Introduction                                                                                                                                                             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

1. 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.
2. 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.
3. 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                                                                                                                

1. 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.
2. 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.

3. 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.
4. 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.
5. 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.’

6. 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.’
7. 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.

ENCEPHALON

FOREBRAIN (PROSENCEPHALON)

SPINAL CORD

HINDBRAIN (RHOMBENCEPHALON)

SPINAL CORD

ECTODERM NEURAL PLATE                    NEURAL GROOVE        NEURAL FOLD

A                                                MID BRAIN                B

PINEAL BODY PARIETAL BODY

CEREBRUM (TELENCEPHALON)

PALLIUM

(MESENCEPHALON) OPTIC LOBES

CEREBELLUM

(METENCEPHALON)

MEDULLA OBLONGATA (MYELENCEPHALON)

SPINAL CORD

ECTODERM

NOTOCHORD

NEURAL CREST NEUROCOEL

GANGLION

OLFACTORY LOBE (RHINENCEPHALON)

CORPUS STRIATUM OPTIC CHIASMA

CRUS CEREBRUM

DIENCEPHALON              C                                 C THALAMUS

NEURAL TUBE

NOTOCHORD                                    D

HYPOPHYSIS PITUITARY BODY

INFUNDIBULUM

Fig. – Stages in the embryonic development of central

nervous system inT.S.

Fig. – Stages in development of brain. A – Anterior end of neural tube in lateral view. B – M.L.S. of embryonic brain to show three primary cerebral vesicles. C – Differentiation of brain from three vesicles.

1. Germinal layer : These are actively dividing cells lining the neural canal. They form the connective tissue lining of neural canal, called ependyma, and also proliferate into mantle layer cells.
2. Mantle layer : It consists of embryonic neurons or nematoblasts, forming the gray matter.
3. Marginal layer : It consists of nerve fibres, mostly surrounded by fatty myelin sheaths, and forms the white matter. Neurons and fibres are supported by a special connective tissue of ectodermal origin, the neuroglia, cells of which become increasingly abundant and diversified in higher vertebrates.

Development of brain : The anterior end of embryonic neural tube is already enlarged forming the embryonic

brain, called encephalon. By differential growth and two constrictions, it is divided into a linear series of three primary cerebral vesicles, termed the forebrain, midbrain and hindbrain. These give rise to the three major divisions of the adult brain – (1) prosencephalon (forebrain), (2) mesencephalon (midbrain), and (3) rhombencephalon (hindbrain).

These further become subdivided into 5 subdivisions.          

The various parts of the adult brain in different vertebrates are formed by modifications. That is, by

OLFACTORY LOBES

CEREBRAL HEMISPHERES

PINEAL BODY

INFUNDIBULUM OPTIC LOBES

CEREBELLUM MEDULLA OBLONGATA

SPINAL CORD

A                                                                      B

RHINENCEPHALON TELENCEPHALON

PROSENCEPHALON DIENCEPHALON

MESENCEPHALON

METENCEPHALON MYELENCEPHALON

C

thickenings and foldings of these 5 subdivisions. The adult brain has a series of cavities, called ventricles, which are in continuation with the central canal of the spinal cord and filled with a cerebro-spinal fluid.

Fig. – Pattern of generalized vertebrate brain. A – Lateral view.

B – Dorsal surface. C – H.L.S. showing ventricles

 Parts of nervous system                                                                                                                                    

Nervous system is divided into three parts

1. Central nervous system (CNS) : In all the vertebrates including man, CNS is dorsal, hollow and non- ganglionated while in invertebrates when present, it is ventral, solid and ganglionated. CNS is formed of two parts : Brain – Upper and broader part lying in the head; and Spinal cord – Lower, long and narrow part running from beginning of neck to trunk.
2. Peripheral nervous system (PNS) : It is formed of long, thin, whitish threads called nerves which extend between CNS and body parts (muscles, glands and sense organs). It controls the voluntary functions of the body. It has cranial and spinal nerves.
3. Autonomic nervous system (ANS) : It is formed of nerve fibres extending upto visceral organs and controls the involuntary functions of visceral organs of body like heart beat, peristalsis etc. It is again formed of two systems: sympathetic and para-sympathetic nervous system which have opposing functions.

Central Nervous System (CNS)              Peripheral Nervous System (PNS)                  Autonomic Nervous System (ANS)

Brain        Spinal cord                    Cranial Nerves         Spinal Nerves

(All Mixed)

Sympathetic Nervous System

Parasympathetic Nervous System

Sensory Nerves (I, II, VIII)

Motor Nerves (III, IV, VI, XI, XII)

Mixed Nerves (V, VII, IX, X)

(i) Central nervous system : Central nervous system is made up of brain and spinal cord. CNS is covered by 3 meninges and its wall has two type of matter.

Types of matter : CNS of vertebrates is formed of two types of matter –

1. Grey matter : It is formed of cell-bodies and non-medullated nerve fibres.

2. White matter : It is formed of only medullated nerve fibres which appear white due to presence of medullary sheath.

Meninges : The meninges are connective tissue membranes which surround the brain and spinal cord of CNS. In the fishes, there is only one meninx called meninx primitiva. In amphibians, reptiles and birds, the brain is covered by two meninges or membranes : inner pia-arachnoid and outer dura mater. In mammals, CNS is covered by three meninges or membranes

1. Duramater (Dura = tough; mater = mother): Outermost, thick, fibrous, 2-layered meninge. The outer layer adheres to skull at many places while the inner layer follows the major convolutions (sulci and gyri) of the brain and spinal cord. Meningeal artery traverses via duramater. The two layers of duramater are widely separated at some places to form the large sinuses called

venous sinus. This drains deoxygenated (= venous) blood from the brain to the large veins that return it to the heard. The space between duramater and the next meninge in succession is called sub-dural space is filled with cerebrospinal fluid and has arachnoid villi in the region of dural space. Similarly the space between the skull and

SEROUS MEMBRANE (RECTICULAR CONNECTIVE TISSUE

FIBROUS          

durameter is called epidural space. Duramater extends in the form of straight sulcus between cerebrum and cerebellum posteriorly. Here it is called tentorium.

(AREOLAR HIGHLY VASCULARISED TISSUE)

SKULL

Fig. – Meninges of brain

2. Arachnoid (= spider-like web) : It is closely related to duramater on its outside and with piameter on the inside. The space between the arachnoid and piameter is called sub-arachnoid space and is filled with cerebro-spinal fluid.
3. Piameter (Pia = soft = tender) : This is the innermost meninge and follows the convolutions of the outer surface of brain and spinal cord. It is highly vascular and penetrates deeply in certain places bringing it with its vasculature and placing it in contact with the ventricles of the brain and neurocoel of spinal cord.

Cerebrospinal fluid : All the ventricles of the brain are continuous and lined by a columnar, ciliated epithelium, the ependyma. They contain lymph-like extracellular fluid called the cerebrospinal fluid (C.S.F.). This fluid is secreted by the choroid plexuses by filtration of blood. The choroid plexuses consist of loose connective tissue of pia mater covered internally by a simple cuboidal epithelim of secretory (glandular) nature. The cerebrospinal fluid slowly flows toward the fourth ventricle by secretion pressure and passes into the spinal cord. Some fluid escapes into the subarachnoid spaces through three pores in the roof of the fourth ventricle in the medulla. From the subarachnoid spaces, the cerebrospinal fluid is transferred to the blood of the venous sinuses. Nervous tissue is without lymphatic vessels.

The cerebro-spinal fluid (CSF) provides

1. Protection to brain from mechanical socks.
2. Optimum physiological fluid environment for neural functions e.g. conduction of nerve impulses, transport of aminoacids, sugars, O₂ etc.

3. ‘Relief’ mechanism for the increase in intracranial pressure that occurs with each arterial pulse of blood to brain.
4. ‘Sink’ like facility for metabolites of brain.
5. The blood CSF barrier for selective transport process between blood and CSF.

Major site of CSF formation is choroid plexus, and mid ventricular wall and sub-arachnoid wall also contribute. CSF is cell free, slightly alkaline, and is isotonic to plasma. Rate of formation of C.S.F is 80 ml/hour approx, 1/2 litre per day. Total amount present in and around CNS is 150 ml it means there is atleast 3 times renewal of C.S.F. every day.

Blood brain barrier facilitate maintenance of stable internal environment. Its acts as physiological and pathological barrier as well. Hydrocephalus : The enlargement of head, a pathological condition characterized by an abnormal accumulation of cerebrospinal fluid resulting headache, vomiting, pain and stiffness of the neck.

• Increased cerebrospinal fluid may result Meningites.
• Meningites may appear due to infection and inflamation of meninges or injury of meninges.
• Infection may be viral, bacterial or both. The most common cause of meningitis in the infection of streptococcus and neumoniae, neisseria meningitidis and haemophilus influenzae.
• Lumber puncture is done for drainage of excess of cerebrospinal fluid during meningitis.
• Cerebro-spinal fluid is formed by choroid plexus (ACP and PCP).

There are three choroid plexus in humans

1. Lateral choroid plexus : It is in the roof of I and II ventricle.
2. Anterior choroid plexus : It is in the roof of III ventricle (diacoel).
3. Posterior choroid plexus or pelochoroida : It is in the roof of IV ventricle.

Oxygen and glucose requirements : Brain controls the functions of our body organs and also provides the qualities of mind – learning, reasoning, and memory. For these activities, brain needs a large and constant energy supply. At any given time, the activities of the brain account

for 20% of the body’s consumption of oxygen and 15% of its consumption of blood glucose. Brain deprived of oxygen for just 5 minutes is permanently damaged. Mental confusion results if brain is deprived of glucose.

(a) Brain (Encephalon) : It is soft, whitish, large sized and slightly flattened structure present inside cranial cavity of cranium of the skull. In man, it is about 1200-1400 gm in weight and has about 10,000 million neurons. Brain is made up of 3 parts

CEREBRUM

SULCI

GYRI          PONS MEDULLA

CEREBELLUM SPINAL

CORD

1. Fore brain (Prosencephalon)                                                Fig. – Main parts of human brain visible from lateral view

1. Olfactory lobe – Rhinencephalon
2. Cerebrum – Telencephalon
3. Diencephalon – Diencephalon

(2)Mid brain (Mesencephalon)

1. Optic lobes – Mesencephalon

(3)Hind brain (Rhambencephalon)

1. Cerebellum – Metencephalon
2. Medulla oblongata – Myelencephalon
   1. Fore brain or Prosencephalon : It forms anterior two-third of brain and is formed of three parts.
      1. Olfactory lobes : These are one pair, small sized, club-shaped, solid, completely covered by cerebral hemisphere dorsally. Each is differentiated into two parts –

1. Olfactory bulb : Anterior, swollen part, and
2. Olfactory tract : Posterior and narrow part which ends in olfactory area of temporal lobe of cerebral hemisphere.

Function : These control the smell.

• It is normal in frog, rabbit and man.
• It is well developed in dog. So power of smell is more in dog.
• These are also well developed in dog fish and name dog fish is on the basis of well developed olfactory lobes.
  2. Cerebrum : (a) Structure is divided into 5 lobes (i) frontal (ii) parietal, (iii) occipital, (iv) temporal and (v) limbic. A lobe called insula is hidden as it lies

deep in the sylvian fissure. The cerebral hemisphere are separated from olfactory lobes by rhinal fissure. The median fissure divides the cerebrum into a right and a left cerebral hemisphere.

A few sulci are well developed and form three deep and wide fissures which divide each cerebral hemisphere into four lobes : anterior frontal lobe, middle parietal lobe,

FRONTAL LOBE

TEMPORAL LOBE HYPOTHALAMUS

PITUITARY GLAND CEREBRAL PEDUNCLE

PONS.

HUMAN BRAIN

PARIETAL LOBE

GYRUS

SULCI CORPUS

CALLOSUM PINEAL GLAND

OCCIPITAL LOBE

CEREBELLUM

posterior occipital lobe and lateral temporal lobe e.g. Fissure lying between the frontal and parietal lobes is central fissure, that lying

MEDULLA OBLONGATA

Fig. – Sagittal section of human brain

between the parietal and occipital lobes is parieto-occipital fissure and that demarcating frontal and parietal lobes from the temporal lobe is lateral or Sylvian fissure. Each cerebral hemisphere is with a fluid-filled cavity called lateral ventricle or paracoel.

FRONTAL LOBE

CENTRAL SULCUS PARIETAL LOBE

LIMBIC LOBE

PARIETO- OCCIPITAL- SULCUS

OCCIPITAL LOBE

CEREBRAL (=MEDIAN) FISSURE

CORPUS CALLOSUM

LATERAL VENTRICLE

INTERNAL CAPSULE

INSULA

LATERAL FISSURE

TEMPORAL LOBE

CORPUS STRIATUM

+ GLOBUS PALLIDUS

= BASAL GANGLION

TEMPORAL LOBE

Fig. – Medial surface of cereberal hemisphere

Fig. – Cross section of cerebrum

Two cerebral hemispheres are interconnected by thick band of transverse nerve fibres called corpus callosum. The peripheral portion of each cerebral hemisphere is formed of grey matter and is called cerebral cortex, while deeper part is formed of white matter and is called cerebral medulla. Cerebral cortex is the highest centre for many sensations and activities and is with a number of sensory areas.

Important areas in the human brain

2. Histology of cerebrum : The whole brain possess grey matter outside and white matter inside around ventricle.
   1. Grey matter : In cerebrum grey matter is very much developed, it is on an average 3.5 mm. thick but at poles its thickness is 1.3 mm. It is thickest at pre central gyrus (4.5 mm thick). Grey matter of cerebrum is called cortex or pallium. Phyllogenetically or evolutionarily cortex is divided into 3 parts –
      1. Allocortex or paleocortex : It is the

SPEECH AREA

OLFACTORY AREA

GENERAL MOTOR AREA

PONS VAROLI

MEDULLA

SOMAESTHETIC AREA

GUSTATORY AREA VISUAL AREA

SPEECH AREA

(READING/LANGUAGE)

CEREBLLUM

SPINAL CORD

cortex of olfactory area of frontal lobe and olfactory

Fig. – Sensory areas of human brain

bulbs. In lower vertebrates (cartilagenous fish) olfact lobes occupy most of the part of cerebrum. So in these animals sense of olfection is very-very much developed. Sense of olfaction is oldest sense.

2. Mesocortex : It is relatively not much older in development.

3. Neocortex or neopallium or isocortex or neencephalon : It is most recent cortex and is developed maximum only in human. It is in prefrontal cortex or prefrontal region (organ of mind), precentral and precentral gyrus etc. The neocortex is having 6 (six) layer of neurons while remaining cortex possess only 5 layers.

The cerebral cortex is having area of about 2200 cm² while the cranial cavity is only 1450 cm³, so to accomodate cerebrum there appears foldings in the cortex. The ridges are called gyrus (or gyri) or convolution while the depression are called sulcus (sulci in plural).

2. White matter : It is inner part of brain. Its fibres are divide into 3 categories –

1. Commissural fibers : These neurons connect gyri of 2 hemispheres, such as corpus callosum. habenular commissure, anterior commissure, posterior commissure.
2. Associate fibres : They connect gyri of same hemispheres.
3. Projection neuron : They are infact assending and descending nerve

GYRUS SULCUS CORTEX

SUB CORTEX WHITE MATTER

tract, they connect one part of brain to another part of brain or to spinal cord. (In spinal cord they were called as columo).

3. Associated     structures    of    cerebrum : Cerebrum has following specific structure.

1. Sub cortex : Nuclei on white matter. It is cluster of grey neurons in depth of white matter, they are formed in whole brain and are named differently.
2. Basal granules or central nucleus : Basal ganglia is the name given to many sub cortical structure

of walls of paracoel, hypothalamus and mid brain –

V1         V2

V3

CAUDATE NUCLEUS

THALAMUS CLAUSTRUM

                   INSULA

PUTAMEN

LENTICULAR

1. Corpus striatum : Corpus striatum is the

                                      GLOBUS

PALLIDUS

NUCLEUS

name given to caudate nucleus and lenticular nucleus.                                           _PITUITARY

Caudate is tail shaped while the lenticular nucleus is

BODY

lenti shaped. The lenticular nucleus is sub-divided in putamen (outer shell) and globus pallidus (ball).

2. Claustrum : It is the name given to grey matter present between insula and patamen.
3. Epistriatum or Amygdaloid body : It is structure present at the end of caudate nucleus.
4. Red nucleus and substantia nigra of mid brain.

The basal ganglia controls automatic movements of skeletal muscles like swinging, walking etc.

3. Corpus callosum : It is the band of white neurons present between both cerebral hemisphere and connect them on medial surface. It is present only mammal. It

has anterior part genu, middle part trunchus and last part splenium.

Below corpus callosum there are two fused band of

TRUNCUS

GENU                                                               SPLENIUM

white neurons called fornix. There anterior part is called column and posterior part is called crura. Between column and genu a membrane is called septum lucidum or septum pellicidum. Septum lucidum encloses a space called V₅ or

SEPTUM PELLICIDUM

ROSTRUM

CRURA COLUMN

FORNIX

Pseudocoel, because it is not possessing C.S.F. i.e. why it is called pseudocoel.

4. Limbic system : It is also called emotional brain or animal brain. Limbic system controlling emotion, animal behaviour like chewing, licking, sniffing,

rage, pain, plessure, anger, sexual feelings, grooming. It has following structure

1. Singulate gyrus : It is a region of pre central gyrus.
2. Hippocampal gyrus : It is a region of temporal lobe near colossomarginal sulcus. These two structure are combinely called limbic lobe.
3. Amygdaloid body : It is the end of caudate nucleus.

OCCIPITAL

HIPPOCAMPAL GYRUS

DENTATE GYRUM

STRAE MEDULARIS

MAMILLARY BODY CAUDATE NUCLEUS

                          SIGNATURE GYRUS

OLFACTORY BULB

             FRONTAL AMYGDALOID BODY

4. Olfactory bulb : They are on the inferior anterior surface of brain. Olfactory nerve ends in these bulb.
5. Mammillary body : They are found in hypothalamus. Olfactory bulb and mammillary body both are centre of olfaction.

From a evolutionary point of view, the cerebral hemisphere are the highly evolved structure and this is manifested by

1. Great increase in the number of feed back circuits between cerebral cortex and sub cortical elements.
2. The ability of man and other primates to perform variety of complex function.
3. The lobe of cerebrum are delineated by fissure and sulci.
4. A corpus callosum connects the left and right cerebral hemisphere. This is a unique property of mammals as it facilitates flow of information between the 2 hemispheres.
5. The cortical layer of cerebrum is thrown into folds (= gyri) separate by sulci. All the larger mammalian brains exhibit well developed gyri. The degree of convolutions of the cortex is a fairly reliable indicator of the evolutionary stages of development of brain. The roof of cerebrum is called pallium while the ventrolateral walls are thick and are called corpora striata.

6. Diencephalon cavity is called, III vertricle or diacoel : The thin roof of this cavity is known as the epithalamus, the thick right and left sides as the thalami, and floor as the hypothalamus.

1. Epithalamus : The epithalamus is not formed of nervous

tissue. It consists of piamater only. Hence, it is of relatively little significance as a nerve centre. Its anterior part is vascular and folded. It is called anterior choroid plexus. Behind this plexus, the epithalamus gives out a short stalk, the pineal stalk which hears a small, rounded body, the pineal body, at its tip.

2. Thalami : A pair of mass of grey matter formes the major part of the wall and floor of diancephalon. Its nuclei have complex connection with the sensory area of the cerebral cortex. It receives and integrates sensory impulses from the eye, ear and skin. It has

nerve connection with motorcortex and act as relay centre. Habenular

LAMINA TERMINALS

DIOCOEL

OPTIC CHIASMA PINEAL BODY

PINEAL STALK

ANTERIOR CHOROID PLEUXES

OPTIC THALAMI HYPOTHALAMUS

INFUNDIBUIUM

PITUITARY (HYPOPHYSIS)

commissure is a band of nerve fivers connecting two thalami. On the

inferior surface of each thalamus there are two rounded bodies of grey matt