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THE CENTRAL NERVOUS SYSTEM
1. Introduction
The human nervous system has two main divisions: the Central nervous system
(CNS), and the Peripheral nervous system (PNS), which includes the somatic
Motor nervous system, and the Sensory nervous system. The CNS consists of
the brain and spinal cord. It acts as the central control region of the human
nervous system, processing information and issuing commands.
The Autonomic Nervous System (ANS) is the command network the CNS uses to
maintain the body's homeostasis. It automatically regulates heartbeat and controls
muscle contractions in the walls of blood vessels, digestive, urinary, and
reproductive tracts. It also carries messages that help stimulate glands to secrete
tears, mucus, and digestive enzymes…
The part of the autonomic nervous system originating in the thoracic and lumbar
regions of the spinal cord that in general inhibits or opposes the physiological
effects of the parasympathetic nervous system, as in tending to reduce digestive
secretions, speeding up the heart, and contracting blood vessels.
Neurons are cells that are specialized to receive, propagate and transmit
electrochemical impulses. In the human brain alone, there are over a hundred
billion neurons. Neurons are diverse with respect to morphology and function.
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2. Overview of the Central Nervous system
Structural Divisions of the nervous system:
1. Central Nervous System (CNS) - the brain and spinal cord.
2. Peripheral Nervous System (PNS) - the nerves, ganglia, receptors, etc.
Functions of the nervous system:
1. Control of effectors, both voluntary and autonomic  muscles (skeletal,
smooth, cardiac), glands
2. Response to stimuli
3. Integration of processes
4. Responsible for conscious thought and perception, emotions, personality,
the mind.
Functional Divisions of the Nervous System:
1. The Voluntary Nervous System - (somatic division) control of wilful
control of effectors (skeletal muscles) and conscious perception.
Mediates voluntary reflexes.
2. The Autonomic Nervous System - control of autonomic effectors smooth muscles, cardiac muscle, glands; responsible for "visceral"
reflexes.
Types of neurons based on function:
1. Motor neurons - these carry a message to a muscle, gland or other
effector. They are said to be efferent, i.e. they carry the message away
from the central nervous system.
2. Sensory neurons - these carry a message in to the CNS; they are afferent,
i.e. going toward the brain or spinal cord.
3. Interneuron (association neuron, connecting neuron); these neurons
connect one neuron with another; e.g. in many reflexes interneurons
connect the sensory neurons with the motor neurons.
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3. Neuron
A. Introduction
A neuron is a particular kind of cell that is specialized for the storage and
transmission of information. Neurons are found in the brain as well as in the
brainstem and spinal cord; they are also the nerve cells which transmit
information to muscles and which register sensory information (e.g. touch
stimuli). There are between 10 billion and 100 billion neurons in the brain; each
neuron may contact about 1000 others. Most neurons have three recognizable
components: a cell body, an axon and dendrites.
The cell body contains the machinery needed to convert nutrients to energy and
keep the cell alive. The dendrites are input areas that contain receptors; each
receptor is specialized to respond to a particular kind of neurotransmitter. If
enough receptors are activated, the neuron may become active itself. In this
case, an electrical charge is generated and passes down the axon, the output
process of the neuron. The axon can vary from a few millimetres to several
cm’s (inches) in length, and can branch widely. When the electrical charge
reaches the tip of the axon, neurotransmitters are released. These
neurotransmitters may in turn activate receptors on the dendrites of
neighbouring neurons, passing the message along.
Neurons come in many sizes: e.g. a single sensory neuron from the fingertip
has an axon that extends the length of the arm, while neurons within the brain
may extend only a few millimetres. Neurons have different shapes depending
on what they do. Motor neurons that control muscle contractions have a cell
body on one end, a long axon in the middle and dendrites on the other end;
sensory neurons have dendrites on both ends, connected by a long axon with a
cell body in the middle.
Neurons generate electrical impulses or changes in voltage of two types: graded
potentials and action potentials. Graded potentials occur when the membrane
potential depolarizes and hypolarizes in a graded fashion relative to the amount
of stimulus that is applied to the neuron. An action potential on the other hand
is an all-or-none electrical impulse. Despite being slower than graded
potentials, action potentials have the advantage of traveling long distances in
axons with little or no decrement.
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B. Anatomy of the Nerve
a. Neurons have three basic parts:
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Cell body: This main part has all of the necessary components of the cell,
such as the nucleus (contains DNA), endoplasmic reticulum and ribosomes
(for building proteins) and mitochondria (for making energy).
Axon: This long, cable-like projection of the cell carries the electrochemical
message (nerve impulse or action potential) along the length of the cell.
(Depending upon the type of neuron, axons can be covered with a thin layer
of myelin, like an insulated electrical wire. Myelin is made of fat, and it
helps to speed transmission of a nerve impulse down a long axon.
Myelinated neurons are typically found in the peripheral nerves (sensory and
motor neurons), while non-myelinated neurons are found in the brain and
spinal cord. )
Dendrites or nerve endings: These small, branch-like projections of the cell
make connections to other cells and allow the neuron to communicate with
other cells or perceive the environment. Dendrites can be located on one or
both ends of the cell.
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b. Neurons also vary with respect to their functions:
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Sensory neurons carry signals from the outer parts of your body (periphery)
into the central nervous system.
Motor neurons carry signals from the central nervous system to the outer
parts (muscles, skin and glands) of your body.
Receptors sense the environment (chemicals, light, sound, touch) and
encode this information into electrochemical messages that are transmitted
by sensory neurons.
Interneurons connect various neurons within the brain and spinal cord.
Interneuron
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c. Structure of a nerve:
 A peripheral nerve is arranged much like a muscle in terms of its connective
tissue. It has an outer covering which forms a sheath around the nerve, called
the epineurium. Often a nerve will run together with an artery and vein and
their connective coverings will merge. Nerve fibers, which are axons,
organize into bundles known as fascicles with each fascicle surrounded by
the perineurium. Between individual nerve fibers is an inner layer of
endoneurium.
 The myelin sheath in peripheral nerves consists of Schwann cells wrapped
in many layers around the axon fibers. Not all fibers in a nerve will be
myelinated, but most of the voluntary fibers are. The Schwann cells are
portrayed as arranged along the axon like sausages on a string. (A more apt
analogy would be like jelly rolls!)
 Gaps between the Schwann cells are called nodes of Ranvier. These nodes
permit an impulse to travel faster because it doesn't need to depolarize each
area of a membrane, just the nodes. This type of conduction is called
saltatory conduction and means that impulses will travel faster in
myelinated fibers than in unmyelinated ones.
d. The myelin-sheath does several things:
1. It provides insulation to help prevent short circuiting between fibers.
Diseases which destroy the myelin sheath lead to inability to control
muscles, perceive stimuli etc. (One such disease is multiple sclerosis, an
autoimmune disorder in which your own lymphocytes attack the myelin
proteins.)
2. The myelin sheath provides for faster conduction.
3. The myelin sheath provides for the possibility of repair of peripheral nerve
fibers. Schwann cells help to maintain the micro-environments of the axons
and their tunnel (the neurilemma tunnel) permits re-connection with an
effector or receptor.
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C. Neuron Function and Potential
- Neurons communicate with one another via synapses. Synapses
are specialized junctions between two cells in close apposition to
one another. In a synapse, the neuron that sends the signal is the presynaptic neuron and the target cell receives that signal is the
postsynaptic neuron or cell. Synapses can be either electrical or
chemical. Electrical synapses are characterized by the formation of
gap junctions that allow ions and other organic compound to
instantaneously pass from one cell to another.
- An action potential in the squid giant axon has been the basis for
much of the current understanding of the ionic bases of action
potentials. Briefly, the model states that the generation of an action
potential is determined by two ions: Na+ and K+. An action potential
can be divided into several sequential phases: threshold, rising
phase, falling phase, undershoot phase, and recovery. Following
several local graded depolarizations of the membrane potential, the
threshold of excitation is reached, voltage-gated sodium channels are
activated, which leads to an influx of Na+ ions.
- As Na+ ions enter the cell, the membrane potential is further
depolarized, and more voltage-gated sodium channels are activated.
Such a process is also known as a positive-feedback loop.
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- As the rising phase reaches its peak, voltage-gated Na+ channels are
inactivated whereas voltage-gated K+ channels are activated,
resulting in a net outward movement of K+ ions, which repolarizes
the membrane potential towards the resting membrane potential.
Repolarization of the membrane potential continues, resulting in an
undershoot phase or absolute refractory period. The undershoot
phase occurs because unlike voltage-gated sodium channels,
voltage-gated potassium channels inactivate much more slowly.
Nevertheless, as more voltage-gated K+ channels become
inactivated, the membrane potential recovers to its normal resting
steady state
- When an impulse reaches the end of an axon, it triggers the
formation of synaptic vesicles at that terminal. Synaptic vesicles are
specialized vacuoles that contain neurotransmitters (such as
acetylcholine).
- The vesicles transport the neurotransmitters to the end of the axon
and release them into the synaptic cleft. These neurotransmitters
attach to receptor sites on the cell membrane of the receiving
neuron. When enough receptor sites are filled, the firing threshold of
the receiving neuron is reached and a depolarization event is
triggered.
Transport of Neurotransmitters, movement of K+ ion and Na+ ion
- Before the neuron depolarizes, it is held steady in its resting
potential. This potential, which is achieved by maintaining a
relatively high concentration of sodium ions outside of the cell
membrane, represents an approximately -70 millivolt discrepancy
between the negatively charged interior and positively charged
exterior.
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- As neurotransmitters attach to the receptor sites and overcome the
firing threshold, small molecular gates open along the cell
membrane allowing the sodium ions to rapidly flood the neuron.
This sudden change in polarity from the influx of positive ions
triggers an action potential that moves like a wave down the axon
triggering another nerve, muscle cell, etc.
- At this point, a different series of molecular gates open which allows
potassium ions to rush out of the neuron. The potassium ions, which
have a positive charge as well, create a negatively charged cell
interior by their absence. This event stops the depolarization process.
The sodium ions are pumped more slowly to the cell exterior by
active transport, resulting in the fully restored resting potential once
again.
- Neurons communicate with one another via synapses. Synapses are
specialized junctions between two cells in close apposition to one
another. In a synapse, the neuron that sends the signal is the presynaptic neuron and the target cell receives that signal is the
postsynaptic neuron or cell. Synapses can be either electrical or
chemical. Electrical synapses are characterized by the formation of
gap junctions that allow ions and other organic compound to
instantaneously pass from one cell to another. Chemical synapses
are characterized by the pre-synaptic release of neurotransmitters
that diffuse across a synaptic cleft to bind with postsynaptic
receptors.
- A neurotransmitter is a chemical messenger that is synthesized
within neurons themselves and released by these same neurons to
communicate with their postsynaptic target cells. A receptor is a
transmembrane protein molecule that a neurotransmitter or drug
binds. Chemical synapses are slower than electrical synapses.
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4. The spinal Cord
A. Anatomy
- The spinal cord is the connection centre for the reflexes as well as
the afferent (sensory) and efferent (motor) pathways for most of the
body below the head and neck. The spinal cord begins at the
brainstem and ends at about the second lumbar vertebra. The
sensory, motor, and interneurons discussed previously are found in
specific parts of the spinal cord and nearby structures.
- Sensory neurons have their cell bodies in the spinal (dorsal root)
ganglion. Their axons travel through the dorsal root into the grey
matter of the cord. Within the grey matter are interneurons with
which the sensory neurons may connect. Also located in the grey
matter are the motor neurons whose axons travel out of the cord
through the ventral root.
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- The white matter surrounds the grey matter. It contains the spinal
tracts which ascend and descend the spinal cord. Surrounding both
the spinal cord and the brain are the meninges, a three layered
covering of connective tissue.
- The dura mater is the tough outer layer.
- Beneath the dura is the arachnoid which is like a spider web in
consistency. The arachnoid has abundant space within and beneath it
(the subarachnoid space) which contains cerebrospinal fluid, as
does the space beneath the dura mater (subdural space). This
cerebrospinal fluid supplies buoyancy for the spinal cord and brain
to help provide shock absorption.
- The pia mater is a very thin layer which adheres tightly to the
surface of the brain and spinal cord. It follows all contours and
fissures (sulci) of the brain and cord.
- At 31 places along the spinal cord the dorsal and ventral roots come
together to form spinal nerves. Spinal nerves contain both sensory
and motor fibers, as do most nerves. Spinal nerves are given
numbers which indicate the portion of the vertebral column in which
they arise. There are 8 cervical (C1-C8), 12 thoracics (T1-T12), 5
lumbar (L1-L5), 5 sacral (S1-S5), and 1 coccygeal nerve. Nerve C1
arises between the cranium and atlas (1st cervical vertebra) and C8
arises between the 7th cervical and 1st thoracic vertebra.
- All the others arise below the respective vertebra or former vertebra
in the case of the sacrum. Since the actual cord ends at the second
lumbar vertebra, the later roots arise close together on the cord and
travel downward to exit at the appropriate point. These nerve roots
are called the cauda equina because of their resemblance to a
horse’s tail.
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B. Plexus
Spinal nerves join together in plexuses. A plexus is an interconnection of fibers
which form new combinations as the "named" or peripheral nerves. There are
four voluntary plexuses (there are some autonomic plexuses which will be
mentioned later): they are the cervical plexus, the brachial plexus, the lumbar
plexus, and the sacral plexus. Each plexus gives rise to new combinations of
fibers as the peripheral nerves. The nerves and plexuses you need to know are:
1. Cervical Plexus:
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The phrenic nerve travels through the thorax to innervate the diaphragm.
2. Brachial Plexus
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Axillary nerve - innervates the deltoid muscle and shoulder, along with
the posterior aspect of the upper arm.
Musculocutaneous nerve - innervates anterior skin of upper arm and
elbow flexors.
Radial nerve - innervates dorsal aspect of the arm and extensors of the
elbow, wrist, and fingers, abduction of thumb.
Median nerve - innervates the middle elbow, wrist and finger flexors,
adducts the thumb.
Ulnar nerve - innervates the medial aspect wrist and finger flexors.
3. Lumbar Plexus
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genitofemoral - to the external genitalia
obturator - to the adductor muscles
femoral - innervates the skin and muscles of upper thigh, including the
quadriceps.
4. Sacral Plexus
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gluteal nerves (superior and inferior) - superior innervates the gluteus
medius and minimus, inferior innervates the gluteus maximus.
Sciatic nerve - the body's largest nerve, consisting of two major
branches, the tibial and common peroneal. Together they innervate most
all of leg including the flexors of the knee, part of adductor magnus,
muscles for plantar flexion, dorsiflexion, and other movements of the foot
and toes.
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Cervical Plexus
Sacral plexus
Brachial Plexus
Lumbar Plexus
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C. The Reflex
A reflex is a direct connection between stimulus and response, which does not
require conscious thought. There are voluntary and involuntary reflexes. It is
the voluntary reflexes we are considering here. As discussed earlier, a reflex
involves at least 2 or 3 neurons. The typical components of a reflex are shown
in. The 3-neuron reflex requires three types of neurons: a sensory, an
interneuron, and a motor neuron. It is also called a withdrawal reflex because
it is commonly involved in withdrawing from painful stimuli. Withdrawing
from painful stimuli does not require thought. But the interneuron does send a
fiber through the spinothalamic tract to the brain where the pain is perceived.
1. The Stretch Reflex:
The stretch reflex in its simplest form involves only 2 neurons, and is therefore
sometimes called a 2-neuron reflex. The two neurons are a sensory and a
motor neuron. The sensory neuron is stimulated by stretch (extension) of a
muscle. Stretch of a muscle normally happens when its antagonist contracts, or
artificially when its tendon is stretched, as in the knee jerk reflex
2. The Deep Tendon Reflex:
Tendon receptors respond to the contraction of a muscle. Their function, like
that of stretch reflexes, is the coordination of muscles and body movements. The
deep tendon reflex involves sensory neurons, interneurons and motor neurons.
The response reverses the original stimulus therefore causing relaxation of the
muscle stimulated (in order to facilitate that the reflex sends excitatory stimuli to
the antagonists causing them to contract - reciprocal activation).
3. The Crossed Extensor Reflex
The crossed extensor reflex is just a withdrawal reflex on one side with the
addition of inhibitory pathways needed to maintain balance and coordination.
A reflex action or reflex is a biological control system linking stimulus to response and
mediated by a reflex arc. Reflexes can be built-in or learned. For example, a person stepping
on a sharp object would initiate the reflex action through the creation of a stimulus, (pain)
within specialized sense receptors located in the skin tissue of the foot. The resulting stimulus
would be transmitted through afferent, or sensory neurons to the spinal cord. This stimulus is
usually processed by an interneuron to create an immediate response to pain by initiating a
motor (muscular) response which is acted upon by muscles of the leg, retracting the foot away
from the object. This retraction would occur as the pain sensation is arriving in the brain
which would process a more cognitive evaluation of the situation.
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D. Human reflexes
Reflex actions seen in adult humans include:
1. The Accommodation reflex is a reflex action of the eye, in response to
focusing on a near object, then looking at distant object (and vice versa),
comprising coordinated changes in vergence, lens shape and pupil size. It is
dependent on cranial nerve II (afferent limb of reflex), higher centres and
cranial nerve III)
2. The Achilles reflex (the ankle jerk reflex) occurs when the Achilles
tendon is tapped while the foot is dorsi-flexed. A positive result would be the
jerking of the foot towards its plantar surface. The Achilles reflex checks if
the S1 and S2 nerve roots are intact and could be indicative of sciatic nerve
pathology. It is classically delayed in hypothyroidism. This reflex is usually
absent in disk herniations at the L5-S1 level.
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3. The Anocutaneous reflex (the anal wink) is the reflexive contraction of
the external anal sphincter upon stroking of the skin around the anus. The
absence of this reflex indicates that there is an interruption of the reflex arc,
which may be in the sensory afferent limb or the motor efferent limb. The
synapse between the afferent and efferent limbs occurs in the lowest sacral
segments of the spinal cord.
4. The Biceps stretch reflex examines the function of the C5 reflex arc and
to a lesser degree the C6 reflex arc. The test is performed by using of a
tendon hammer to quickly depress the biceps brachii tendon as it passes
through the cubital fossa. Specifically, the test activates the stretch receptors
inside the biceps brachii muscle which communicates mainly with the C5
spinal nerve and partially with the C6 spinal nerve to induce a reflex
outcome. The reflex results in flexion of the antebrachium.
5. The Brachioradialis reflex (supinator reflex) is testing the reflex motor
arc associated with the brachioradialis muscle. The supinator reflex is tested
by striking the lower end of the radius just above the wrist with a tendon
hammer. This normally causes contraction of the brachioradialis and hence
flexion of the elbow.
6. The Corneal reflex (the blink reflex): is an automated involuntary
blinking of the eyelids elicited by stimulation (such as touching or a foreign
body) of the eyeball's cornea. The evolutionary purpose of this reflex is to
protect the eyes from foreign bodies.The reflex is mediated by: the
nasociliary branch of the ophthalmic nerve (V1) of the 5th Cranial nerve
(Trigeminal) sensing the stimulus on the cornea, and the 7th Cranial nerve
(Facial nerve) initiating the motor response. The examination of the corneal
reflex is a part of some neurological tests in particular the coma exam.
7. The Crossed extensor reflex: When the reflex occurs the flexors in the
withdrawing limb contract and the extensors relax, while in the other limb
the opposite occurs
8. The H-reflex is a reflectory reaction of muscles after electrical
stimulation of sensory fibers in their innervating nerves (for example, those
located behind the knee).
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9. The patellar reflex or knee jerk is a monosynaptic reflex. Striking the
patellar tendon with a tendon hammer just below the patella stretches the
quadriceps tendon. This stimulates stretch sensory receptors (most
importantly, muscle spindles) that trigger an afferent impulse in a sensory
nerve fiber of the femoral nerve leading to the lumbar region of the spinal
cord. There, the sensory neuron synapses directly with a motor neuron that
conducts an efferent impulse to the quadriceps femoris muscle, triggering
contraction. This contraction, coordinated with the relaxation of the
antagonistic flexor hamstring muscle causes the leg to kick. It has been
thought that this type of reflex helps maintain the upright posture. The
patellar tendon reflex tests the function of the femoral nerve and spinal cord
segements L2-L4.
The Patellar reflex
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10. The Babinski reflex or the Plantar reflex: The lateral side of the sole of
the foot is rubbed with a blunt implement so as not to cause pain, discomfort or
injury to the skin; the instrument is run from the heel along a curve to the
metatarsal pads. There are three responses possible:
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Flexor: the toes curve inward and the foot everts; this is the response
seen in healthy adults.
Indifferent: there is no response.
Extensor: the hallux (large toe) extends upward, and the other toes fan
out; Babinski's sign.
o
o
The extensor response can indicate damage of the spinal cord in the thoracal or
lumbar region, or brain disease. Occasionally, a pathological plantar reflex is the
first (and only) indication of a serious disease process, and a clearly abnormal
plantar reflex often prompts detailed neurological investigations, including CT
scanning of the brain or MRI of the spine, as well as lumbar puncture for the
study of cerebrospinal fluid.
Young babies (less than a few months of age) will also show an extensor response.
A baby's smaller toes will fan out, and their big toe wills dorsiflex slowly. This
happens because the corticospinal pathways (that run from the brain down the
spinal cord) are not fully myelinated at this age, so the reflex is not inhibited by
the cerebral cortex.
11. Pupillary reflex: The pattern of papillary response to light can help
determine which of the cranial nerves is damaged. There are two types of
response assessed for each eye:
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Direct pupillary reflex: whether each pupil constricts with light shone
into the that eye
Consensual pupillary reflex: whether each pupil constricts with light
shone into the other eye
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Test for nerve damage
- Emergency room physicians often assess the pupillary reflex because
it is useful for gauging brain stem function. Normally, pupils react
(constrict) equally. Lack of the pupillary reflex or an abnormal
pupillary reflex can be caused by optic nerve damage, oculomotor
nerve damage, brain death and depressant drugs, such as
barbiturates. The optic nerve is responsible for the afferent limb of the
pupillary reflex, or in other words, senses the incoming light. The
oculomotor nerve is responsible for the efferent limb of the pupillary
reflex; in other words, it drives the muscles that constrict the pupil.
- Normally, each pupil should constrict with light shone into either eye.
On testing each reflex for each eye, several patterns are possible.
12. Scratch reflex is a reflex transmitted by very sensitive nerve endings
near the surface of the skin via the spinal cord, and is a reflex frequently
inherited by mammals. The nerve signal includes positioning to pinpoint the
location of the itch, and the effect of the reflex is either an affective
sensation or even an involuntary action to make a scratching movement that
usually relieves the itch. The scratch reflex is a reflex to help an organism
protect and rid its body of parasites and other irritants.
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In case of intense itch, scratching to relieve it can cause pain,
and such pain signals are believed to suppress the itch signals
due to a lateral inhibition effect, as the dorsal column system
transmits critical sensations much faster than the cruder
spinothalamic tract that deals with less accurate types of
sensations, such as itch, also at a slower pace.
13. The Tendon reflex (or T-reflex) is an feedback mechanism that controls
increasing muscle tension by causing muscle relaxation before tension force
becomes so great it may damage the muscle
14. The Vestibulo-Ocular Reflex (VOR) is a reflex eye movement that
stabilizes images on the retina during head movement by producing an eye
movement in the direction opposite to head movement, thus preserving the
image on the center of the visual field. For example, when the head moves to
the right, the eyes move to the left, and vice versa.
15. The Caloric reflex test is a test of the vestibulo-ocular reflex. It is used
by physicians to assess brain stem function and may be part of an evaluation
to determine brain death.
It involves putting cold or hot water into the external auditory canal.
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If the water is cold (4o C) the eyes turn toward the ipsilateral ear, with
horizontal nystagmus (quick horizontal eye movements) to the contra
lateral ear.
If the water is warm (44oC) the eyes turn toward the contra lateral ear,
with horizontal nystagmus to the ipsilateral ear.
Absent reactive eye movement suggests brain death.
Cold water = nystagmus to the opposite side
Warm water = nystagmus to the same side
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5. Parasympathetic - and Sympathetic Nervous System
A. The Parasympathetic Nervous System and the Sympathetic Nervous
System (SNS) as a part of the Autonomic Nervous System.
Sympathetic Nervous System is the portion of the autonomic nervous
system concerned with non-volitional preparation of the organism for
emergency situations; it is frequently referred to as the "fight or flight"
system, as it has a stimulatory effect on organs and physiological systems.
In contrast the parasympathetic nervous system has been described as the
"rest and digest" system because it has a relaxing effect on many organs.
The Sympathetic Nervous System is controlled by bunches of nerves called
ganglions. One large ganglion, called the stellate ganglion, helps control
nerves in the upper body. In the lower body, nerves are controlled by
several ganglions that make up the sympathetic chain.
The Parasympathetic Nervous System is sometimes called the rest and
digests system; the parasympathetic system conserves energy as it slows
the heart rate, increases intestinal and gland activity, and relaxes sphincter
muscles in the gastrointestinal tract.
The Parasympathetic Nervous System is a portion of the visceral
(autonomic) branch of the PNS (peripheral nervous system). The regions of
the body associated with this division are the cranial and sacral regions of
the spinal cord. Specifically, cranial nerves III, VII, IX, X (vagus n.) and
in the sacral region (spinal nerves exiting from the sacrum) the spinal
nerves S2 to S4.
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1. Parasympathetic Nervous System
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Dilates blood vessels leading to the GI tract, increasing blood flow.
This is important following the consumption of food, due to the
greater metabolic demands placed on the body by the gut.
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The parasympathetic nervous system can also constrict the
bronchiolar diameter when the need for oxygen has diminished.
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During accommodation, the parasympathetic nervous system causes
constriction of the pupil and lens.
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The parasympathetic nervous system stimulates salivary gland
secretion, and accelerates peristalsis, so, in keeping with the rest and
digests functions; appropriate PNS activity mediates digestion of
food and indirectly, the absorption of nutrients.
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Is also involved in erection of genitals, via the pelvic splanchnic
nerves 2-4.
2. Sympathetic Nervous System
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Diverts blood flow away from the gastro-intestinal (GI) tract and
skin via vasoconstriction.
Blood flow to skeletal muscles, the lung is not only maintained, but
enhanced (by as much as 1200%, in the case of skeletal muscles).
Dilates bronchioles of the lung, which allows for greater alveolar
oxygen exchange.
Increases heart rate and the contractility of cardiac cells (myocytes),
thereby providing a mechanism for the enhanced blood flow to
skeletal muscles.
Dilates pupils and relaxes the lens, allowing more light to enter the
eye.
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6. NEUROTRANSMITTERS
A. Definition
Neurotransmitters are chemicals that allow the movement of information
from one neuron across the gap between it and the adjacent neuron. The
release of neurotransmitters from one area of a neuron and the recognition
of the chemicals by a receptor site on the adjacent neuron causes an
electrical reaction that facilitates the release of the neurotransmitter and its
movement across the gap.
The three major categories of substances that act as neurotransmitters:
1. Amino acids (primarily glutamic acid, GABA [= Gamma-aminobutyric
acid], aspartic acid & glycine),
2. Peptides (vasopressin, somatostatin, neurotensin, etc.) and
3. Monoamines (norepinephrine, dopamine & serotonin) plus
acetylcholine.
The major "workhorse" neurotransmitters of the brain are glutamic acid
(=glutamate) and GABA. The monoamines & acetylcholine perform
specialized modulating functions, often confined to specific structures. The
peptides perform specialized functions in the hypothalamus or act as cofactors elsewhere in the brain.
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There are many neurotransmitters in the central nervous system, the
peripheral nervous system has only two: acetylcholine and norepinephrine.
Some examples of neurotransmitter action:
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Acetylcholine: Acetylcholine is particularly important in the
stimulation of muscle tissue. After stimulation, acetylcholine degrades
to acetate and choline, which are absorbed back into the first neuron to
form another acetylcholine molecule. The poison curare blocks
transmission of acetylcholine. Some nerve gases inhibit the breakdown
of acetylcholine, producing a continuous stimulation of the receptor
cells, and spasms of muscles such as the heart.
Epinephrine (adrenaline) and norepinephrine: These compounds are
secreted principally from the adrenal gland. Secretion causes an
increased heart rate and the enhanced production of glucose as a ready
energy source (the "fight or flight" response).
Dopamine: Dopamine facilitates critical brain functions and, when
unusual quantities are present, abnormal dopamine neurotransmission
may play a role in Parkinson's disease, certain addictions, and
schizophrenia.
Serotonin: Synthesized from the amino acid tryptophan, serotonin is
assumed to play a biochemical role in mood and mood disorders,
including anxiety, depression, and bipolar disorder.
Aspartate: An amino acid that stimulates neurons in the central
nervous system, particularly those that transfer information to the area
of the brain called the cerebrum.
Oxytocin: A short protein (peptide) that is released within the brain,
ovary, and testes. The compound stimulates the release of milk by
mammary glands, contractions during birth, and maternal behavior.
Somatostatin: A peptide, which is inhibitory to the secretion of growth
hormone from the pituitary gland, of insulin, and of a variety of
gastrointestinal hormones involved with nutrient absorption.
Insulin: A peptide secreted by the pancreas that stimulates other cells to
absorb glucose.
The actions of some drugs mimic those of naturally occurring
neurotransmitters. The pain-regulating endorphins, for example, are similar
in structure to heroin and codeine, which fill endorphin receptors to
accomplish their effects. The wakefulness that follows caffeine consumption is
the result of its blocking the effects of adenosine, a neurotransmitter that
inhibits brain activity.
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Acetylcholine - voluntary movement of the muscles
Norepinephrine - wakefulness or arousal
Dopamine - voluntary movement and emotional arousal
Serotonin - memory, emotions, wakefulness, sleep and temperature
regulation
GABA (gamma aminobutyric acid) - motor behaviour
Glycine - spinal reflexes and motor behaviour
Neuromodulators - sensory transmission-especially pain
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