Nervous System for class 12

About Nervous System -Structure

In all the multicellular animals above the level of sponges, the system meant to perceive stimuli detected by the receptors, to transmit these to various body parts, and to effect responses through effectors, is called nervous system. In vertebrates, it is highly specialized and plays at least three vital roles
(i) Response to stimuli : By responding to all sorts of stimuli, it acquaints the organism with them so that the organism may react and orient itself favourably in the surrounding     environment.
(ii) Coordination : Along with endocrine system, the nervous system also serves to coordinate and integrate the activities of various parts of the body so that they act harmoniously as a unit. This makes possible the integrated control of the internal body environment (homeostasis). However, the nervous system brings about rapid coordination by means of nerves, whereas the endocrine system does so gradually and slowly by secreting hormones into blood.
(iii) Learning : By accumulating memories from past experiences, in higher vertebrates at least, the nervous system serves as a centre for learning. The branch of medical science dealing with the structure (anatomy), functions (physiology) and diseases (pathology) of nervous system is called neurology.

Nervous system in various animals

(i) Coelenterata : True nerve cell or ganglion cells occur for the first time in coelenterates. They are derived from interstitial cells of epidermis, forming nerve net or nerve plexus below whole epidermis.
(ii) Platyhelminthes : Nervous system of planarians marks the beginning of a centralized nervous system encountered in higher animals. That is made up of brain or cerebral ganglia, two lateral longitudinal nerve chords, numerous peripheral nerves and transverse commissures or connectives. This is sometimes called the ladder type of nervous system. In addition to the centralized nervous system planaria also possesses a sub-epidermal nerve net like that of coelenterates. Brain receives stimuli from the sense organs and conveys them to different parts of body. Special receptors, as found in turbellarians, are lacking in tapeworm. However numerous free sensory nerve-endings are present throughout the body specially in the scolex.
In Nematoda (e.g. ascaris) these system made up of central nervous system, peripheral nervous system and rectal nervous system. Rectal nervous system more developed in male.
(iii) Annelida : Nervous system well developed and concentrated. It consists of three parts : central nervous system, peripheral nervous system and sympathetic nervous system, central N.S. made up of Nerve ring and ventral nerve cord. Nerves are of mixed type, consisting of both afferent (sensory) and efferent (motor) fibres.
 (iv) Arthopoda : The nervous system of prawn or arthopods is of the annelidan type. However it is somewhat larger and has more fusion of ganglia. It consists of (i) The central nervous system including brain connected with a ventral ganglionated nerve cord through a pair of circum-oesophageal commissures, (ii) The peripheral nervous system including nerves and (iii) The sympathetic nervous system.
(v) Mollusca : In gastropodes (e.g. pila) consists of paired ganglia, commissures and connective uniting them and nerves running from these central organs to all parts of the body. It has various type of ganglia as cerebral, buccal, pleuro-pedal, supraintestinal and visceral etc. In palecypoda nervous system is greatly reduced due to sluggish and sedentary mode of life and there is little evidence of the brain. But in cephalopoda shows a high grade of organization attained only by some insects and arachnids among the other invertebrates.’
(vi) Echinodermata : Echinodermates has simple and primitive type nervous system.  It has the form of a nerve net, consisting of nerve fibres and a few ganglion cells, all confined to the body wall except the visceral nerve plexus situated in the gut wall. At certain places the nervous tissue is concentrated to form distinct nerve cords. It is made up of (i) Superficial or ectoneural nervous system (ii) Hyponeural or deep nervous system (iii) Aboral or coelomic nervous system and (iv) Visceral nervous system.’
(vii)Hemichordata : Nervous system is of primitive type resembling that of coelenterates and echinodermates.
Chordates : Nervous system well developed and formed by ectoderm. It is formed by CNS, peripheral nervous system and autonomous N.S.
Development of central nervous system in human
The central nervous system of vertebrates includes the brain and the spinal cord. These are derived from a longitudinal mid-dorsal ectodermal thickening of the embryo, called the meduallary or neural plate. This neural plate or neural groove is converted by fusion into a closed mid-dorsal longitudinal neural tube lying above the notochord. Histologically, the embryonic neural tube exhibits three zones of cells.

nervous system

Difference between Frog and Mammal brain




No arachnoid layer

Arachnoid layer present.

No fatty tissue in epidural space.

Completely filled with fatty tissue.

No subarachnoid space.


Olfactory lobe comparatively larger and in

Much smaller, visible only on anterior ventral

front of cerebrum.

side of cerebrum.

Cerebrum smaller, cylindrical and without

Much larger, divided into many lobes by

any divisions of lobes and no gyri and sulci.

many fissures, and with larger number of gyri

and sulci at the surface.

Corpus callosum absent hence no genu,splenium and fornix.

All present

Pineal body well developed with pineal stalk.

Only as vestigial part, in some present in embryonic stage only.

No intermediate commissure and habenular commissure.

Both present in the diocoel.

Infundibulum present with pituitary gland.

No infundibulum, but a pair of mammillary bodies present.

Only cross-fibres present in optic chiasma

Both cross fibres and straight fibres are present.

Two large Optic lobes (corpora bigemina)

Four small optic lobes (corpora quadrigemina)

with optocoel present.

without any cavity.

Cerebellum as very small, transverse ridge on

Much larger, divided into five lobes with

the antero-dorsal side of medulla oblongata

numerous gyri and sulci at the surface, filled

and without any lobes or gyri and sulci and no arbor vitae.

with white matter as arbor vitae.

Pons varoli absent.

Present as prominent part on the ventral side of cerebellum.


Parts of nervous system


nervous system


Differences between Cerebrum and Cerebellum




(1) It is the largest part of the brain, forming four-fifths of its weight.

(1) It is the second largest part of the brain, forming one-eighth of its mass.

(2) It covers the rest of the brain.

(2) It covers the medulla oblongata only.

(3) It is a part of the forebrain.

(3) It is a part of the hindbrain.

(4) It consists of 2 cerebral hemispheres each comprising 4 lobes : frontal, occipital, parietal, temporal.

(4) It consists of two cerebellar hemispheres and a median vermis.

(5) It encloses 2 lateral ventricles.

(5) It is solid.

(6) White matter does not form arbor vitae.

(6) White matter form arbor vitae.

(7) It initiates voluntary movements, and is a seat of will, intelligence, memory etc.

(7) It maintains posture and equilibrium.


Subdivisions, parts and associated strcutures of a vertebrate brain






Associated strcutures


(1) Telencephalon


I Ventricle (Rhinocoel)

Olfactory bulbs

Olfactory tracts

Olfactory lobes

Palaeocortex on pallium


Cerebral hemispheres

II or Lateral Ventricles

nervous system

Corpora striata or basal ganglia

Corpus callosum

Neocortex on pallium


(I) Prosencephalon (Forebrain)

(2) Diencephalon

Epithalamus (roof)

nervous system


Pineal apparatus

Parapineal or parietal


Thalamus (sides)


Hypothalamus (floor)


Hypothalamic nuclei

Optic chiasma

Median eminence

Infundibular stalk


Saccus vasculosus

Mamillary bodies

Anterior choroid plexus

(II) Mesencephalon (Midbrain)

Crura cerebri (floor)

Iter or cerebral aqueduct

nervous system

Cerebral peduncles

(III) Rhombencephalon (Hind brain)

(1) Metencephalon



Trapezoid body


(2) Myelencephalon

Medulla oblongata

IV Ventricle (Metacoel)

Restiform bodies


Salient or mammalian features of human brain

The salient or mammalian features in the human brain are
(1) Relatively small, solid olfactory lobes.
(2) Very large cerebral hemispheres divided into lobes and with highly folded surface, fully cover the rest of the brain.
(3) Corpus callosum interconnecting the cerebral hemispheres.
(4) Very small pineal body.
(5) A pair of mammillary bodies joined to hypothalamus.
(6) Relatively small, solid optic lobes divided into 4 corpora quadrigemina.
(7) Large, solid cerebellum, with highly folded surface and divided into lobes.
(8) Pons varolii present anterior to the cerebellum.

Tela choroidea is the term used for epithalamus and piamater fused.

Tela choroidea is made up of epithelium and blood vessels.

Ataxia mean lacks of muscle coordination. Damage to cerebellum is characterized by ataxia.

Dyslexia involves an inability of an individual to comprehend written language.

Multiple sclerosis is the destruction of myelin sheath of neurons of CNS.

An American scientist Roger Sperry got Nobel Prize in 1981 for his outstanding work on split brain theory.

Parkinson’s disease or Paralysis agitans is a defect of brain.

Parkinsonism is characterised by tremors and progressive rigidity of limbs caused by a degeneration of brain neurons and a neurotransmitter called dopamine.

Avian brain has large sized optic lobes to see the objects on the earth while flying so is called eye brain, while fish brain has large sized olfactory lobes to smell the prey from a distance so is called nose brain.

In fishes : Cerebrum is not differentiated in two cerebral hemispheres.

Hypothalamus has additional lobes to note pressure changes.

In reptilian brain, pineal eye (parietal body) present in front of pineal body.

In birds, instictive behaviour is well developed so corpora striata are well developed.

Grey matter of spinal cord of frog is rectangular white it is butterfly-shaped in mammals.

Central canal : Cavity of spinal cord.

Optic bigemina : Two optic lobes in brain and are found from fishes to birds.

Optic lobes of man are solid and have no optocoel but those of frog have optocoel.

Optic tectum : Dorsal thick wall of optic lobe.

Cerebellum is also called little brain.

Thalami of diencephalon act as relay centres as well as gate keepers of brain.

Optic chiasma is meant for binocular vision.

Olfactory lobes of human brain have no rhinocoel while those of frog have rhinocoel.

Man and birds are less dependent upon smell so olfactory lobes are small sized but are large sized in cartilage fishes (dog fish), dogs and reptiles as are more dependent upon smell.

Cerebellum is large sized in fishes, birds and rabbit due to their multidirectional movements and increased dependency on balance.

Stimulus for hunger : In February 1998, an American scientist Dr. Masashi Yanagisawa reported that a drop of sugar level in blood stimulates the apettite centres of lateral hypothalamus to release oraxin hormone (Gr. Oraxis = hunger)

which stimulates hunger.

Nervous disorders

Agnosia : Failure to recognize;

Alexia : Failure to read;Agraphia : Failure to write;

Aphasia : Failure to speak (due to injury to Broca’s area)

Analgesia : Loss of sensation of pain;

Anaesthesia : Loss of feeling;

Insomnia : Inability to sleep;

Amnesia : Partial or complete loss of memory;

Coma : Complete loss of consciousness.

Aproxia : Inability to carry out purposeful movements.

Multiple sclerosis : Progressive degenerative disease of CNS and is characterized by many hard scar tissues.

Caudal equamma : Bundle of roots in last segment of spinal cord.

Brain stem : Diancephalon + mid brain + pons medulla.

Cerebro vascular accident (C.V.A) or stroke : Blocking of blood supply of a part of brain.

Alzeimer : It is the disease appearing usually after 65 year. It is characterized by dementia usually. Usually in this disease is ACH producing neurons of cerebral cortex and hippocampal lobe are degenerated. It is also seen that a amyloid protein is accumulated in the brain. It is the matter of research.

Comissure : The band of neurons connecting similar structure of brain or spinal cord.

Connective : The band of neurons connecting two different structure of brain and spinal cord.

Associate fibres : If joining fiber are joining two similar structure in same halves then, they are called associate fiber.

Spinal cord : Present in spinal canal or vertebral canal of vertebral column. It is extended from foramen magnum to between I and II lumber vertebra. Spinal cord is swollen in cervical and lumber region which are called cervical and lumber enlargement.

nervous system

Structure of spinal cord

Conus medullaris : It is last tapering ends of spinal cord, its ciliated central canal is called Vth ventricle.
Cauda equine : Nearly upto birth the length of spinal cord corresponds the length of V.C. but after birth there is vertically no growth of spinal cord but vertebral column grow upto I lumber vertebra in adult. Spinal nerve come out through their respective intervertebral foramen, form horse tail hair like cluster below conus medullaris it is called cauda equine.
Filum terminales : It is extension of piamater below conus up to coccyx. In frog spinal cord also extends upto end of vertebral column.
Cisterna terminalis : It is last dilation of subarachnoid space below 1st lumbar vertebra. It is a proper site for lumber puncture or spinal tap, which is done to drain C.S.F out (5 to 10 ml). This C.S.F is used in diagnosing many diseases of CNS like meningitis, cyphalis, inter cranial pressure, menningococcal inferaction etc.
Meninges : Like brain, spinal cord is also enclosed with in three membranes. In this case duramater does not remain attached with the vertebra, instead there is a space between duramater and vertebra called epidural space. The epidural space is filled with a fluid. The distribution of duramater and piamater in spinal cord is the same as that of brain.

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Cranial nerves of mammal at a glance









Olfactory Nerves


Olfactory lobe

Sensory epithelium of olfactory sacs

Receive stimuli from the sensory epithelium of olfactory sac and carry them to olfactory lobes


Optic nerves


Optic lobes

Retina in Eyes

Stimulus of light is carried to optic lobes


Occulomotor nerves


Crura cerebri

Eye ball muscles, except superior oblique muscle

Carry the impulses from crura cerebri to the eye muscles


Trochlear nerves


From in between the optic lobes and cerebellum

Superior oblique muscle of eye ball

Carry the impulses from the brain to superior oblique muscles of the eye


Trigeminal nerves


From the gassarion galglia situated on the lateral side of medulla oblongata


Ophthalmic nerve



Skin of lips






Upper lip, skin of nose, lower eye lid.

Carry the stimuli from these organs to brain


Mandibular nerve



Lower lip and skin of jaw

Carry the stimuli from these organs to brain


Abducens nerves



Eye muscles

Carry the impulses from the brain (medulla) to eye muscles


Facial nerves


Behind trigeminal nerve, from geniculate ganglion




In the roof of mouth cavity

Carry the impulses from roof of mouth cavity


Hyoman dibular


Muscles of low jaw, muscles of neck and pinna (external ear)

Carry the impulses from brain muslces of lower jaws, neck and pinna.




In salivary glands and taste buds

Receives the stimuli from the taste buds and carry the stimulus to salivary gland.


Auditory nerves




Vestibular nerve



Utriculus, sacculus, semicircular canals and Cochlea.

Receives impulses from the internal ear and carry to brain.


Cochlear nerve





Glossopharyngeal nerve



Taste buds present in tongue and muslces of oesphagus

Carry sound impulses to brain, to muscles of oesophagus and carry the taste impulse of tongue to the brain


Vagus nerve


After arising from medulla, 9thand 10th cranial nerves unite to form vagus nerve but become separate and divide into branches


Superior laryngeal nerve



Carry the impulse to muscle of glottis


Recurrent laryngeal nerve





Cardiac nerve


Heart Muscles

From brain to heart muscles




In the abdominal cavity, in stomach and lungs.

Carry impulse from these organs to brain and from brain to muscles of these organs.


Depresser nerve



Carry the impulse to diaphragm


Spinal accessory



Muscles of neck and shoulders

From brain to muscles of neck and shoulder


Hypoglossal nerve



Muscles of tongue and neck

From brain to their muscles

nervous system

The brain reaches between 75 and 80 per cent of adult size within the first two years, and its full size of the age fo 6 years.

Each neuron of brain can connect with as many as 25,000 other cells of body.

Neuroglia are as many as ten times of the neurons. These suppport and nourish the neurons.

Corpus callosum is about 10 cm long and is with about 2000 million densely packed nerve fibres.

Somatosensory area lies in parietal lobe just behind the central sulcus and registers messages from the body and the sense organs.

Cerebral cortex is greyish-brown coloured as it is made up of ten billion (109) pyramidal spindle and stellate neurons. It contains about 10 percent of all the neurons of brain.

There are three general kinds of cortex in the cerebral cortex : sensory, motor and associative.

Cerebrospinal fluid (CSF) is similar to blood plasma except that it has less amount of proteins and cholesterol. It is about 80-150 ml.

Cerebrospinal fluid is alkaline and has a specific gravity of about 1.005.

Fore brain forms about two-third of brain, while cerebellum forms about one-eighth of brain.

Anterior commissure : A band of nerve fibres connecting two cerebral hemispheres.

Cavities of various lobes of brain are called ventricles e.g.

Lateral ventricle (Paracoel) = Cerebral hemisphere

(1st ventricle is paracoel of right cerebral hemisphere while 2nd ventricle is paracoel of left cerebral hemisphere)

3rd ventricle or diacoel = Diencephalon

4th ventricle or myelocoel = Medulla oblongata.

Broca’s area : Motor speech area lying in the frontal lobe.

Apetite centre (to initiate feeding) and satiety centre (to stop feeding) are present in hypothalamus. These have glucoreceptor neurons to note glucose level in blood.

Cerebellar cortex is formed of 3 layers. Its middle layer is formed of large, flask shaped, Purkinje cells.

Central canal : Cavity of spinal cord.

Brain stem formed of mid brain, pons varolii and medulla oblongata.

Reticular formation conencts the thalamus with the major nerves in the spinal cord and acts and gatekeeper of consciousness.

Brown spot : Pineal body of tadpole larva of frog lies as brown spot on head between two eyes and is supposed to be vestigial 3rd eye.

Stimulus for hunger : In February 1998, an American scientist Dr. Masashi Yanagisawa reported that a drop of suger level in blood stimulates the apetite centres of lateral hypothalamus to release oraxin hormone (Gr. oraxis = hunger) which stimulates hunger.

Avian brain has large sized optic lobes to see the objects on the earth while flying so is called eye brain, while fish brain has large sized olfactory lobes to smell the prey from a distance so is called nose brain.

In reptilian brain, pineal eye (parietal body) is present in front of pineal body.

Vagus is the longest cranial nerve, while Pathetic (IVth) is thinnest and smallest cranial nerve.

Gasserian ganglion lies on 5th cranial nerve, while Geniculate ganglion lies on 7th cranial nerve.

Reflex action was first discovered by Marshal Hall (1833).

I.P. Pavlov is commonly called, ‘‘Father of conditioned reflex.’

A polarized nerve fibre is about 50-100 times more permeable to K+ than Na+.

Sodium conductance : Permeability of the neuronal or muscle membrane to Na+ ions

Spike potential : Peak of action potential; potential difference is +45 to + 50 mV.

Glands of Swammerdam : Calcareous bodies at the emerging of spinal nerves in frog.

Neuritis : Inflammation of nerve/nerves.

Neuralgia : Pain travelling along a nerve.

There is no middle ear in cyclostomes, cartilage and bony fishes.

In middle ear of amphibians, reptiles and birds, there is only one ear ossicle i.e. stapes. It is cartilaginous in frog but is bony in reptiles and birds.

There is only internal ear in cyclostomes and fishes. Internal ear has only two semicircular canal in Petromyzon but one in hag fish while fishes have three semicircular canals but there is no curs commune.

Sea sickness : Tendency of nausea and vomiting due to unusual stimulation of semicircular canals by the movement of ship in sea.

Periotic bone : Bone surrounding internal ear.

Porphyropsin : Pigment in the rods of fresh-water fishes.

Human ear is insensitive to ultrasonic sound waves while bats use these sound waves for navigation during flight.

Middle ear increases the force of sound waves by 22 times (2.2 times by size difference of eardrum and fenestra

Tympanum : Specialized hearing organ of insects like grasshoppe

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