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7/30/2019 The Nervous System is a Complex Network of Nerves and Cells That Carry Messages to and From The
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The nervous system is a complex network of nerves and cells that carry messages to and
from thebrainand spinal cord to various parts of the body.
The nervous system includes both the Central nervous system and Peripheral nervous
system. The Central nervous system is made up of the brain and spinal cord and The
Peripheral nervous system is made up of the Somatic and the Autonomic nervous systems.
The Central nervous system
The central nervous system is divided into two major parts: the brain and the spinal cord.
The brain
The brain lies within the skull and is shaped like a mushroom. The brain consists of four
principal parts:
thebrainstem
the cerebrum
the cerebellum
the diencephalon
The brain weighs approximately 1.3 to 1.4 kg. It has nerve cells called the neurons andsupporting cells called the glia.
There are two types of matter in the brain: grey matterand white matter. Grey matter
receives and stores impulses. Cell bodies of neurons and neuroglia are in the grey matter.
White matter in the brain carries impulses to and from grey matter. It consists of the nerve
fibers (axons).
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The brain stem
The brain stem is also known as the Medulla oblongata. It is located between the pons and
the spinal cord and is only about one inch long.
The cerebrum
The cerebrum forms the bulk of thebrainand is supported on the brain stem. The
cerebrum is divided into two hemispheres. Each hemisphere controls the activities of the
side of the body opposite that hemisphere.
The hemispheres are further divided into four lobes:
Frontal lobe
Temporal lobes
Parietal lobe
Occipital lobe
The cerebellum
This is located behind and below the cerebrum.
The diencephalon
The diencephalon is also known as the fore brain stem. It includes
thethalamusandhypothalamus. The thalamus is where sensory and other impulses go and
coalesce.
The hypothalamus is a smaller part of the diencephalon
Other parts of the brain
Other parts of the brain include the midbrain and the pons:
the midbrain provides conduction pathways to and from higher and lower centers
the pons acts as a pathway to higher structures; it contains conduction pathways
between the medulla and higher brain centers
The spinal cord
The spinal cord is along tube like structure which extends from thebrain. The spinal cord is
composed of a series of 31 segments. A pair of spinal nerves comes out of each segment.
The region of the spinal cord from which a pair of spinal nerves originates is called the
spinal segment. Both motor and sensory nerves are located in the spinal cord.
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The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and
weighs about 35-40 grams. It lies within the vertebral column, the collection of bones (back
bone).
Other parts of the central nervous system
The meninges are three layers or membranes that cover the brain and the spinal cord. The
outermost layer is the dura mater. The middle layer is the arachnoid, and the innermost
layer is the pia mater. The meninges offer protection to the brain and the spinal cord by
acting as a barrier against bacteria and other microorganisms.
The Cerebrospinal Fluid (CSF) circulates around the brain and spinal cord. It protects and
nourishes the brain and spinal cord.
Neurons
The neuron is the basic unit in the nervous system. It is a specialized conductor cell that
receives and transmits electrochemical nerve impulses. A typical neuron has a cell body and
long arms that conduct impulses from one body part to another body part.
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There are three different parts of the neuron:
the cell body
dendrites
axon
Cell body of a neuron
The cell body is like any other cell with a nucleus or control center.
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Dendrites
The cell body has several highly branched, thick extensions that appear like cables and are called
dendrites. The exception is a sensory neuron that has a single, long dendrite instead of many dendrites.
Motor neurons have multiple thick dendrites. The dendrite's function is to carry a nerve impulse into the
cell body.
Axon
An axon is a long, thin process that carries impulses away from the cell body to another neuron or tissue.
There is usually only one axon per neuron.
Myelin Sheath
The neuron is covered with theMyelinSheath or Schwann Cells. These are white segmented covering
around axons and dendrites of many peripheral neurons. The covering is continuous along the axons or
dendrites except at the point of termination and at the nodes of Ranvier.
The neurilemma is the layer of Schwann cells with a nucleus. Its function is to allow damaged nerves to
regenerate. Nerves in thebrainand spinal cord do not have a neurilemma and, therefore cannot recover
when damaged.
Types of neuron
Neurons in the body can be classified according to structure and function. According to structure neurons
may be multipolar neurons, bipolar neurons, and unipolar neurons:
Multipolar neurons have one axon and several dendrites. These are common in the brain andspinal cord
Bipolar neurons have one axon and one dendrite. These are seen in the retina of the eye, the inner
ear, and the olfactory (smell) area.
Unipolar neurons have one process extending from the cell body. The one process divides with one
part acting as an axon and the other part functioning as dendrite. These are seen in the spinal cord.
The Peripheral nervous system
The Peripheral nervous system is made up of two parts:
Somatic nervous system Autonomic nervous system
Somatic nervous system
The somatic nervous system consists of peripheral nerve fibers that pick up sensory information or
sensations from the peripheral or distant organs (those away from the brain like limbs) and carry them to
the central nervous system.
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These also consist of motor nerve fibers that come out of the brain and take the messages for movement
and necessary action to the skeletal muscles. For example, on touching a hot object the sensory nerves
carry information about the heat to the brain, which in turn, via the motor nerves, tells the muscles of the
hand to withdraw it immediately.
The whole process takes less than a second to happen. The cell body of the neuron that carries theinformation often lies within thebrainor spinal cord and projects directly to a skeletal muscle.
Autonomic Nervous System
Another part of the nervous system is the Autonomic Nervous System. It has three parts:
the sympathetic nervous system
the parasympathetic nervous system
the enteric nervous system
This nervous system controls the nerves of the inner organs of the body on which humans have no
conscious control. This includes the heartbeat,digestion, breathing (except conscious breathing) etc.
The nerves of the autonomic nervous system enervate the smooth involuntary muscles of the (internal
organs) and glands and cause them to function and secrete their enzymes etc.
The Enteric nervous system is the third part of the autonomic nervous system. The enteric nervous
system is a complex network of nerve fibers that innervate the organs within the abdomen like the
gastrointestinal tract,pancreas, gall bladder etc. It contains nearly 100 million nerves.
Neurons in the peripheral nervous system
The smallest worker in the nervous system is the neuron. For each of the chain of impulses there is one
preganglionic neuron, or one before the cell body organglion, that is like a central controlling body for
numerous neurons going out peripherally.
The preganglionic neuron is located in either the brain or the spinal cord. In the autonomic nervous
system this preganglionic neuron projects to an autonomic ganglion. The postganglionic neuron then
projects to the target organ.
In the somatic nervous system there is only one neuron between the central nervous system and the
target organ while the autonomic nervous system uses two neurons.
Transmission of nerve impulses
A nerve impulse is essentially an electrical stimulus that travels over the cell's membrane. It passes
through the axons and dendrites of the neurons. It travels via the dendrites from the skin and then
reaches the cell body, axon, axon terminals and the Synapse of the neuron.
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The Synapse is the junction between two neurons where the impulse moves from one to the other. At the
synapse neurotransmitters are present. These are chemical transmitters of messengers that transmit the
impulse. They include Acetylcholine and Noradrenaline.
The impulse continues to the next dendrite, in a chain reaction till it reaches the brain that in turn instructs
the skeletal muscles to work.
The Reflex Arc
These reflexes are automatic, involuntary responses. They may or may not involve the brain for example
blinking does not involve the brain. The Reflex arc is the main functional unit of the nervous system that
helps a person react to a stimulus.
Functions of different parts of the nervous system
Different parts of the nervous system have different functions. They can be outlined as follows.
Functions of the brain
The brain is made up of several parts. Each part has a certain function:
Cerebral Cortex
Thought , voluntary movement , language, reasoning and perception are the major functions of the
cerebral cortex.
Cortex literally means "bark" (of a tree) in latin and is so termed because it is a sheet of tissue that makes
up the outer layer of thebrain.
The thickness of the cerebral cortex is between 2 to 6 mm. The right and left sides of the cerebral cortex
are connected by a thick band of nerve fibers called the "corpus callosum."
The cortex has numerous grooves and bumps to increase its surface area. A bump or bulge on the cortex
is called a gyrus (the plural of the word gyrus is "gyri") and a groove is called a sulcus (the plural of the
word sulcus is "sulci").
Cerebellum
The major functions of the cerebellum are maintenance of movement, balance and posture. The word
"cerebellum" comes from the Latin word for "little brain." It is divided into two parts or hemispheres and
has a cortex that covers the hemispheres.
Hypothalamus
The hypothalamus regulates the body temperatures, emotions and hunger, thirst and controls the
circadian rhythms.
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This pea sized organ is in control of body temperature. It acts like a "thermostat" by sensing changes in
body temperature and sends out signals to adjust the temperature.
Brain stem or Medulla oblongata
This area is vital for life as it controls breathing, heart rateand blood pressure. The brain stem comprisesof the medulla, pons, tectum, reticular formation and tegmentum.
Thalamus
Works by integrating sensory information and motor information. The thalamus receives sensory
information and relays this information to the cerebral cortex.
The cerebral cortex also sends information to the thalamus which then transmits this information to other
areas of the brain and spinal cord.
Limbic System
This part of thebrainincludes amygdala, thehippocampus, mammillary bodies and cingulate gyrus.
These help in controlling the emotional response. The hippocampus is also important for learning and
memory.
Basal Ganglia
This part works in maintaining balance and movements. It includes structures like the globus pallidus,
caudate nucleus, subthalamic nucleus, putamen and substantia nigra.
Midbrain
This part of the brain has sites controlling vision, hearing, eye movement and general body movement.
The structures that are part of the midbrain are superior and inferior colliculi and red nucleus.
Functions of the Cerebrospinal nervous system
This system has 12 pairs of cranial nerves. These are attached to the brain and have specific functions.
Each cranial nerve leaves the skull through an opening at its base.
The nerves and their functions include:
1. Olfactory for smell
2. Optic - Sight
3. Oculomotor - Movement of the eyeball, lens, and pupils
4. Trochlear Movement of the Superior oblique muscle of the eye
5. rigeminal Innervates the eyes, cheeks and the jaw areas and controls chewing
6. Abducens - Moves the eye outward
7. Facial - Controls muscles of the face, scalp, ears; controls salivary glands; receives taste
sensation from the anterior two-thirds of the tongue
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8. Acoustic Hearing and maintenance of balance
9. Glossopharyngeal Taste sensation from the back of the tongue and throat
10. Vagus Innervates the chest and abdominal organs
11. Spinal Accessory - movement of head and shoulders
12. Hypoglossal - Controls muscles of tongue
Functions of the Autonomic nervous system
The autonomic nervous system is divided into sympathetic and parasympathetic nervous systems. These
two systems have opposite effects on the same set of organs.
The sympatheticnervous systemis important during an emergency and is associated with fight or flight
reaction. The energy is directed away from digestion, there is dilation of pupils, increased heart rate,
increased perspiration and salivation, increased breathing etc.
The parasympathetic nervous system is associated with a relaxed state. The pupils contract, energy is
diverted for digestion of food, heart rate slows etc.
Overview
Maya is walking and is about to cross the street. However, she saw the red light just ahead for the pedestrian
crossing. She waited until the light for crossing changes to green.
Donnie is in his English class. While his teacher is explaining about the correct grammar usage his mind is
continuously drifting on his date the following with the girl he likes and how he wish that things would turn out the
better after their time together. Then suddenly, his mind is back at the moment hoping that his teacher had not
noticed his brief departure.
In both situations, it is clearly noted that the brain has always been on the go. The brain performs a lot of things for a
human body to function well such as controlling what a person thinks and feel, learning and remembering things, the
way a person moves and talk, and some other things such as the beating of the heart, digestion of the food and etc.
Thus, it is very important to have an understanding on how this amazing brain or the nervous system works.
Definition
The nervous system is an organ system that contains a network of specialized cells called neurons. This is the
master controlling and communicating system of the body. It coordinates the action of an animal and transmits
signals between the different parts of the body. Every thought, movement and emotions reflect the activity of the
nervous system.
Functions of the NERVOUS SYSTEM
1. To monitor changes that takes place inside and outside the body. The nervous system utilizes the million
sensory receptors to carry out this function. Any changes or stimuli occurring are noted by the nervous system
and the gathered data is now called a sensory input.
2. Another important function of the nervous system is to process and interpret the sensory input or gathered
data. It is the working of this system to make decision about what should be done at each moment. This is theprocess known as INTEGRATION.
3. As the nervous system has reached a decision of what response and appropriate action to be done in response
to the stimuli, it then effects a response by activating muscles or glands through motor output.
These functions of the nervous systems works hand in hand. Lets take a look at the situation given above about
Maya. While Maya is walking and is about to cross the street she saw the red light for pedestrian crossing just ahead,
this is the sensory input. Her nervous system then processes and integrates this information that the red light for
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The nervous system derives its name from nerves, which are cylindrical bundles of fibers that emanate from the brain
and central cord, and branch repeatedly to innervate every part of the body. Even though it is complex, nervous
tissue is made up of two principal types of cells namely, the supporting cells and the neurons.
SUPPORTING CELLS
The supporting cells in the CNS are lumped together as NEUROGLIA or GLIAL CELLS.Glial Cells are non-
neuronal cells that provide support and nutrition, maintain homeostasis, form myelin and participate in signaltransmission in the nervous system. In the human brain, it is estimated that the total number of glia roughly equals
the number of neurons, although the proportions vary in different brain areas.
The functions of glial cells are:
1. to support neurons and hold them in place
2. to supply nutrients to neurons
3. to insulate neurons electrically
4. to destroy pathogens and remove dead neurons
5. to provide guidance cues directing the axons of neurons to their targets
Characteristics of Glial Cells:
1. Lumped together.
2. Not able to transmit impulses.3. Never lose their ability to divide.
The CNS glia include:
Astrocytes. These are star-shaped cells that account nearly half of the neural tissue. Astrocytes form a living
barrier between capillaries and neurons and play a role in making exchanges between the two. This is to prevent
harmful substances in the blood from entering the neurons. Aside from that, astricytes are also important in
controlling the chemical environment in the brain. This is done by picking up excess ions and recapturing
released neurotransmitters.
Microglia. These are spiderlike phagocytes that dispose debris including dead brain cells and bacteria.
Ependymal cells. These cells line the cavities of the brain and the spinal cord. Aside from lining the cavities of
certain organs, these cells are very important in helping the CSF through their cilia to circulate and fill those
cavities and form a protective cushion around the CNS.
Oligodendrocytes. These are glial cells that wrap their flat extensions tightly around the nerve fibers, producing
fatty insulating coverings called myelin sheaths.
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NEURONS
Anatomy of the Neuron
The nervous system is defined by the presence of a special type of cellthe neuron (sometimes called neurone or
nerve cell). Neurons can be distinguished from other cells in a number of ways, but their most fundamental property
is that they communicate with other cells via SYNAPSES, which are membrane-to-membrane junctions containing
molecular machinery that allows rapid transmission of signals, either electrical or chemical. Many types of neuron
possess an AXON, a protoplasmic protrusion that can extend to distant parts of the body and make thousands ofsynaptic contacts. Axons frequently travel through the body in bundles called nerves.
Cell body the metabolic center of the neuron. This part of neuron contains the usual organelles except for the
centrioles. It contains a nucleus and cytoplasm. Where it is most distinct from cells of other types is that out of the
cell body, long threadlike projections emerge. Over most of the cell there are numerous projections that branch
out into still finer extensions. This is well protected and is located in the bony skull or vertebral column and is
essential to well-being of the nervous system. The cell body carries out most of the metabolic functions of a
neuron.
Nissl substance and Neurofibrils found in the cell body that is essential in maintaining cell shape.
Dendrites neuron processes that covey incoming messages TOWARD the cell body.
Axons neuron processes that generate nerve impulses AWAY from the cell body.
Axon hillock a cone-like region of the cell body where the axon arises.
Axon terminals - located at the terminal end of the axons that contains tiny vesicles or membranous sacs that
contains chemicals called neurotransmitters. When impulses reach the axon terminals, they stimulate the release
of neurotransmitters into the extracellular spaces.
Synaptic cleft a tiny gap that separates axon terminal from the next neuron.
Myelin a whitish, fatty material that covers long nerve fibers. It has a waxy appearance that protects and
insulates the fibers and increases the rate of nerve impulses.
Schwann cells myelinates the axon outside the nervous system. Schwann cells are specialized supporting cells
that enclose themselves tightly around the axon jelly-roll fashion.
Myelin sheath a tight coil of wrapped membranes created after the Schwann cells enclose the axon.
Neurilemma part of the Schwann cell external to the myelin sheath.
Nodes of Ranvier gaps or indentations between the myelin sheaths.
Classification of Neurons
Functional Classification of Neurons
Even in the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a
wide variety of morphologies and functions. These include SENSORY NEURONS that transmute physical stimuli
such as light and sound into neural signals, and MOTOR NEURONS that transmute neural signals into activation of
muscles or glands; however in many species the great majority of neurons receive all of their input from other
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neurons and send their output to other neurons. An ITERNEURON is always found completely within the CNS and
conveys messages between parts of the system
In addition to neurons, nervous tissue contains glial cells such as the Schwann cells covering the neurons with
sheath. These cells maintain the tissue by supporting and protecing the neurons. They also provide nutrients to
neurons and help to keep the tissue free of debris. The neurons require a great deal of energy for the maintenance of
the ionic imbalance between themselves and their surrounding fluids, which is constantly in flux as a result of theopening and closing of channels through the neuronal membranes.
Structural Classification of Neurons
Multipolar neurons These are several processes extending from the cell body. All motor and association
neurons are multipolar and this is the most common structural type.
Bipolar neurons These are neurons with two processes an axon and a dendrite. Bipolar neurons are rare in
adults and are only found in some special sense organs such as the eye or nose where they act in sensory
processing as receptor cells.
Unipolar neurons These neurons have single process emerging from the cell body. it is very short and divides
almost immediately into proximal (central) and distal (peripheral) processes.
Neurons are dynamically polarized, so that information flows from the fine dendrites into the main dendrites and then
to the cell body, where it is converted into all-or-none signals, the action potentials, which are relayed to other
neurons by the axon, a long wire-like structure. The neuron is actually a very poor conductor; the signal drops to 37%
of its original strength in only about 0.15 mm. Thus it needs amplification all along its length in the form of sodium-
potassium pumps and gates.
Sodium ions rush into the neurons from the extracellular fluid, resulting in a transient change in the voltage difference
between the neuron and the surrounding environment. The action potential travels like a wave from the cell body
down the neuron via the repeating amplifications. Thus, the action potential enables the neuron to communicate
rapidly with other neurons over sizable distances, sometime more than a meter away with a speed from 20 -200
m/sec. When the action potential reaches an axon terminal, it causes the terminals to secrete a chemical messenger
(neurotransmitter), generally an amino acid or its derivative, which binds to receptors in the post -synaptic neurons on
the far side of the synaptic cleft. When the postsynaptic potential has reached a specific value an action potential is
triggered and the signal is passed to the next neuron.
THE CENTRAL NERVOUS SYSTEM
The Central Nervous System (CNS) is composed of the brain and spinal cord. The CNS is surrounded by bone-skull
and vertebrae. Fluid and tissue also insulate the brain and spinal cord. During embryonic development, the brain first
forms as a tube, the anterior end of which enlarges into three hol low swellings that form the brain, and the posterior of
which develops into the spinal cord.
Anatomy of the CNS
Brain
When a message comes into the brain from anywhere in the body, the brain tells the body how to react. For example,
if you accidentally touch a hot stove, the nerves in your skin shoot a message of pain to your brain. The brain sends a
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message back telling the muscles in your hand to pull away. Luckily, this neurological relay race takes a lot less time
than it just took to read about it. Considering everything it does, the human brain is incredibly compact, weighing just
3 pounds. Its many folds and grooves, though, provide it with the additional surface area necessary for storing all of
the bodys important information.
The four main regions of the brain are:
Cerebral hemispheres Diencephalon
Brain stem
Cerebellum
Cerebral Hemispheres
The paired cerebral hemispheres are the most superior part of the brain and are collectively called the cerebrum.
1. Gyri or gyrus (singular) elevated ridges of tissue found on the entire surface of the cerebral hemisphere.
2. Sulci or sulcus (singular) shallow grooves that separates the gyri.
3. Fissures deeper groves which separates the larger regions of the brain. The cerebral hemispheres are
separated by a single deep fissure called the LONGITUDINAL FISSURE.
The cerebrum, the largest part of the human brain, is divided into left and right hemispheres connected to each other
by the corpus callosum. The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex,
the most recently evolved region of the vertebrate brain. The cortex in each hemisphere of the cerebrum is between 1
and 4 mm thick. Folds divide the cortex into four lobes: occipital, frontal, parietal and temporal. No region of the brain
functions alone, although major functions of various parts of the lobes have been determined.
The occipital lobe (back of the head) receives and processes visual information. The temporal lobe receives
auditory signals, processing language and the meaning of words. The parietal lobe is associated with the sensory
cortex and processes information about touch, taste, pressure, pain, and heat and cold. The frontal lobe conducts
three functions:
1. motor activity and integration of muscle activity
2. speech
3. thought processes
Language comprehension is found in Wernickes area. Speaking ability is in Brocas area. Damage to Brocas area
causes speech impairment but not impairment of language comprehension. Lesions in Wernickes area impair abilityto comprehend written and spoken words but not speech. The remaining parts of the cortex are associated with
higher thought processes, planning, memory, personality and other human activities.
Diencephalon
The diencephalon or interbrain sits atop the brainstem and is enclosed by the cerebral hemispheres. The major
structures of the diencephalon are:
1. Thalamus The thalamus is a relay station for sensory impulses passing upward the sensory cortex.
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2. Hypothalamus Plays a role in body temperature regulation, water balance and metabolism. It is also the center
for many drives and emotion such as thirst, appetite, sex, pain and pleasure. Aside from that, the hypothalamus
regulates the pituitary gland and produces two hormones of its own.
3. Epithalamus The epithalamus contains the pineal body and the choroid plexuses. The choroid plexuses form
thecerebrospinal fluid.
Brain StemThe brain stem is about the size of a thumb in diameter and is approximately 3 inches long. It provides a pathway for
ascending and descending tracts. The structures of the brain stem are:
1. Midbrain The midbrain, located underneath the middle of the forebrain, acts as a master coordinator for all the
messages going in and out of the brain to the spinal cord. It is composed primarily of two bulging fiber tracts
called the cerebral peduncles, which convey ascending and descending impulses.
2. Pons the pons have an important nuclei in the control of breathing.
3. Medulla oblongata most inferior part of the brain stem. It contains many nuclei that regulate vital visceral
activities. The medulla oblongata contains centers that control heart rate, BO, breathing, swallowing, vomiting and
others.
4. Reticular Formation the neurons of the reticular formation are involved in the motor control of the visceral
organs. A special group of reticular formation neurons, the reticular activating system (RAS) plays a role in
consciousness and the awake/sleep cycles.
Cerebellum
The cerebellum is the third part of the hindbrain, but it is not considered part of the brain stem. Functions of the
cerebellum include fine motor coordination and body movement, posture, and balance. This region of the brain is
enlarged in birds and controls muscle action needed for flight.
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.
Nerves divide many times as they leave the spinal cord so that they may reach all parts of the body. The thickest
nerve is 1 inch thick and the thinnest is thinner than a human hair. Each nerve is a bundle of hundreds or thousands
of neurons (nerve cells). The spinal cord runs down a tunnel of holes in your backbone or spine. The bones protect it
from damage. The cord is a thick bundle of nerves, connecting your brain to the rest of your body.
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Peripheral Nervous System
The Peripheral Nervous System contains only nerves and connects the brain and spinal cord (CNS) to the rest of the
body. The axons and dendrites are surrounded by a white myelin sheath. Cell bodies are in the central nervous
system (CNS) or ganglia. Ganglia are collections of nerve cell bodies.Cranial nervesin the PNS take impulses to
and from the brain (CNS). Spinal nerves take impulses to and away from the spinal cord. There are two majorsubdivisions of the PNS motor pathways: the somatic and the autonomic.
Two main components of the PNS:
1. sensory (afferent) pathways that provide input from the body into the CNS.
2. motor (efferent) pathways that carry signals to muscles and glands (effectors).
Most sensory input carried in the PNS remains below the level of conscious awareness. Input that does reach the
conscious level contributes to perception of our external environment.
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Autonomic Nervous System
The Autonomic Nervous System is that part of PNS consisting of motor neurons that control internal organs. It has
two subsystems. The autonomic system controls muscles in the heart, the smooth muscle in internal organs such as
the intestine, bladder, and uterus. TheSympathetic Nervous System is involved in the fight or flight response.
The Parasympathetic Nervous System is involved in relaxation. Each of these subsystems operates in the reverse
of the other (antagonism). Both systems innervate the same organs and act in opposition to maintain homeostasis.
For example: when you are scared the sympathetic system causes your heart to beat faster; the parasympathetic
system reverses this effect.
Motor neurons in this system do not reach their targets directly (as do those in the somatic system) but rather connect
to a secondary motor neuron which in turn innervates the target organ.
Somatic Nervous System
The Somatic Nervous System (SNS) includes all nerves the muscular system and external sensory receptors.
External sense organs (including skin) are receptors. Muscle fibers and gland cells are effectors. The reflex arc is an
automatic, involuntary reaction to a stimulus. When the doctor taps your knee with the rubber hammer, she/he istesting your reflex (or knee-jerk). The reaction to the stimulus is involuntary, with the CNS being informed but not
consciously controlling the response. Examples of reflex arcs include balance, the blinking reflex, and the stretch
reflex.
Sensory input from the PNS is processed by the CNS and responses are sent by the PNS from the CNS to the
organs of the body.
Motor neurons of the somatic system are distinct from those of the autonomic system. Inhibitory signals, cannot be
sent through the motor neurons of the somatic system.
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