucose
Conversion of glucose into glycogen
Kidney
Decreased urine secretion
Increased urine secretion
Adrenal medulla
Norepinephrine and
epinephrine secreted
No hormone secreted
Bladder
Wall relaxed
Sphincter closed
Wall contracted
Sphincter relaxed
Senses
Input to the nervous system is in the form of our five senses: pain, vision, taste,
smell, and hearing. Vision, taste, smell, and hearing input are the special senses.
Pain, temperature, and pressure are known as somatic senses. Sensory input begins
with sensors that react to stimuli in the form of energy that is transmitted into an
action potential and sent to the CNS.
Ear, eye, nose, skin and tongue are the five senses of organs.
References
1)
http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/Bio%20102/Bio%20102%20lectures/Nervous
%20System/nervous1.htm
2)
Notes for Diploma (by Dr. Penjor and Nidup)
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