CHNB Nervous System

Nervous systemThe nervous system is the master controlling and communicating system of the body which apart from

maintaining homeostasis also controls the emotions, thoughts, actions, perceptions, behavior, movements

and various other aspects of the human body.

The major function of the nervous system is to regulate and maintain homeostasis (i.e. maintain the

essential physiological processes within normal limits). To achieve this, the nervous system is supported

by the endocrine system, which is second important regulating system. However, inspite of the similar

target (i.e. homeostasis), both these systems differ in their method of achieving it. While the nervous

system controls with rapid electrical nerve impulses, the endocrine glands produce hormones that are

released into the blood stream and brings about delayed response.

Functions of the nervous system:

The three basic functions of the nervous system include:

1. Sensory function: The nervous system comprises several sensory receptors that function to sense the

external stimuli such as touching a hot object or internal stimuli such as changes in the blood pressure etc.

this sensory information is transferred to the brain and spinal cord by means of specialized neurons

termed as afferent or sensory neurons.

2. Integrative function: on receiving the sensory information, the nervous system performs the

integrative function. In simpler terms, it processes, interprets and stores some of this information and

decides upon the next course of action.

3. Motor function: Upon the integration of the sensory information, the motor response is transferred to

the effector organs which could be muscles or glands. Such responses are carried from the brain and

spinal cord to the effector organs by neurons termed motor or efferent neurons.

4. Homeostasis: The nervous system plays an important role in the maintenance of homeostasis. This

function depends on the ability of the nervous system to detect, interpret, and respond to changes in

internal and external conditions. In response, the nervous system can stimulate or inhibit the activities of

other systems to help maintain a constant internal environment.

5. Mental activity: The brain is the center of mental activities, including consciousness, thinking,

memory, and emotions.

Structural classification:

Page 1 of 29

CHNB Nervous System

The structural classification has two subdivisions, the central nervous system and the peripheral nervous

system.

Fig: Organizational flow chart showing that the central nervous system receives input via sensory fibers and issue commands via motor fibers. The sensory and

motor fibers together form the nerves that constitute the peripheral nervous system.

The Central Nervous system (CNS) consists of the brain and spinal cord, which occupy the dorsal

cavity and act as the integrating and command centers of the nervous system. They interpret incoming

sensory information and issue instructions based on past experiences and present conditions.

The Peripheral Nervous System (PNS), the part of the nervous system outside the CNS, consists of

mainly 31 pairs of spinal nerves and 12 pairs of cranial nerves. These nerves serve as communication

lines. They link all parts of the body carrying impulses from the sensory receptors to the CNS and from

the CNS to the appropriate glands or muscles.

The PNS is further divided into somatic nervous system and autonomic nervous system (ANS).

a) Somatic Nervous System: it is also known as voluntary nervous system because it involves skeletal

muscles which are usually under voluntary control. However, exception to this is the stretch reflex of

skeletal muscles which are involuntary in nature.

b) Autonomic Nervous System (ANS): This division of the PNS is mainly concerned with all those

physiological functions that are not under voluntary control. These include digestive functions, smooth

muscle contractions, glandular activities etc. moreover, ANS is further divided as sympathetic and

parasympathetic nervous system.Page 2 of 29

CHNB Nervous System

Histological aspects of nervous tissue: The human nervous tissue based on the properties irritability (i.e. the ability to generate impulses) and

conductivity (i.e. the ability to conduct the impulses) basically divided into two types, the neurons and

neuroglia.

The neurons or the nerve cells are the structural and functional unit of nervous system. On the other

hand, the neuroglia is basically providing nutrition to the neurons.

Neuroglia(Glial cells or Glia): The neuroglia basically helps to provide support, nutrition and protection to the neurons. They are

smaller and much more abundant than neurons. The glial cells differ from the neurons in two aspects.

Firstly, they are incapable of generating and transmitting nerve impulses as neuron do. Secondly, they are

capable of multiplying and dividing. Therefore in case of any injury or damage to the neurons, their space

is occupied by the growing neuroglia. Basically there are six types of neuroglia, four are found in the

CNS while the remaining two are present in the PNS.

CNS Neuroglia:

1. Astrocytes:

The astrocytes are star shaped cells with numerous processes, the terminal ends of which are swollen

that enable to attain effective contact between the neurons and the capillaries.

Functions:

Astrocytes by forming the blood brain barrier (BBB) provide optimal chemical environment to the CNS

by preventing the entry of harmful substances from the circulation into the neurons.

The astrocytes also have a role to play in the developing embryo where they help to modulate growth.

Page 3 of 29

NeurogliaCNS NeuragliaAstrocytesOligodendrocytesMicrogliaEpendymal cellsPNS Neurogliasatellite cellsschwann cells

CHNB Nervous System

These regulate the formation of synapses among the neurons and thereby help in the process of learning

and memorizing.

Astrocytes provide a secondary energy source for neurons. Although glucose from the blood supplies

most of the energy needs of neurons, astrocytes play an energy supporting role by producing lactic

acid from stored glycogen. The lactic acid is transported to neurons, which use it to produce ATP.

2. Microglia: These are spider shaped cells with abundant projections. Their basic function is

phagocytosis in response to inflammation. They phagocytize necrotic tissue, microorganisms, and other

foreign substances that invade the CNS. Numerous microglia migrate to areas damaged by infection,

trauma, or stroke. A pathologist can identify these damaged areas in the CNS during an autopsy because

large numbers of microglia are found in them.

3.Ependymal cells: The shape of these cells ranges from cubical to columnar and bear cilia on their

surfaces. These cells line the cavities of the brain and spinal cord. The beating of their cilia helps to

circulate the CSF that fills those cavities and forms a protective cushion around the CNS.

4. Oligodendrocytes: Oligodendrocytes have cytoplasmic extensions that can surround axons. If the

cytoplasmic extensions wrap many times around the axons, they form myelin sheaths. A single

oligodendrocyte can form myelin sheaths around portions of several axons.

These cells are mainly responsible for forming myelin sheath over numerous axons of the CNS neurons. It helps in insulating the axons and also enhances the rate of conduction of nerve impulses. These neurons with a myelin sheath are termed as myelinated neurons.

PNS Neuroglia:

Page 4 of 29

CHNB Nervous System

1. Satellite cells:

Satellite cells surround neuron cell bodies. They provide support and nutrition to the neuron cell bodies,

and they protect neurons from heavy metal poisons, such as lead and mercury, by absorbing them and

reducing their access to the neuron cell bodies.

2. Schwann cells:

The Schwann cells are indulged in myelination. Schwann cells help to form a sheath over about 20 such

axons. These cells also help in the regeneration of axons of the PNS. There, the axon regeneration in the

CNS is relatively difficult.

Neuron or Nerve cell

The nerves are highly specialized cells, which enable intercommunication within the body by means of

nerve impulses. The length of the neurons may vary, some propagate nerve impulses to a distance as short

as 1mm, while certain neurons run the entire stretch of the body i.e., extending from the brain stem to the

feet. These can propagate the nerve impulses at an amazing speed varying between 0.5-130 m/sec.

Components of a neuron

Inspite of structural differences among the neurons, all of them share the following three components.

a) Cell body or soma

b) Axon

c) Dendrites

a) Cell body or Soma or perikaryon: The cell body of neuron resembles a normal cell except for

centriole. Due to this, the neurons are amitotic i.e., cannot undergo cell division. The cell body is a region

where all the metabolic functions take place. The cell bodies of all the neurons are located in the periphery

of the brain and in the centre of the spinal cord. Therefore, the cell body constitutes the grey matter of the

nervous system. The various components of cell body are,

Nucleus: Each neuron cell body contains a single relatively large and centrally located nucleus with a

prominent nucleolus.

Neurofibrils: These are composed of numerous fine filaments and function to provide shape and

support to the neuron.

Page 5 of 29

CHNB Nervous System

Nissel Bodies: These are basically rough endoplasmic reticulam, which synthesizes new proteins that

are utilized for growth of the neurons and for repair of the damaged axons in the peripheral nervous

system.

b) Axon: Axon of a neuron is a single, long, cylindrical shaped structure that arises from a cone like

region of cell body called the axon hillock, carries the nerve impulses away from the soma and transmits it

to either another neuron or an effector (muscle, glands or organs). An axon can remain as a single structure

or can branch to form axon collaterals or side branches. Cellular organelles include mitochondria,

neurofibrils, microtubules etc. However, the rough endoplasmic reticulum is absent hence the axon cannot

indulge in protein synthesis. All axons branch profusely at their terminal end, forming hundreds to

thousands of axon terminals. These terminals contain hundreds of tiny vesicles that contain chemicals

called neurotransmitters. Most long nerve fibers are covered with a whitish, fatty material, called myelin,

which gives waxy and white appearance. Myelin protects and insulates the fibers and increases the

transmission rate of nerve impulses. Since the myelin sheath is formed by many individual Schwann cells,

it has gaps called nodes of ranvier at regular intervals.

c) Dendrites: Dendrites are short, often highly branched cytoplasmic extensions. Many dendrite surfaces

have small extensions called dendritic spines, where axons of other neurons form synapses with the

dendrites. Dendrites are the input part of the neuron. When stimulated, they generate small electric

currents (i.e., graded potentials), which are conducted to the neuron cell body and to the axon hillock,

where an action potential can be generated. Axons are the output part of the neuron. Action potentials are

conducted along the axon to the presynaptic terminal, where they stimulate the release of

neurotransmitters.

Classification of neuronsNeurons are classified according to their function or structure. The functional classification is based on

the direction in which action potentials are conducted.

Sensory or afferent neurons conduct action potentials toward the CNS.

Motor or efferent neurons conduct action potentials away from the CNS toward muscles or glands.

Interneurons or association neurons conduct action potentials from one neuron to another within the

CNS.

The structural classification scheme is based on the number of processes that extend from the neuron

cell body. The three major categories of neurons are multipolar, bipolar, and unipolar.

Multipolar neurons have many dendrites and a single axon. Most of the neurons within the CNS and

motor neurons are multipolar.

Page 6 of 29

CHNB Nervous System

Bipolar neurons have two processes: one dendrite and one axon. Bipolar neurons are located in some

sensory organs, such as in the retina of the eye and in the nasal cavity.

Unipolar neurons have a single process extending from the cell body. This process divides into two

branches a short distance from the cell body. One branch extends to the CNS, and the other branch extends

to the periphery and has dendrite like sensory receptors. The two branches function as a single axon. The

sensory receptors respond to stimuli, resulting in the production of action potentials that are transmitted to

the CNS. Most sensory neurons are unipolar neurons.

Groupings of nervous tissue:

The collection of nervous tissue is called by different names when present either in the CNS or in the PNS.

1. Ganglion: Its plural is ganglia and it represents the collection of cell bodies of the neurons in the PNS.

These ganglia are found associated with both cranial and spinal nerves.

2. Nuclei: It constitutes the collection of cell bodies of the neurons in the CNS.

3. Nerve: A nerve is a collection of numerous axons of the PNS. Examples of such nerves are cranial and

spinal nerves which help in communication between the nervous system and the body cells.

4. Tract: The collection of axons of the CNS is called as tract. The numerous tracts help to associate the

neurons of the brain with the spinal cord and to propagate action potential from one area of grey matter in

the CNS to another.

Page 7 of 29

CHNB Nervous System

5. White matter: Bundles of parallel axons with their associated white myelin sheaths are called white

matter. In the brain, the white matter constitutes for the inner regions, while in the spinal cord, it makes

up the peripheral regions.

6. Gray matter: Collections of neuron cell bodies, dendrites, axon terminals, neuroglia and unmyelinated

axons are grayer in color and are called gray matter.

Neurohumoral TransmissionThe basic property of the nerve cells that renders them highly effective is irritability and conductivity.

In simpler terms, the nerve cells upon stimulation respond by generating a nerve impulse and transmit

the information even to distantly located body cells.

Graded Potentials

A stimulus applied to the plasma membrane of a cell normally causes a change in the resting

membrane potential called a graded potential. Graded potentials are so called because the potential

change can vary from small to large. Graded potentials are also called local potentials because they are

confined to a small region of the plasma membrane.

A graded potential can be either a depolarization or a hyperpolarization. A change in membrane

permeability to Na+, K+, or other ions can produce a graded potential. For example, if a stimulus causes

gated Na+ channels to open, the diffusion of a few Na+ into cells results in depolarization. If a stimulus

causes gated K+ channels to open, the diffusion of a few K+ out of the cell results in hyperpolarization.

The magnitude of graded potentials can vary from small to large, depending on the stimulus strength or

on summation. For example, a weak stimulus can cause a few gated Na+ channels to open. A few Na+

diffuse into the cell and cause a small depolarization. A stronger stimulus can cause a greater number of

gated Na+ channels to open.

A greater number of Na+

diffusing into the cell causes a larger

depolarization Summation of graded

potentials occurs when the effects

produced by one graded potential are

added onto the effects produced by

another graded potential.

Graded potentials spread, or are conducted, over the plasma membrane in a decremental fashion. That is, they rapidly decrease in magnitude as they spread over the surface of the plasma membrane.

Page 8 of 29

CHNB Nervous System

Action Potentials

When a graded potential causes depolarization of the plasma membrane to a level called

threshold, a series of permeability changes occurs that results in an action potential. The generation of

action potentials is dependent on graded potentials. Depolarizing graded potentials that reach threshold

produce an action potential. Hyperpolarizing graded potentials can never reach threshold and do not

produce action potentials. Thus, depolarizing graded potentials can potentially activate a cell by causing

the production of an action potential, whereas hyperpolarizing graded potentials can prevent the

production of an action potential. The magnitude of a depolarizing graded potential affects the likelihood

of generating an action potential. For example, a weak stimulus can produce a small depolarizing graded

potential that does not reach threshold and therefore does not cause the production of an action potential.

A stronger stimulus, however, can produce a larger depolarizing graded potential that reaches threshold,

resulting in the production of an action potential.

Depolarization

As soon as a threshold depolarization is reached, many voltage-gated Na+ channels open. Each

voltage-gated Na+ channel has two voltage-sensitive gates, called activation gates and inactivation

gates. When the plasma membrane is at rest, the activation gates of the voltage-gated Na+ channel are

closed, and the inactivation gates are open. Because the activation gates are closed, Na+ cannot diffuse

through the channels. When the graded potential reaches threshold (-55mV), the change in the

membrane potential causes many of the activation gates to open, and Na+ can diffuse through the Na+

channels into the cell. When the plasma membrane is at rest, voltage-gated K+ channels are closed.

When the graded potential reaches threshold, the voltage-gated K+ channels begin to open at the same

time but they open more slowly. The membrane potential changes from -55mV to 30mV.

Repolarization

As the membrane potential approaches its maximum depolarization, the change in the potential

diff erence across the plasma membrane causes the inactivation gates in the voltage-gated Na+ channels

to begin closing,. During repolarization, the voltage-gated K+ channels, which started to open along with Page 9 of 29

CHNB Nervous System

the voltage-gated Na+ channels, continue to open. Consequently, the permeability of the plasma

membrane to Na+ decreases, and the permeability to K+ increases. The decreased diffusion of Na+ into

the cell and the increased diffusion of K+ out of the cell cause repolarization.

At the end of repolarization, the decrease in membrane potential causes the activation gates in

the voltage-gated Na+ channels to close and the inactivation gates to open. Although this change does

not affect the diffusion of Na+, it does return the voltage-gated Na+ channels to their resting state.

Hyperpolarization

In many cells, a period of hyperpolarization follows repolarization. The hyperpolarization

exists because the increased K+ permeability that develops during repolarization lasts slightly longer

than the time required to bring the membrane potential back to its resting level. As the voltage-gated K+

channels close, the original resting membrane potential is reestablished. During an action potential, a

small number of Na+ diffuse into the cell and a small number of K+ diffuse out of the cell. The Na+–

K+ pump restores normal resting ion concentrations by transporting these ions in the opposite direction

of their movement during the action potential. That is, Na+ are pumped out of the cell and K+ are

pumped into the cell.

Refractory Period

Once an action potential is produced at a given point on the plasma membrane, the sensitivity

of that area to further stimulation decreases for a time called the refractory period. The first part of the

refractory period, during which complete insensitivity exists to another stimulus, is called the absolute

refractory period.

The second part of the refractory period, called the relative refractory period, follows the absolute refractory period. A stronger-than threshold stimulus can initiate another action potential during the relative refractory period.

Page 10 of 29

CHNB Nervous System

Page 11 of 29

CHNB Nervous System

Synaptic transmission:

1. When a nerve impulse arrives at a synaptic end of a presynaptic axon.

2. The depolarizing phase of the nerve impulse opens voltage gated Ca2+ channels, which are present in

the membrane of synaptic end bulbs. Because calcium ions are more concentrated in the extracellular

fluid, Ca2+ flows inward through the opened channels.

3. An increase in the concentration of Ca2+ inside the presynaptic neuron serves as a signal that triggers

exocytosis of the synaptic vesicles and

are released into the synaptic cleft. Each

synaptic vesicle contains several

thousand molecules of neurotransmitter.

4. The neurotransmitter molecules

diffuse across the synaptic cleft and

bind to neurotransmitter receptors in

the postsynaptic neuron’s plasma

membrane.

5. Binding of neurotransmitter

molecules to their receptors on ligand-

gated channels opens the channels and

allows particular ions to flow across the

membrane.

6. As ions flow through the opened

channels, the voltage across the

membrane changes. This change in

membrane voltage is a postsynaptic

potential. Depending on which ions the channels admit, the postsynaptic potential may be a depolarization

or a hyperpolarization. For example, opening of Na+ channels allows inflow of Na+, which causes

depolarization. However, opening of Cl- or K+ channels causes hyperpolarization. Opening Cl-channels

permits Cl- to move into the cell, while opening the K+ channels allows K+ to move out—in either event,

the inside of the cell becomes more negative.

7. When a depolarizing postsynaptic potential reaches threshold, it triggers an action potential in the axon

of the postsynaptic neuron.

Page 12 of 29

CHNB Nervous System

CENTRAL NERVOUS SYSTEMBrain

Brain is the major organ of the central nervous system. The weight of an adult human brain is about 1.5

kg. it is a spongy mass having a yellowish –white colour inside and tan grey colour outside.

Protective Coverings: (Meninges)

Three connective tissue membranes, the meninges surround and protect the spinal cord and the brain.

Cranial meninges are the protective coverings of the brain that are continuous with the spinal meninges.

The cranial meninges consisting of three layers. They are,

1. Dura mater (Outermost layer):

This is the most superficial and thickest fibrous tissue layer of the cranial meninges, consisting of two

layers. These layers form a fused layer around the brain and only at venous sinuses they provide a space

so that the venous blood can drain into it. The dura matter forms three extensions which separate various

parts of the brain. The two cerebral hemispheres are separated by falx cerebri while the cerebellar

hemispheres are separated by falx cerebella and cerebrum and cerebellum are separated by tenetorium

cerebella.

Arachnoid Mater (Middle layer):

The arachnoid matter is a serous membrane present in between the dura matter and pia matter& subdural

space and subarachnoid space. The cerebro spinal fluid is present in the subarachnoid space. Arachnoid

matter along with the dura matter extends inside the brain forming the fissures. It ends at the second

sacral vertebra by joining with the dura mater.

2. Pia mater (Innermost layer):

Pia matter is a connective tissue comprising of smaller blood vessels that forms a sheath over the convex

surfaces of the brain. They extend into the brain forming folds that dip into each fissure of the brain.

Blood Brain Barrier (BBB):

All the substances cannot gain entry into the brain. The entry of substances like harmful toxins, pathogens

etc., is restricted by abarrier known as Blood Brain Barrier (BBB). It consists of endothelial cells of

blood capillaries that are tightly joined by intercellular junctions and are lined by foot processes of

astrocytes. Hence, the entry of a substance into the brain is possible when it passes through the cells but

not between the cells. The substances that can readily cross the BBB are glucose, oxygen, water, carbon

dioxide and lipid soluble substances like nicotine, caffeine etc., while amino acids, antibiotics and water

soluble drugs cannot cross the BBB. Blood brain barrier helps in providing optimal chemical environment

for normal function of the brain and protects it from disturbances in fluid composition.

Ventricles of the Brain

The ventricles are the cavities filled with cerebrospinal fluid. The ventricles present in the brain are,

Page 13 of 29

CHNB Nervous System

1. Two lateral ventricles (Right and left)

2. Third ventricle

3. Fourth ventricle

The lateral ventricles are curved shaped cavities present within the lobes of cerebrum, each lying on

either side of median plane inferior to corpus callosum. The septum lucidum separates the left and right

ventricles and the interventricular foramina connects the lateral ventricles with the third ventricle. The

third ventricle is a narrow cavity present between the thalamus and hypothalamus, cerebral aqueduct

connects the third and fourth ventricles. The third ventricle serves as a route for transporting

cerebrospinal fluid from lateral ventricles to fourth ventricle. The fourth ventricle is present inferior to the

third ventricle and behind the pons and cerebellum. It extends into medulla oblongata and becomes

continuous with central canal of spinal cord.

Cerebrospinal Fluid (CSF)

Cerebrospinal fluid is a clear liquid which nourishes and protects the brain from injuries. Choroid

plexuses are a group of specialized capillaries that surround the ventricles of the brain, cerebrospinal fluid

is formed by choroid plexus of lateral, third and fourth ventricles of the brain. About 140 ml of CSF is

present in an adult human and the rate of formation of CSF is 500-700 ml per day. The composition of

CSF is glucose, proteins, lactic acid, urea, and uric acid, cations like Na+,K+,Mg+2 and Ca+2 ions, anions

like Cl- and HCO3- ions.

Functions:

1. It lubricates the membranes.

2. It provides mechanical protection by acting as a shock absorber.

3. It provides chemical protection by optimizing the chemical environment.

4. It supplies nutrients and removes wastes waste products from the brain.

Parts of the brain

Page 14 of 29

BrainBrain stemMedulla oblongataPonsMid BraincerebellumDiencephalon ThalamusHypothalamusEpithalamusCerebrum

CHNB Nervous System

Brainstem

The brainstem consists of the medulla oblongata, pons, and midbrain. It connects the spinal cord and

cerebellum to the remainder of the brain, and 10 of the 12 pairs of cranial nerves arise from it. In general,

the posterior part of the brainstem contains ascending tracts from the spinal cord, cerebellum, and cranial

nerves, whereas the anterior part of the brainstem contains descending tracts involved with motor control.

The brainstem contains several nuclei involved in vital body functions, such as the control of heart rate,

blood pressure, and breathing. Damage to small areas of the brainstem can cause death, whereas damage

to relatively large areas of the cerebrum or cerebellum often do not cause death.

Medulla Oblongata

The medulla oblongata is the most inferior portion of the brainstem and is continuous with the spinal

cord. It extends from the level of the foramen magnum to the pons. In addition to ascending and

descending nerve tracts, the medulla oblongata contains discrete nuclei with specific functions, such as

the regulation of heart rate, blood vessel diameter, breathing, swallowing, vomiting, coughing, sneezing,

hiccuping, balance, and coordination The posterior portion of medulla oblongata relay the proprietary

impulses to the cerebellum.

Function:

1. It is involved in the conscious control of skeletal muscles.

2. Most of the fibers coming from higher areas cross to the opposite side, or decussate,

this decussation accounts for the fact that each half of the brain controls the opposite half of the

body.

3. It conveys nerve impulses between the brain and spinal cord and controls the vital

functions by housing the vital centers like

a. Cardiovascular center

b. Respiratory center

c. Vasomotor center

d. Reflex center.

Pons

The pons is the part of the brainstem just superior to the medulla oblongata. The length of the pons is 2.5

cm. The term pons means bridge, and it describes both the structure and the function of the pons.

Functions:

1. Pons relay information between the cerebrum and the cerebellum.

2. The transverse axons provide connection between two parts (right and left) of

cerebellum.

Page 15 of 29

CHNB Nervous System

3. Pons contain the accessory breathing centers i.e., pneumonotaxic and apneustic areas

that regulate the transient period.

4. It involved in regulating the auditory and vestibular responses. It maintains sleep and

arousal patterns.

Midbrain

The midbrain, just superior to the pons and is 2.5 cm in length. is the smallest region of the brainstem.

The midbrain contains nuclei involved in the coordination of eye movements and in the control of pupil

diameter and lens shape. The midbrain consists largely of ascending tracts from the spinal cord to the

cerebrum and descending tracts from the cerebrum to the spinal cord or cerebellum.

Cerebellum

The cerebrum is the second largest part of the brain. About half of the total neurons present in the brain

are present in it. Anatomically, it is present posterior to the pons and medulla oblongata and below the

posterior part of the cerebrum. Cerebellum looks like a butterfly- shaped organ. The cerebellum is

involved in regulating the body posture, fine motor coordination and in maintaining balance and

equilibrium of the body. It controls muscle tone and rapid muscular activities like running, talking, typing

etc.

Diencephalon

The diencephalon forms the central part of the forebrain. It extends from the brain stem to the cerebrum

and surrounds the third ventricle. Diencephalon is enclosed by cerebral hemispheres and constitutes three

paired structures composed of grey matter, namely, thalamus, hypothalamus and epithalamus.

Thalamus

Thalamus is a paired large mass of grey matter, having length of about 3 cm. it is situated on lateral walls

of third ventricle. It forms the 80% of the diencephalon.

Functions:

Page 16 of 29

CHNB Nervous System

1. Thalamus acts as a relay station in transmitting sensory impulses from the spinal cord

and brain stem to the primary sensory areas of the cerebral cortex.

2. The sensations of vision, taste, hearing, touch, propioception and vibrations are carried

to specific areas of cerebral cortex.

3. Thalamus forms the center for perception of sexual sensations.

4. It serves as a center for integration of motor functions.

5. Thalamus control emotional and its behavioral aspects along with the hypothalamus and

prefrontal cortex.

Hypothalamus

The hypothalamus is the most inferior part of the diencephalon. Although a small portion of the brain, it

connects to many other parts of the brain and spinal cord and is involved with autonomic, endocrine,

emotional (limbic system), and basic body functions.

1. Autonomic nervous system (ANS). The hypothalamus is a major integrating center for controlling the

autonomic nervous system, helping control heart rate, blood vessel diameter, urine release from the

urinary bladder, and the movement of food through the digestive tract.

2. Endocrine system. The hypothalamus regulates the pituitary gland which influences functions as

diverse as metabolism, reproduction, responses to stressful stimuli, and urine production.

3. Limbic system. The hypothalamus is part of the limbic system and functions with other parts of the

brain to affect mood, motivation, and emotions. Sexual behavior and pleasure, feeling relaxed, rage, and

fear are related to hypothalamic functions

4. Basic body functions. The hypothalamus is involved with many basic body functions. It plays a central

role in the control of body temperature, activating sweat glands for cooling and shivering for heating the

body. Hypothalamic nuclei are involved in the control of thirst, hunger, and sexual arousal.

5. It maintains circadian rhythm (biological clock).

Cerebrum

The cerebrum accounts for the largest portion of total brain weight, which is about 1200 g in females and

1400 g in males. The cerebrum is divided into left and

right hemispheres by a longitudinal cerebral fissure.

Each hemisphere containing one of the lateral ventricles.

Deep within the brain the hemispheres are connected by a

mass of white matter called the corpus callosum. The falx

cerebri separates the two hemispheres and penetrates to

the depth of the corpus callosum. The superficial part of

the cerebrum is composed of nerve cell bodies or grey

Page 17 of 29

CHNB Nervous System

matter, forming the cerebral cortex and the deeper layer consists of nerve fibers or white matter. The most

conspicuous features on the surface of each hemisphere are numerous folds called gyri (jī′rī, sing. gyrus,

circle), which greatly increase the surface area of the cortex. The grooves between the gyri are called sulci

(su˘l′sī, sing. Sulcus means ditch).

Lobes of the cerebrum: Each cerebral hemisphere is further divided into four lobes which are named

according to the bones of the cranium that cover them. They are the frontal lobe, parietal lobe, temporal

lobe and occipital lobe. The frontal lobe and parietal lobe are separated from each other by the central

sulcus. Temporal lobe is separated from the frontal lobe by lateral cerebral sulcus or fissure. Occipital lobe

is separated from the parietal lobe more posterior by parieto- occipital sulcus.

Cerebral Cortex

The cerebral cortex is a region of gray matter that forms the outer rim of the cerebrum. Although only 2–

4 mm thick, but due to the presence of gyri its surface area is increased by three times. It consists of

billions of neurons arranged in six layers. It accounts for about 40% of the total weight of brain. Lobes of

the Cerebrum

Each cerebral hemisphere can be further subdivided into four lobes. The lobes are named after the bones

that cover them: frontal, parietal, temporal, and occipital lobes. The central sulcus separates the frontal

lobe from the parietal lobe. The lateral cerebral sulcus (fissure) separates the frontal lobe from the

temporal lobe. The parieto- occipital sulcus separates the parietal lobe from the occipital lobe.

Functions of the cerebrum

It helps to us to be aware of ourselves and the surroundings. It is associated sensory perception like pain,

taste, smell, touch, sound etc. it initiates and regulates skeletal muscle contraction. It is also involved in

mental functions such as memory, intelligence, reasoning, thinking, etc.

Functional areas of the cerebrum

The cerebral cortex is mainly composed of three functional areas, namely sensory areas, motor areas and association areas.

The right cerebral hemisphere is concerned with the sensory and motor functions of right side of the body.

Page 18 of 29

CHNB Nervous System

Though both the cerebral hemispheres are symmetrical, their functions are not entirely equal.I.Sensory areas

1. Primary somatosensory area (post central area) is the area behind the central sulcus. Here

sensations of pain, temperature, pressure and touch, knowledge of muscular movements and the position

of joints are perceived. The sensory area of the right hemisphere receives impulses the left side of the

body and vice versa. In sensory area of the cerebrum the body is represented upside down, i.e. the cells

nearest the vertex control the feet and those in the lowest part control the head, neck, face and fingers.

The size of the areas representing different parts of the body proportional to the extent of sensory

innervations.

2. Somatosensory association area lies behind the

sensory area (post central area) and includes greater

part of the cerebrum. It is posterior to and receives

input from the primary somatosensory area, as well as

from the thalamus and other parts of the brain. This

area permits you to determine the exact shape and

texture of an object by feeling it. Another role of the

somatosensory association area is the storage of

memories of past somatic sensory experiences,

enabling you to compare current sensations with

previous experiences. For example, the somatosensory

association area allows you to recognize objects such

as a pencil and a paperclip simply by touching them.

3. The primary visual area, located at the posterior

tip of the occipital lobe mainly on the medial surface.

It is the largest area of all the sensory areas. It receives

visual input from the retina of the eye through the

optic nerve (2nd cranial nerve) and integrates as well as

recognizes as visual impressions.

4. The primary auditory area, lies immediately

below the lateral sulcus within the temporal lobe. The

cells receive and interpret impulses transmitted from the inner ear by the vestibulocochlear (auditory)

nerves.

5. The primary gustatory area (Taste area), located at the base of the somatosensory area superior to

the lateral cerebral sulcus in the parietal lobe, receives impulses form glossopharyngeal, facial and vagus

Page 19 of 29

CHNB Nervous System

nerves (cranial nerves VII, IX and X) and is involved in gustatory (taste) perception and taste

discrimination.

6. The primary olfactory area (smell area), located deep within the temporal lobe on, receives

impulses form the nose via the olfactory nerves and is involved in olfactory (smell) perception.

II. Motor Areas1. The primary motor area is located in the

precentral gyrus (anterior to the central

sulcus) of the frontal lobe. The nerve cells are

pyramid-shaped (Bet’z cells) and they initiate

the contraction of voluntary muscles. A nerve

fiber from the Betz’s cell passes downwards

through the internal capsule to the medulla

oblongata where it crosses to the opposite

level in the spinal cord. At the appropriate

level in the spinal cord the nerve impulse

crosses a synapse to stimulate a second

neuron which terminates at the motor end-

plate of a muscle fiber. More cortical area is

devoted to those muscles involved in skilled,

complex, or delicate movement. For instance,

the cortical region devoted to muscles that

move the fingers is much larger than the

region for muscles that move the toes.

2. The premotor area, located slightly

anterior to the precentral area in the frontal lobe. This area is mainly associated with the regulation of

learned motor skills of a patterned manner like typing, playing a musical instrument etc. by sending the

stimulatory nerve impulses to the primary motor area. It is regarded as the memory bank of motor

activities.

3. Broca’s speech area, located in the frontal lobe close to the lateral cerebral sulcus, is involved in

the articulation of speech. In most people, Broca’s speech area is localized in the left cerebral

hemisphere.

Neural circuits established between Broca’s speech area, the premotor area, and primary motor area

activates muscles of the larynx, pharynx, and mouth and breathing muscles. The coordinated

contractions of your speech and breathing muscles enable you to speak your thoughts.

Page 20 of 29

CHNB Nervous System

Association Areas

The association areas of the cerebrum consist of large areas of the occipital, parietal, and temporal lobes

and of the frontal lobes anterior to the motor areas. Association areas are connected with one another by

association tracts and include the following:

1. The visual association area, located in the occipital lobe, receives sensory impulses from the

primary visual area and the thalamus. It relates present and past visual experiences and is essential for

recognizing and evaluating what is seen. For example, the visual association area allows you to

recognize an object such as a spoon simply by looking at it.

2. The facial recognition area, receives nerve impulses from the visual association area. This area

stores information about faces, and it allows you to recognize people by their faces. The facial

recognition area in the right hemisphere is usually more dominant than the corresponding region in the

left hemisphere.

3. The auditory association area, located inferior and posterior to the primary auditory area in the

temporal cortex, allows you to recognize a particular sound as speech, music, or noise.

4. Wernicke’s (posterior language) area, a broad region in the left temporal and parietal lobes,

interprets the meaning of speech by recognizing spoken words. It is active as you translate words into

thoughts. The regions in the right hemisphere that correspond to Broca’s and Wernicke’s areas in the

left hemisphere also contribute to verbal communication by adding emotional content, such as anger or

joy, to spoken words.

5. The prefrontal cortex (frontal association area) is an extensive area in the anterior portion of the

frontal lobe that is well-developed in humans. This area has numerous connections with other areas of the

Page 21 of 29

CHNB Nervous System

cerebral cortex, thalamus, hypothalamus, limbic system, and cerebellum. The prefrontal cortex is

concerned with the makeup of a person’s personality, intellect, complex learning abilities, recall of

information, initiative, judgment, foresight, reasoning, conscience, intuition, mood, planning for the

future, and development of abstract ideas. A person with bilateral damage to the prefrontal cortices

typically becomes rude, inconsiderate, incapable of accepting advice, moody, inattentive, less creative,

unable to plan for the future, and incapable of anticipating the consequences of rash or reckless words or

behavior.

The cerebral white matter

The cerebral medulla consists of tracts that connect areas

of the cerebral cortex to each other or to other parts of the

CNS. The fibers in these tracts fall into three main

categories. (1) Association fibers connect areas of the

cerebral cortex within the same hemisphere. (2)

Commissural fibers connect one cerebral hemisphere to

the other. The largest bundle of commissural fibers

connecting the two cerebral hemispheres is the corpus

callosum. (3) Projection fibers are between the cerebrum

and other parts of the brain and spinal cord. The projection

fibers form the internal capsule.

Basal nuclei

Within the two cerebral hemispheres, deep in the white matter. The most important function of the basal

nuclei is to assist in controlling initiation and termination and regulating the intensity of the movements

executed by the cerebral cortex, mainly subconscious movements of skeletal muscles like arm swinging

while walking, true laughing after a joke, etc.

Cranial nerves

The 12 pairs of cranial nerves primarily serve the head and neck. Only one pair (the vagus nerves)

extends to the thoracic and abdominal cavities. The cranial nerves are numbered in order, and in most

cases their names reveal the most important structures they control.

Most cranial nerves are mixed nerves. However, three pairs, the optic, olfactory and vestibulocochlear

nerves are purely sensory in function.

Useful mnemonics for remembering the names of the cranial nerves in order are Oh, Oh,

Oh, to touch and feel very good velvet, ah,

“Oh Oh Oh, To Touch And Feel Very Green Vegetables, ah heaven.

Page 22 of 29

CHNB Nervous System

Nerves in Order Function

I Olfactory (sensory) Sense of Smell

II Optic (sensory) Sense of vision

III Oculomotor (motor)

Moving the eye ball and focusingRegulating the size of the pupil

IV Trochlear (motor) Moving the eye ball

V Trigeminal (mixed)

ChewingSensation from the face

VI Abducens (motor) Movement of eyes laterally.

VII Facial (mixed)Activates the muscles of facial expression and the lacrimal and salivary glands.Carries sensory impulses from the taste buds of anterior tongue.

VIII

Vestibulocochlear(sensory)

Vestibular branch transmits impulses for the sense of balance, and cochlear branch transmits impulses for the sense of hearing.

IX Glossopharyngeal (mixed)

Carries sensory input from taste buds of the posterior tongue and baroreceptors of the carotid artery.Supplies motor fibers to the pharynx (throat) that promote swallowing and saliva production.

X Vagus (mixed)

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.

XI Accessory (motor) Motor fibers that activate the trapezius & sternocleidomastoid muscles of the neck and back.

XII Hypoglossal (motor) Tongue muscles and movements.

Spinal Cord

The cylindrical spinal cord, is a glistering white continuation of the brain stem. The length of the

adult spinal cord ranges from 42 to 45 cm. The spinal cord connects to the brain at the level of the

foramen magnum and extends inferiorly in the vertebral canal to level L1–L2 of the vertebral column.

Like the brain, the spinal cord is cushioned and protected by meninges. The spinal cord is extremely

important to the overall function of the nervous system. The spinal cord gives rise to 31 pairs of spinal

nerves, which exit the vertebral column through intervertebral and sacral foramina. It is the major

communication link between the brain and the PNS. The spinal cord participates in the integration of

incoming information and produces responses through reflex mechanisms.

Cross section of the spinal cord

A cross section of spinal cord reveals regions of white matter that surround an inner core of gray matter.

The white matter of the spinal cord consists primarily of bundles of myelinated axons of neurons. Two

grooves penetrate the white matter of the spinal cord and divide it into right and left sides. The anterior

median fissure is a wide groove on the anterior (ventral) side. The posterior median sulcus is a narrow

furrow on the posterior (dorsal) side. The gray matter of the spinal cord is shaped like the letter H or a Page 23 of 29

CHNB Nervous System

butterfly; it consists of dendrites and cell bodies of neurons, unmyelinated axons, and neuroglia. In the

center of the gray commissure is a small space called the central canal; it extends the entire length of the

spinal cord and is filled with cerebrospinal fluid. At its superior end, the central canal is continuous with

the fourth ventricle (a space that contains cerebrospinal fluid) in the medulla oblongata of the brain.

Anterior to the gray commissure is the anterior (ventral) white commissure, which connects the white

matter of the right and left sides of the spinal cord.

The gray matter on each side of the spinal cord is subdivided into regions called horns. The posterior

(dorsal) gray horns contain cell bodies and axons of interneurons as well as axons of incoming

sensory neurons. Between the posterior and anterior gray horns are the lateral gray horns, which are

present only in thoracic and upper lumbar segments of the spinal cord. The lateral gray horns contain

autonomic motor nuclei, which regulate the activity of cardiac muscle, smooth muscle, and glands.

The white matter of the spinal cord, like the gray matter, is organized into regions. The anterior and

posterior gray horns divide the white matter on each side into three broad areas called columns: (1)

anterior (ventral) white columns, (2) posterior (dorsal) white columns, and (3) lateral white columns.

Functions of spinal Cord:

1. Firstly, by utilizing the services of ascending and descending tracts, the spinal cord acts as a relay station to convey impulses from the periphery to the brain and vice versa.

2. The spinal cord gives rise to 31 pairs of spinal nerves that enable in connecting the brain and spinal cord to various sensory and effector organs of the body.

3. The spinal cord is also the site of origin of origin of autonomic motor neurons that innervate the involuntary smooth muscle.

4. The spinal cord is also involved in spinal reflexes where it functions as important reflex centre.

Page 24 of 29

CHNB Nervous System

5. Moreover, certain lower centers such as the micturition defecation centers are located in the spinal cord.

Reflexes and reflex arcs:

Reflexes are defined as rapid, autonomic, predictable and involuntary responses to a particular stimulus. For example, instant removal of the hand after touching a hot object is a reflex.

The neural pathway involved in the production of a reflex is known as a reflex arc.

Components of reflex arc:

The five basic components of a reflex arc are as follows.

1. Receptor: Site of the stimulus action.

2. Sensory neuron: Transmits afferent impulses to the CNS.

3. Integration center: In simple reflex arcs, may be a single synapse between a sensory neuron and a

motor neuron (monosynaptic reflex). More complex reflex arcs involve multiple synapses with

chains of interneurons (polysynaptic reflex). The integration center is always within the CNS.

4. Motor neuron: Conducts efferent impulses from the integration center to an effector organ.

5. Effector: Muscle fiber or gland cell that responds to the efferent impulses (by contracting or

secreting).

Types of reflex arcs1. Monosynaptic reflex arc: It is a reflex arc which involves only one synapse between a sensory

and a motor neuron.2. Disynaptic reflex arc: It is a type of reflex arc which has an interneuron between the sensory

and motor neurons. Therefore, the disynaptic reflex arc involves three neurons and two synapses.

3. Poly/ Multisynaptic reflex arc: This type of reflex arc involves more than one interneuron. Such reflex arcs are very complex in nature.

Page 25 of 29

CHNB Nervous System

Autonomic nervous system (ANS), which adjusts our basic life support systems without our conscious

control. The autonomic nervous system (ANS) makes routine homeostatic adjustments in

physiological systems. The ANS coordinates cardiovascular, respiratory, digestive, urinary, and

reproductive functions. It adjusts internal water, electrolyte, nutrient, and dissolved gas concentrations

in body fluids—without instructions or interference from the conscious mind. The autonomic nervous

system contributes to homeostasis by responding to subconscious visceral sensations and exciting or

inhibiting smooth muscle, cardiac muscle, and glands.

A continual flow of nerve impulses from autonomic sensory neurons in visceral organs and blood

vessels propagate into integrating centers in the central nervous system (CNS). Then, impulses in

autonomic motor neurons propagate to various effector tissues, thereby regulating the activity of

smooth muscle, cardiac muscle, and many glands. Centers in the hypothalamus and brain stem do

regulate ANS reflexes.

The main input to the ANS comes from autonomic sensory neurons. Mostly, these neurons are

associated with interoceptors, which are sensory receptors located in blood vessels, visceral organs,

muscles, and the nervous system that monitor conditions in the internal environment. Examples of

interoceptors are chemoreceptors that monitor blood CO2 level and mechano/ baroreceptors that detect

the degree of stretch in the walls of organs or blood vessels.

Autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing

(inhibiting) ongoing activities in their effector tissues (cardiac muscle, smooth muscle, and glands).

Changes in the diameter of the pupils, dilation and constriction of blood vessels, and adjustment of the

rate and force of the heartbeat are examples of autonomic motor responses. Most autonomic responses

cannot be consciously altered to any great degree. You probably cannot voluntarily slow your

heartbeat to half its normal rate.

Organization of the ANS

Compares the organization of the somatic and autonomic nervous systems. Both are efferent divisions

that carry motor commands. The SNS controls skeletal muscles, and the ANS controls visceral

effectors such as smooth muscle, glands, cardiac muscle, and adipocytes. The primary structural

difference between the two is that in the SNS, motor neurons of the central nervous system exert direct

control over skeletal muscles. In the ANS, by contrast, most autonomic motor pathways consist of two

motor neurons in series, that is, one following the other. The first neuron has its cell body in the CNS.

Its myelinated axon extends from the CNS to an autonomic ganglion. A ganglion is a collection of

neuronal cell bodies in the PNS.) The cell body of the second neuron is also in that same autonomic

Page 26 of 29

CHNB Nervous System

ganglion; its unmyelinated axon extends directly from the ganglion to the effector (smooth muscle,

cardiac muscle, or a gland).

Divisions of the ANS

The names of the two main subdivisions of the ANS are sympathetic division and parasympathetic

division. The two divisions work in several ways:

1. Most often, these two divisions have opposing effects. If the sympathetic division causes excitation,

the parasympathetic causes inhibition.

2. The two divisions may also work independently. Only one division innervates some structures.

3. The two divisions may work together, with each controlling one stage of a complex process.

In general, the sympathetic division dominates only during exertion, stress, or emergency, and the

parasympathetic division predominate under resting conditions.

Page 27 of 29

CHNB Nervous System

In the sympathetic division, or thoracolumbar division, preganglionic fibers emerge from the thoracic

and superior lumbar segments of the spinal cord and forms synapse with postganglionic neuron in

ganglia near the spinal cord. In this division, the preganglionic fibers are short, and the postganglionic

fibers are long. The sympathetic division prepares the body for the “fight or flight” response. This

response readies the body for a crisis that may require sudden, intense physical activity. An increase in

sympathetic activity generally stimulates tissue metabolism and increases alertness.

We can summarize this general pattern of responses to increased levels of sympathetic activity as

follows:

1. heightened mental alertness

2. increased metabolic rate

3. reduced digestive and urinary functions

4. activation of energy reserves

5. increased respiratory rate and dilation of respiratory passageways

6. increased heart rate and blood pressure, and

7. activation of sweat glands.

Page 28 of 29

CHNB Nervous System

In the parasympathetic division, or craniosacral division, preganglionic fibers originate in the

brain stem and the sacral segments of the spinal cord. They synapse in ganglia very close to (or

within) the target organs. In this division, the preganglionic fibers are long, and the postganglionic

fibers are short. The parasympathetic division stimulates visceral activity. This division has been

called the anabolic system, because its stimulation leads to a general increase in the nutrient content

of the blood. In response to this increase, cells throughout the body absorb nutrients and use them

to support growth and cell division and to create energy reserves in the form of lipids or glycogen.

It brings about the state of “rest and digest”. The overall pattern of responses to increased

parasympathetic activity is as follows:

1. decreased metabolic rate.

2. decreased heart rate and blood pressure

3. increased secretion by salivary and digestive glands

4. increased motility and blood flow in the digestive tract, and

5. stimulation of urination and defecation.

The Primary Neurotransmitters of the ANS

Acetylcholine and Norepinephrine are the Primary neurotransmitters of the ANS.

Acetylcholine. Acetylcholine (ACh) is the transmitter released by the preganglionic nerve terminals of

both the sympathetic and the parasympathetic divisions. Parasympathetic postganglionic neurons

release Ach at the synapse with the effectors. The postganglionic sympathetic neurons to the sweat

glands and to some blood vessels in skeletal muscle also use ACh as the neurotransmitter.

Norepinephrine. The catecholamine norepinephrine (NE) is the neurotransmitter for postganglionic

synapses of the sympathetic division. The synapses that utilize NE receptors can also be activated by

the closely related compound epinephrine (adrenaline), which is released into the general circulation

by the adrenal medulla— hence, the original designation of these type receptors as adrenergic.

Page 29 of 29