The Nervous System is a Complex Network of Nerves and Cells That Carry Messages to and From The

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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Documents

The Nervous System is a Complex Network of Nerves and Cells That Carry Messages to and From The