biology-reviewer-nervous-system

NERVOUS SYSTEM

Functions of the Nervous System

To carry out its normal role, the nervous system has three overlapping functions.

1. Monitoring changes

Much like a sentry, it uses its millions of sensory receptors to monitor changes occurring both inside and outside the body; these changes are called stimuli, and the gathered information is called sensory input.

2. Interpretation of sensory input

It processes and interprets the sensory input and decides what should be done at each moment, a process called integration.

3. Effects responses

It then effects a response by activating muscles or glands (effectors) via motor output.

4. Mental activity

brain is the center of mental activity, including consciousness, thinking, and memory.

5. Homeostasis

It can help stimulate or inhibit the activities of other systems to help maintain a constant

internal environment.

Nervous System Structure

The structural classification, which includes all of the nervous system organs, has two subdivisions-

the central nervous system and the peripheral nervous system.

Central nervous system (CNS).

The CNS consists of the brain and spinal cord, which occupy the dorsal body cavity and act as the integrating and command centers of the nervous system

Peripheral nervous system (PNS).

The PNS, the part of the nervous system outside the CNS, consists mainly of the nerves that extend

from the brain and spinal cord.

Supporting Cells

Supporting cells in the CNS are “lumped together” as

neuroglia, which literally means “nerve glue”.

Neuroglia

Neuroglia includes many types of cells that generally

support, insulate, and protect the delicate neurons;

in addition, each of the different types of neuroglia,

also simply called either glia or glial cells, has

special functions.

Astrocytes.

These are abundant, star-shaped cells that account

for nearly half of the neural tissue; astrocytes form a

living barrier between the capillaries and neurons

and play a role in making exchanges between the

two so they could help protect neurons from harmful

substances that might be in the blood

Microglia.

These are spiderlike phagocytes that dispose of debris including dead brain cells and bacteria.

Ependymal cells

Ependymal cells are glial cells that

line the central cavities of the brain

and the spinal cord; the beating of

their cilia helps to circulate

the cerebrospinal fluid that fills those

cavities and forms a protective

cushion around the CNS.

Oligodendrocytes

These are glia that wraps their flat

extensions tightly around the nerve

fibers, producing fatty insulating

coverings called myelin sheaths.

Schwann cells

Schwann cells form the myelin sheaths

around nerve fibers that are found in the

PNS.

Satellite cells

Satellite cells acts as protective cushioning cells.

Cell body

The cell body is the metabolic center of the neuron; it

has a transparent nucleus with a conspicuous nucleolus;

the rough ER, called Nissl substance,

and neurofibrils are particularly abundant in the cell

body.

Processes

The armlike processes, or fibers, vary in length from

microscopic to 3 to 4 feet; dendrons convey incoming

messages toward the cell body, while axons generate

nerve impulses and typically conduct them away from

the cell body.

Axon hillock

Neurons may have hundreds of branching dendrites,

depending on the neuron type, but each neuron has only

one axon, which arises from a conelike region of the cell

body called the axon hillock.

Axon Terminals

Terminals contain hundreds of tiny vesicles, or

membranous sacs that contain neurotransmitters.

Synaptic cleft

Each axon terminal is separated from the next neuron

by a tiny gap called the synaptic cleft.

Myelin sheaths

Most long nerve fibers are covered with a whitish, fatty

material called myelin, which has a waxy appearance;

myelin protects and insulates the fibers and increases

the transmission rate of nerve impulses.

Nodes of Ranvier

Because the myelin sheath is formed by many individual

Schwann cells, it has gaps, or indentations, called nodes

of Ranvier.

Functional classification

Functional classification groups neurons according to the

direction the nerve impulse is traveling relative to the CNS;

on this basis, there are sensory, motor,

and association neurons.

Sensory neurons

Neurons carrying impulses from sensory receptors to the

CNS are sensory, or afferent, neurons; sensory neurons keep

us informed about what is happening both inside and outside

the body.

Motor neurons

Neurons carrying impulses from the CNS to the viscera and/or muscles and glands are motor, or efferent, neurons.

Interneurons

The third category of neurons is known as the interneurons

or association neurons; they connect the motor and sensory

neurons in neural pathways.

Structural classification

Structural classification is based on the number of

processes extending from the cell body.

Multipolar neuron

If there are several processes, the neuron is a multipolar

neuron; because all motor and association neurons are

multipolar, this is the most common structural type.

Bipolar neurons

Neurons with two processes- an axon and a dendrite- are

called bipolar neurons; these are rare in adults, found only

in some special sense organs, where they act in sensory

processing as receptor cells.

Unipolar neurons

Unipolar neurons have a single process emerging from the

cell’s body, however, it is very short and divides almost

immediately into proximal (central) and distal (peripheral)

Central Nervous System

During embryonic development, the CNS first appears as a simple tube, the neural tube, which extends down the dorsal median plan of the developing embryo’s body.

Brain

Because the brain is the largest and most complex mass of nervous tissue in the body, it is commonly discussed in terms of its four major regions – cerebral hemispheres, diencephalon, brain stem, and cerebellum.

Cerebral Hemispheres

The paired cerebral hemispheres, collectively called the cerebrum, are the most superior part of the brain, and together are a good deal larger than the other three brai

Gyri

The entire surface of the cerebral hemispheres exhibits elevated ridges of tissue called gyri, separated by shallow grooves called sulci.

Fissures

Less numerous are the deeper grooves of tissue called fissures, which separate large regions of the brain; the cerebral hemispheres are separated by a single deep fissure, the longitudinal fissure.

Lobes

Regions of the cerebral hemisphere

Each cerebral hemisphere has three basic regions: a

superficial cortex of gray matter, an internal white matter,

and the basal nuclei.

Cerebral cortex

Speech, memory, logical and emotional response, as

well as consciousness, interpretation of sensation, and

voluntary movement, are all functions of neurons of

the cerebral cortex.

Parietal lobe

The primary somatic sensory area is located in the

parietal lobe posterior to the central sulcus; impulses

traveling from the body’s sensory receptors are

localized and interpreted in this area.

Occipital lobe

The visual area is located in the posterior part of the

occipital lobe.

Temporal lobe

The auditory area is in the temporal lobe bordering

the lateral sulcus, and the olfactory area is found deep

inside the temporal lobe.

Frontal lobe

The primary motor area, which allows us to consciously

move our skeletal muscles, is anterior to the central

sulcus in the front lobe.

Pyramidal tract

The axons of these motor neurons form the major

voluntary motor tract- the corticospinal or pyramidal

tract, which descends to the cord.

Broca’s area

A specialized cortical area that is very involved in our

ability to speak, Broca’s area, is found at the base of the

precentral gyrus (the gyrus anterior to the central

sulcus).

Speech area

The speech area is located at the junction of the

temporal, parietal, and occipital lobes; the speech area

allows one to sound out words.

Cerebral white matter

The deeper cerebral white matter is composed of fiber

tracts carrying impulses to, from, and within the cortex.

Corpus callosum

One very large fiber tract, the corpus callosum, connects

the cerebral hemispheres; such fiber tracts are

called commissures.

Fiber tracts

Association fiber tracts connect areas within a

hemisphere, and projection fiber tracts connect the

cerebrum with lower CNS centers.

Basal nuclei

There are several islands of gray matter, called the basal

nuclei, or basal ganglia, buried deep within the white

matter of the cerebral hemispheres; it helps regulate

voluntary motor activities by modifying instructions sent

to the skeletal muscles by the primary motor cortex.

Left Brain, Right Brain

The human brain is split into two hemispheres, right and left. They are joined together by the corpus callosum, a bundle of nerve fibers located in the middle of the brain.

The left hemisphere is associated with

language functions, such as

formulating grammar and vocabulary

and containing different language

centers (Broca’s and Wernicke’s area).

The right hemisphere is associated

with more visuospatial functions such

as visualization, depth perception, and

spatial navigation. These left and right

functions are the cases in most

people, especially right-handed

people.

Thalamus

The thalamus, which encloses the shallow third

ventricle of the brain, is a relay station for

sensory impulses passing upward to the sensory

cortex.

Hypothalamus

The hypothalamus makes up the floor of the

diencephalon; it is an important autonomic

nervous system center because it plays a role in

the regulation of body temperature, water

balance, and metabolism; it is also the center for

many drives and emotions, and as such, it is an

important part of the so-called limbic system or

“emotional-visceral brain”; the hypothalamus

also regulates the pituitary gland and produces

two hormones of its own.

Mammillary bodies

The mammillary bodies, reflex centers

involved in olfaction (the sense of smell),

bulge from the floor of the hypothalamus

posterior to the pituitary gland.

Epithalamus

The epithalamus forms the roof of the

third ventricle; important parts of the

epithalamus are the pineal body (part of

the endocrine system) and the choroid

plexus of the third ventricle, which forms

the cerebrospinal fluid.

Midbrain

The midbrain extends from the mammillary bodies to

the pons inferiorly; it is composed of two bulging

fiber tracts, the cerebral peduncles, which convey

descending and ascending impulses.

Corpora quadrigemina

Dorsally located are four rounded protrusions called

the corpora quadrigemina because they remind some

anatomists of two pairs of twins; these bulging nuclei

are reflex centers involved in vision and hearing.

Pons

The pons is a rounded structure that protrudes just

below the midbrain, and this area of the brain stem

is mostly fiber tracts; however, it does have

important nuclei involved in the control of breathing.

Medulla Oblongata

The medulla oblongata is the most inferior part of

the brain stem; it contains nuclei that regulate

vital visceral activities; it contains centers that

control heart rate, blood pressure, breathing,

swallowing, and vomiting among others.

Reticular Formation

Extending the entire length of the brain stem is a

diffuse mass of gray matter, the reticular

formation; the neurons of the reticular formation

are involved in 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 large, cauliflower-like cerebellum projects dorsally from under the occipital lobe of the cerebrum.

Structure

Like the cerebrum, the cerebellum has two

hemispheres and a convoluted surface; it also has

an outer cortex made up of gray matter and an

inner region of white matter.

Function

The cerebellum provides precise timing for

skeletal muscle activity and controls our balance

and equilibrium.

Coverage

Fibers reach the cerebellum from the equilibrium

apparatus of the inner ear, the eye, the

proprioceptors of the skeletal muscles and

tendons, and many other areas.

Protection of the Central Nervous System

Nervous tissue is very soft and delicate,

and the irreplaceable neurons are

injured by even the slightest pressure, so

nature has tried to protect the brain and

the spinal cord by enclosing them within

the bone (the skull and vertebral

column), membranes (the meninges), and

a watery cushion (cerebrospinal fluid).

Dura Mater

The outermost layer, the leathery dura mater, is

a double-layered membrane where it surrounds

the brain; one of its layers is attached to the

inner surface of the skull, forming

the periosteum (periosteal layer); the other,

called the meningeal layer, forms the outermost

covering of the brain and continues as the dura

mater of the spinal cord.

Falx Cerebri

In several places, the inner dural membrane

extends inward to form a fold that attaches the

brain to the cranial cavity, and one of these folds

is the falx cerebri.

Tentorium Cerebelli

The tentorium cereberi separates the

cerebellum from the cerebrum.

Arachnoid Mater

The middle layer is the weblike arachnoid

mater; its threadlike extensions span

the subarachnoid space to attach it to the

innermost membrane.

Pia Mater

The delicate pia mater, the innermost

meningeal layer, clings tightly to the surface

of the brain and spinal cord, following every

fold.

Contents

The CSF contains less protein and more vitamin C, and glucose.

Choroid plexus

CSF is continually formed from the blood by the choroid

plexuses; choroid plexuses are clusters of capillaries hanging

from the “roof” in each of the brain’s ventricles.

Function

The CSF in and around the brain and cord forms a watery

cushion that protects the fragile nervous tissue from blows

and other trauma.

Normal Volume

CSF forms and drains at a constant rate so that its normal

pressure and volume (150 ml-about half a cup) are maintained.

Lumbar Tap

The CSF sample for testing is obtained by a procedure called

lumbar or spinal tap; because the withdrawal of fluid for

testing decreases CSF fluid pressure, the patient must remain

in a horizontal position (lying down) for 6 to 12 hours after the

procedure to prevent an agonizingly painful “spinal headache”.

The Blood-Brain Barrier

No other body organ is so absolutely dependent on a constant internal environment as is the brain, and so the blood-brain barrier is there to protect it.

Spinal Cord

The cylindrical spinal cord is a glistening white continuation of the brain stem.

Length

The spinal cord is approximately 17 inches (42

cm and can expand of 45cm for men) long.

Major Function

The spinal cord provides a two-way

conduction pathway to and from the brain,

and it is a major reflex center (spinal reflexes

are completed at this level).

Location

Enclosed within the vertebral column, the

spinal cord extends from the foramen

magnum of the skull to the first or second

lumbar vertebra, where it ends just below the

ribs.

Meninges

Like the brain, the spinal cord is cushioned and

protected by the meninges; meningeal coverings

do not end at the second lumbar vertebra but

instead, extend well beyond the end of the spinal

cord in the vertebral canal.

Spinal Nerves

In humans, 31 pairs of spinal nerves arise from

the cord and exit from the vertebral column to

serve the body area close by.

Cauda Equina

The collection of spinal nerves at the inferior end

of the vertebral canal is called cauda equina

because it looks so much like a horse’s tail.

Projections

The two posterior projections are the dorsal,

or posterior, horns; the two anterior projections

are the ventral, or anterior, horns. Central Canal

The gray matter surrounds the central canal of

the cord, which contains CSF.

Dorsal Root Ganglion

The cell bodies of sensory neurons, whose

fibers enter the cord by the dorsal root, are

found in an enlarged area called dorsal root

ganglion; if the dorsal root or its ganglion is

damaged, the sensation from the body area

served will be lost.

Dorsal Horns

The dorsal horns contain

interneurons.

Ventral Horns

The ventral horns of gray matter

contain cell bodies of motor neurons

of the somatic nervous system, which

send their axons out the ventral root

of the cord.

Spinal Nerves

The dorsal and ventral roots fuse to

form the spinal nerves.

Regions

Because of the irregular shape of the gray

matter, the white matter on each side of the cord

is divided into three regions- the dorsal, lateral,

and ventral columns; each of the columns

contains a number of fiber tracts made up of axon

with the same destination and function.

Sensory tracts

Tracts conducting sensory impulses to the brain

are sensory, or afferent, tracts.

Motor tracts

Those carrying impulses from the brain to

skeletal muscles are motor, or efferent, tracts.

Structure of a Nerve

A nerve is a bundle of neuron fibers found outside the CNS.

Endoneurium

Each fiber is surrounded by a delicate connective tissue

sheath, an endoneurium.

Perimeurium

Groups of fibers are bound by a coarser connective

tissue wrapping, the perineurium, to form fiber bundles,

or fascicles.

Epineurium

Finally, all the fascicles are bound together by a tough

fibrous sheath, the epineurium, to form the cordlike

nerve.

Mixed nerves

Nerves carrying both sensory and

motor fibers are called mixed nerves.

Sensory nerves

Nerves that carry impulses toward

the CNS only are called sensory, or

afferent, nerves.

Motor nerves

Those that carry only motor fibers

are motor, or efferent, nerves.

Cranial Nerves

The 12 pairs of cranial nerves primarily serve the head and the neck. M represents Motor

Nerves, S represents Sensory nerves, and B represents both nerves.

Olfactory (S)

Its function is purely sensory, and it carries impulses for the sense of smell.

Optic (S)

Fibers arise from the retina of the eye. Its function is purely sensory and carries impulses for vision.

Oculomotor (M)

Fibers run from the midbrain to the eye; it supplies motor fibers to most of the eye movement.

Trochlear (M)

Fibers run from the midbrain to the eye; it

supplies motor fibers for one external eye

muscle.

Trigeminal (B)

Fibers emerge from the pons and form three

divisions that run to the face; it conducts sensory

impulses from the skin of the face and mucosa of

the nose and mouth; also contains motor fibers

that activate the chewing muscles.

Abducens (M)

Fibers leave the pons and run to the eye; it

supplies motor fibers that helps roll the eyes.

Facial (B)

Fibers leave the pons and run to the face; it activates the

muscles of facial expression and the lacrimal and

salivary glands; which carry sensory impulses from the

taste buds of the anterior tongue.

Vestibulocochlear(S)

Fibers run from the equilibrium and hearing receptors of

the inner ear to the brain stem. Its function is purely

sensory for the sense of hearing.

Glossopharyngeal (B)

Fibers emerge from the medulla and run to the throat; it

supplies motor fibers to the pharynx (throat) that

promote swallowing and saliva production; it carries

sensory impulses from the taste buds of the posterior

tongue.

Vagus (B)

The fibers carry sensory impulses from and motor

impulses to the pharynx, larynx, and the abdominal and

thoracic viscera; most motor fibers are parasympathetic

fibers that promote digestive activity and help regulate

heart activity.

Accessory (M)

Fiber arise from the medulla and superior spinal cord and

travel to muscles of the neck and back. Hypoglossal (M)

Fibers run from the medulla to the tongue; motor fibers

control tongue movements.

Spinal Nerves and Nerve Plexuses

The 31 pairs of human spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord.

Composition

It is composed of a specialized group of

neurons that regulate cardiac muscle,

smooth muscles, and glands.

Function

At every moment, signals flood from the

visceral organs into the CNS, and the

automatic nerves make adjustments as

necessary to best support body activities.

Divisions

The ANS has two arms: the sympathetic

division and the parasympathetic division.

Anatomy of the Parasympathetic Division

The parasympathetic division allows us to “unwind” and conserve energy.

Preganglionic neurons

The preganglionic neurons of the parasympathetic division

are located in brain nuclei of several cranial nerves- III, VII,

IX, and X (the vagus being the most important of these) and in

the S2 through S4 levels of the spinal cord.

Craniosacral division

The parasympathetic division is also called the craniosacral

division; the neurons of the cranial region send their axons

out in cranial nerves to serve the head and neck organs.

Pelvic splanchnic nerves

In the sacral region, the preganglionic axons leave the spinal

cord and form the pelvic splanchnic nerves, also called the

pelvic nerves, which travel to the pelvic cavity.

Anatomy of the Sympathetic Division

The sympathetic division mobilizes the body during

extreme situations and is also called the

thoracolumbar division because its preganglionic

neurons are in the gray matter of the spinal cord

from T1 through L2.

Ramus communicans

The preganglionic axons leave the cord in the

ventral root, enter the spinal nerve, and then pass

through a ramus communicans, or small

communicating branch, to enter a sympathetic chain

ganglion.

Sympathetic chain

The sympathetic trunk, or chain, lies along the

vertebral column on each side.

Splanchnic nerves

After it reaches the ganglion, the axon may

synapse with the second neuron in the

sympathetic chain at the same or a

different level, or the axon may through

the ganglion without synapsing and form

part of the splanchnic nerves.

Collateral ganglion

The splanchnic nerves travel to the viscera

to synapse with the ganglionic neuron,

found in a collateral ganglion anterior to

the vertebral column.

Nerve Impulse

Neurons have two major functional properties: irritability, the ability to respond to a stimulus and convert it into a nerve impulse, and conductivity, the ability to transmit the impulse to other neurons, muscles, or glands.

Electrical conditions of a resting neuron’s membrane

The plasma membrane of a resting, or inactive, neuron is

polarized, which means that there are fewer positive ions

sitting on the inner face of the neuron’s plasma membrane

than there are on its outer surface; as long as the inside

remains more negative than the outside, the neuron will

stay inactive.

Action potential initiation and generation

Most neurons in the body are excited by neurotransmitters

released by other neurons; regardless of what the

stimulus is, the result is always the same- the

permeability properties of the cell’s plasma membrane

change for a very brief period.

Depolarization

The inward rush of sodium ions changes the polarity of

the neuron’s membrane at that site, an event called

depolarization.

Graded potential

Locally, the inside is now more positive, and the outside is less positive, a

situation called graded potential.

Nerve impulse

If the stimulus is strong enough, the local depolarization activates the

neuron to initiate and transmit a long-distance signal called an action

potential, also called a nerve impulse; the nerve impulse is an all-or-

none response; it is either propagated over the entire axon, or it doesn’t

happen at all; it never goes partway along an axon’s length, nor does it

die out with distance as do graded potential.

Repolarization

The outflow of positive ions from the cell restores the electrical

conditions at the membrane to the polarized or resting, state, an event

called repolarization; until a repolarization occurs, a neuron cannot

conduct another impulse.

Saltatory conduction

Fibers that have myelin sheaths conduct impulses much faster because

the nerve impulse literally jumps, or leaps, from node to node along the

length of the fiber; this occurs because no electrical current can flow

across the axon membrane where there is fatty myelin insulation.

The Nerve Impulse Pathway

How the nerve impulse actually works is detailed below.

Resting membrane electrical conditions

The external face of the membrane is slightly positive; its

internal face is slightly negative; the chief extracellular

ion is sodium, whereas the chief intracellular ion

is potassium; the membrane is relatively permeable to

both ions.

Stimulus initiates local depolarization

A stimulus changes the permeability of a “patch” of the

membrane, and sodium ions diffuse rapidly into the cell;

this changes the polarity of the membrane (the inside

becomes more positive; the outside becomes more

negative) at that site.

Depolarization and generation of an action potential

If the stimulus is strong enough, depolarization causes

membrane polarity to be completely reversed and an

action potential is initiated.

Propagation of the action potential

Depolarization of the first membrane patch causes

permeability changes in the adjacent membrane, and the

events described in (b) are repeated; thus, the action

potential propagates rapidly along the entire length of the

membrane.

Repolarization

Potassium ions diffuse out of the cell as the membrane

permeability changes again, restoring the negative charge

on the inside of the membrane and the positive charge on

the outside surface; repolarization occurs in the same

direction as depolarization.

Communication of Neurons at Synapses

The events occurring at the synapse are arranged below.

• Arrival

The action potential arrives at the axon terminal.

• Fusion

The vesicle fuses with the plasma membrane.

• Release

Neurotransmitter is released into the synaptic cleft.

• Binding

The neurotransmitter binds to a receptor on receiving neuron’s end.

• Opening

The ion channel opens.

• Closing

Once the neurotransmitter is broken down and released, the ion channel close.