Ana and Physio of the Nervous System

8/3/2019 Ana and Physio of the Nervous System

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Overview

Maya is walking and is about to cross the street. However, she saw the red light just ahead forthe 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 usagehis mind is continuously drifting on his date the following with the girl he likes and how he wishthat 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 andfeel, 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 havean 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 theaction 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 systemutilizes 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 sensoryinput.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 aboutwhat should be done at each moment. This is the process known as INTEGRATION.

3. As the nervous system has reached a decision of what response and appropriate action tobe 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 situationgiven 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 thenprocesses and integrates this information that the red light for pedestrian crossing means

stop. The nervous system then sends motor output to Mayas legs and feet and your feet stopswalking response.

Structural Classification of the Nervous system


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Structurally, the nervous system is classified into the central nervous system and the peripheralnervous system.

y Central nervous system. The CNS consists of the brain and the spinal cord. These organsoccupy the dorsal body cavity and act as the INTEGRATING and COMMANDC

ENTERS of the nervous system. It is theCNS that interprets an incoming sensoryinformation and sends and instruction basing on the past experience and current

condition.

y Peripheral Nervous System. The PNS is consisting of the nerves that extend from thebrain and the spinal cord. It is the part of the nervous system outside the CNS. There are

varieties of nerves. The spinal nerves carry impulses to and from the spinal cord. Thecranial nerves, on the other hand, carry impulses to and from the brain. These nerves

serve as the communication lines of the body.

Functional Classification of the Nervous System

The functional classification of the nervous system is only concerned about the structures of the

peripheral nervous system (PNS). The PNS in this classification is divided into two principalsubdivisions:

y Sensory or afferent division. This subdivision is composed of the nerve fibers that conveyimpulses to the central nervous system (CNS) from the sensory receptors. These sensory

receptors are located in the different parts of the body. With the presence of these sensoryfibers the CNs is constantly informed of the events going on both inside and outside the

body.

1. The fibers responsible for delivering impulses from the skin, skeletal muscles and jointsare called the somatic sensory fibers.

2. Fibers that transmit impulses from the visceral organs are called the visceral sensoryfibers.

y Motor or efferent division. This division is responsible for carrying impulses from theCNS to the effector organs, muscles and glands. In response these impulses, activatemuscles and glands and they effect a motor response. The two classification of motor or

efferent division are:


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1. Somatic nervous system. This subdivision is also referred as the voluntary nervoussystem. The somatic NS allows a person to consciously or voluntarily control apersons

skeletal muscles.2. Autonomic nervous system (ANS). The ANS regulates the events that are automatic or

INVOLUNTARY such as the activity of the smooth and cardiac muscles and glands. The

two parts of the ANS are the sympathetic and the parasympathetic systems.

Function and Structure of the Nervous System

If you think of the brain as a central computer that controls all bodily functions, then the nervous

system is like a network that relays messages back and forth from the brain to different parts ofthe body. It does this via the spinal cord, which runs from the brain down through the back and

contains threadlike nerves that branch out to every organ and body part.

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 cellsnamely, 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 signal transmission in the nervous system. In the human brain, it isestimated 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 place2. to supply nutrients to neurons3. to insulate neurons electrically4. to destroy pathogens and remove dead neurons5. to provide guidance cues directing the axons of neurons to their targets


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Characteristics of Glial Cells:

1. Lumped together.2. Not able to transmit impulses.3. Never lose their ability to divide.

The CNS glia include:

y 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 inmaking 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 controllingthe chemical environment in the brain. This is done by picking up excess ions and

recapturing released neurotransmitters.y Microglia. These are spiderlike phagocytes that dispose debris including dead brain cells

and bacteria.y

Ependymal cells. These cells line the cavities of the brain and the spinal cord. Asidefrom 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.y Oligodendrocytes. These are glial cells that wrap their flat extensions tightly around the

nerve fibers, producing fatty insulating coverings called myelin sheaths.

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 ofways, but their most fundamental property is that they communicate with other cells viaSYNAPSES, which are membrane-to-membrane junctions containing molecular machinery that

allows rapid transmission of signals, either electrical or chemical. Many types of neuron possessan AXON, a protoplasmic protrusion that can extend to distant parts of the body and make

thousands of synaptic contacts. Axons frequently travel through the body in bundles callednerves.


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y Cell body the metabolic center of the neuron. This part of neuron contains the usualorganelles 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 projectionsemerge. 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 outmost of the metabolic functions of a neuron.y Nissl substance and Neurofibrils found in the cell body that is essential in maintaining

cell shape.y Dendrites neuron processes that covey incoming messages TOWARD the cell body.y Axons neuron processes that generate nerve impulses AWAY from the cell body.y Axon hillock a cone-like region of the cell body where the axon arises.y 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 extracellularspaces.

ySynaptic cleft a tiny gap that separates axon terminal from the next neuron.

y Myelin a whitish, fatty material that covers long nerve fibers. It has a waxy appearancethat protects and insulates the fibers and increases the rate of nerve impulses.

y Schwann cells myelinates the axon outside the nervous system. Schwann cells arespecialized supporting cells that enclose themselves tightly around the axon jelly-rollfashion.

y Myelin sheath a tight coil of wrapped membranes created after the Schwann cellsenclose the axon.

y Neurilemma part of the Schwann cell external to the myelin sheath.y 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 SENSORYNEURONS 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


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in many species the great majority of neurons receive all of their input from other neurons andsend 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 t issue by supporting and protecing the neurons.They also provide nutrients to neurons and help to keep the tissue free of debris. The neuronsrequire 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 the opening and closing ofchannels through the neuronal membranes.

Structural Classification of Neurons

y Multipolar neurons These are several processes extending from the cell body. All motorand association neurons are multipolar and this is the most common structural type.

y Bipolar neurons These are neurons with two processes an axon and a dendrite.B

ipolar neurons are rare in adults and are only found in some special sense organs suchas the eye or nose where they act in sensory processing as receptor cells.y 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 actionpotentials, 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 about0.15 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 inthe 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 actionpotential 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 nextneuron.

THE CENTRAL NERVOUS SYSTEM

The Central Nervous System (CNS) is composed of the brain and spinal cord. The CNS issurrounded 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


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enlarges into three hollow swellings that form the brain, and the posterior of which develops intothe 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 messageof pain to your brain. The brain then sends a 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 aboutit. 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 necessaryfor storing all of the bodys important information.

The four main regions of the brain are:

y Cerebral hemispheresy Diencephalony Brain stemy 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 thecerebral 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.


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The cerebrum, the largest part of the human brain, is divided into left and right hemispheresconnected 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 vertebratebrain. 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 lobeis 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 activity2. speech3. thought processes

Language comprehension is found in Wernickes area. Speaking ability is inB

rocas area.Damage to Brocas area causes speech impairment but not impairment of languagecomprehension. Lesions in Wernickes area impair ability to comprehend written and spoken

words but not speech. The remaining parts of the cortex are associated with higher thoughtprocesses,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 thesensory cortex.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 andproduces two hormones of its own.

3. Epithalamus The epithalamus contains the pineal body and the choroid plexuses. Thechoroid plexuses form the cerebrospinal fluid.


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Brain Stem

The brain stem is about the size of a thumb in diameter and is approximately 3 inches long. Itprovides 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 mastercoordinator for all the messages going in and out of the brain to the spinal cord. It iscomposed 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. ReticularFormation the neurons of the reticular formation are involved in the motor

control of the visceral organs. A special group of reticular formation neurons, thereticular 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, andbalance. This region of the brain is enlarged in birds and controls muscle action needed for flight.

SpinalCord

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 comprisethe 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.


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Nerves divide many times as they leave the spinal cord so that they may reach all parts of thebody. 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 tunnelof 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.

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 nerves in the PNS take impulses to and from the brain (CNS). Spinal nervestake impulses to and away from the spinal cord. There are two major subdivisions 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. Inputthat 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 ofPNS consisting of motor neurons that controlinternal 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. The Sympathetic

Nervous System is involved in the fight or flight response. The Parasympathetic NervousSystem 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 beatfaster; the parasympathetic system reverses this effect.

Motor neurons in this system do not reach their targets directly (as do those in the somaticsystem) 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 externalsensory receptors. External sense organs (including skin) are receptors. Muscle fibers and glandcells 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 is testing your reflex (or knee-jerk). Thereaction 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 thestretch reflex.


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Sensory input from the PNS is processed by the CNS and responses are sent by the PNS from theCNS 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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Ana and Physio of the Nervous System