Central Nervous System
The structures of the CNS arise from its embryological components. 
Prosencephalon 
(i) Becomes the thalamus and hypothalamus (diencephalon) and 
(ii) The cerebral cortex, corpus striatum, hippocampus and amygdala (telencephalon).  
Mesencephalon 
Becomes mid brain. 
Rhombencephalon develops into    
(i) The medulla (myelencephalon) 
(ii) The pons and cerebellum (metencephalon). 

BRAIN 
    Meninges
        The brain is surrounded by three protective coats of connective tissue besides the bony cranium. These are known as meninges (singular, meninx). 
(i) Piamater : It is the inner meninx. It is very thin, highly vascular, and closely invests the brain. It is covered by simple squamous epithelium. 
(ii) Arachnoid Mater or Membrane: It is the middle meninx. It is also thin but is nonvascular. It is covered with simple squamous epithelium on both (intemal and extemal) surfaces. There is a narrow space between the pia mater and the arachnoid membrane. It is called the subarachnoid space. It contains cerebrospinal fluid and is crossed by a number of connective tissue strands. 
(iii) Duramater : It is the outer meninx. It is thick, tough and lines the cranial cavity. Its internal surface is covered with simple squamous epithelium. A very narrow space also exists between the dura mater and the arachnoid membrane. It is called the subdural space. It contains a little fluid which is not the cerebrospinal fluid.

The adult human brain contains more than 100 billion neurons and almost 10 times neuroglia cells. The brain is divided into three main sections: 
(i) Fore brain; (ii) Mid brain; (iii) Hind brain    
Fore brain: It consists of two main parts, the cerebrum and the diencephalon.    
(a) Cerebrum. By far the largest and most highly developed part of the brain is cerebrum. It is divided into two hemispheres by a prominent longitudinal fissure. The two hemispheres are connected by a bundle of transverse fibres called corpus callosum. The anterior part of corpus callosum is curved and is called genu while the posterior part is called splenium. Each cerebral hemisphere is divided into four lobes. These are the frontal at the front, the parietal towards the top of the head, the temporal on the side and the occipital at the rear.

Human brain in side view showing location of certain functions

Cerebral cortex : 
The outer surface of cerebrum, called the cortex, is a layer only 2-4 millimetres thick. 
Because the six layers of it are packed with ten billion (109) pyramidal, spindle and stellate neurons with a greyish brown appearance, it is referred to as grey matter.

The cerebral cortex contains roughly 10 percent of all neurons of brain.

Much of the neural activities occur here, e.g., from the touch of a feature to the movement of an arm.

Unlike mouse brain, human brain is greatly convoluted.

These convolutions or folds consist of sulci (sing. Sulcus : small groove), fissures (large grooves), and gyri (sing. Gyrus : buldge between adjacent sulci or fissures).

These greatly enlarge the surface area of the cortex.

In fact, two-thirds of the surface of the cortex is hidden in the sulci and fissures.

Thus, their presence triples the area of the cerebral cortex.

Beneath this run millions of axons comprising nerve fibre tracts, connecting the neurons of cerebral cortex with those located elsewhere in the brain.

The large concentration of myelin gives this tissue an opaque white appearance.

Hence, they are referred by the term white matter.

By examining the effect of injuries or lesions and the effect of electrical stimulation on the behaviour, it has been possible to map roughly the location of its various associative activities on the cerebral cortex.

Each area is referred to as a specialised cortex.

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

(b) Diencephalon:

The diencephalon contains the epithalamus, thalamus and hypothalamus.

Epithalamus is thin, non nervous roof of diencephalon.

Its anterior region is folded and fused with piamater to form anterior choroid plexus.

This is responsible for formation of CSF.

Above it is present pineal stalk bearing the pineal body at the top of it.

Pineal body is endocrine gland and also taken as vestige of 3rd eye.

The thalamus directs sensory impulses from the lower parts of the brain and spinal cord to appropriate parts of the cerebrum.

Limited sensory awareness of pain, temperature, touch and pressure is provided by the thalamus.

Hypothalamus:

As the name implies, hypothalamus nestles at the base of the thalamus, and so of the brain.

Although relatively small, just 4 grams, about 1/300 of the total brain mass is highly vascularised.

It integrates and controls the visceral activities.

The hypothalamus, through its connection with brain stem, maintains homeostasis and the body's internal equilibrium, specialising in involuntary behaviour control.

The nuclei in it signal the body to eat, drink, get angry, keep cool, and make love and so on.

Hyothalamus organises behaviour related to survival of species: fighting, feeding, fleeing and mating.

 It keeps body temperature at roughly 37ºC by means of a complex thermostat system.

It also influences respiration and heartbeat and sends out signals to correct them when they are wrong.

Through connections with the pituitary gland, the hypothalamus controls growth and sexual behaviour.

It also controls many more functions.

Basal Ganglia:

The inside of human brain is not so densely packed, but there are all kinds of different collections of neurons, called nuclei, each with its specific functions.

These control different body activities automatically.

Basal ganglia is a collection of subcortical nuclei in the forebrain, at the base of the cortex.

The largest nucleus in it is the corpus striatum.

It regulates planning and execution of stereotyped movements.

Other basal ganglia perform at subconscious level learned pattern of movements like slow and fast pedalling; slow and fast writing/typing etc.

Destruction of dopamine secreting pars compacta part of basal nucleus called substantia nigra leads to paralysis agitansl parkinson's disease. Huntington's chorea is due to degeneration of GABA secreting neurons of corpus striatum and acetylcholine secreting neurons of other parts.

Limbic System :

Flared like a wish bone a ring or fork, through extensive neural links with the cerebrum and the brain stem below, constitute what is called limbic (meaning lip-like) system.

This system sends out signals to the rest of the brain and the body which have great effect on your behaviour.

Limbic system includes hypothalamus, amygdala, hippocampus, septum, anterior nucleus of thalamus and portion of basal ganglia.

Amygdala :

Above the hypothalamus, attached to the interior lips of both forks, is almond-shaped amygdala.

This bulge of neurons is like a defense castle controlling the moods, especially anger and rage.

Various regions of the amygdala play important role in emotional behaviour, such as aggression and remembering fear.

Hippocampus:

Taking its name from the Greek for "sea horse", whose shape roughly resembles, the hippocampus make the swollen lower lip of the limbic fork.

The remarkable organ deals with a strange mix of signals about smells and memories.

The hippocampus functions as a kind of index for recall of an event with its associated memory.

The hippocampus converts information from short-term to long-term memory, essential in learning.

The septum linked to the hypothalamus contains yet another emotion centre for sexual arousal.

Mid Brain

It has two structures -corpora quadrigemina and cruracerebri.

(a)    Corpora Quadrigemina :

It contains 4 lobes, therefore, corpora quadrigemina.

Its principal structures are superior colliculi and inferior colliculi.

The superior pair of colliculi receive sensory impulses from the eyes and muscles of the head and control visual reflexes.

For example, they control and coordinate the movement of the head and eyes, to fix and focus on an object.

The inferior pair of colliculi receive sensory impulses from the ears and muscles of the head and control auditory reflexes such as the movement of the head to locate and detect the source of a sound.

Median section of human brain

(b) Crura cerebri (Cerebral peduncle) :

These are two heavy fibrous tracts on the inferior side of mid brain and connect hind brain with fore brain.

Crura cerebri is involved in controlling muscle tone and modifying some motor activities.

These relay sensory as well as motor impulses between fore brain and hind brain.

Hind Brain

Consist of cerebellum, Pons and Medulla.

Cerebellum:

To the rear of the brain and placed under the cerebrum, is the second largest part of the brain, called the cerebellum that means simply "little cerebrum".

Wedged between cerebral hemispheres and brainstem, cerebellum is made up of two cerebellar hemispheres.

Like the cerebrum, the cerebellum has its grey matter on the outside, comprising of three layers of cells and fibres.

The middle layer contains characteristically large-flask-shaped Purkinje cells.

Tree-like themselves with myriad of dendrites, purkinje cells rank among the most complex of all neurons.

The white and grey matter form arbor vitae.

Central portion of the cerebellum has worm like appearance as it is narrow and furrowed.

It is called Vermis.

Three paired bundles of myelinated nerve fibres, called cerebellar peduncles, form communication pathways between the cerebellum and other parts of the CNS.

The superior cerebellar peduncles connect the cerebellum to the midbrain, the middle cerebellar peduncles communicate with the pons, and the inferior cerebellar peduncles consist of pathways between the cerebellum and the medulla oblongata, as well as spinal cord.

Cerebellum does not initiate movement but modulates or reorganises motor commands.

Cerebellum's unconscious directions and cerebrum's conscious instructions determine how and when to move body parts.

The cerebellum is vital to the control of rapid muscular activities, such as running, typing and even talking.

All the activities of the cerebellum are involuntary, but may involve learning in their early stages.

Pons, (Latin meaning : the bridge) forms the floor of the brain stem.

It serves as a neuronal link between the cerebral cortex and the cerebellum.

It has pneumotaxic centre, the switch off centre for inspiration.

Medulla oblongata, literally meaning oblong marrow, is the posterior most part that connects the spinal cord and various parts of the brain.

It has with it breathing centre, cardiovascular centre, vomiting centre.

Vagus nerve arises from medulla.

Its roof is thin and non-nervous and constitute posterior choroid plexus.

Below the plexus, the roof has three opening, a pair of lateral apertures called foramina luschka and a single median foramina Magendie.

These apertures connect external and internal components of CSF of brain.

Most of the sensory as well as motor nerve tracts cross over to the other side in medulla, therefore, right half of cerebrum controls left half of body and vice a versa.

Reticular formation that connects to the thalamus and major nerves in the spinal cord, is the gatekeeper to consciousness.

Brain Stem: It is the area of the brain between the thalamus and the spinal cord and includes medulla, pons and midbrain. Diencephalon mayor may not be included.

Ventricles of the Brain and Cerebrospinal Fluid

The ventricles consist of four hollow, fluid filled spaces inside the brain.

A lateral ventricle lies inside each hemisphere of the cerebrum.

Each lateral ventricle is connected to the third ventricle by an interventricular formen (foramen of Monro).

The third ventricle consists of a narrow channel between the hemispheres through the area of the thalamus.

It is connected by the cerebral aqueduct or aqueduct of Sylvius or iter in the midbrain portion of the brain stem to the fourth ventricle in the pons and medulla.

The fourth ventricle continues with the central canal of the spinal cord.

Three openings in the roof of the fourth ventricle, a pair of lateral apertures (foramina of Luschka) and a median aperture (foramen of Magendie) allow cerebrospinal fluid to move upward to the subarachnoid space that surrounds the brain and spinal cord.

Schematic representation of the Ventricles of Human brain

The cerebrospinal fluid is secreted by anterior choroid plexus and posterior choroid plexus and is found inside the ventricles of the brain, the central canal of the spinal cord.

The cerebrospinal fluid acts as a shock absorber for the brain and spinal cord and may also contribute to nourish brain tissue it contains protein, glucose, chloride and water.

Spinal Cord

It is an elongated cylindrical structure which lies in the neural canal of the vertebral column and is continued with the \ medulla oblongata through foramen magnum of the skull.

It measures about 45 cm in length.

It extends down upto first lumbar vertebra where it tapers to a point called conus medularis/conus terminalis.

However, the meninges of the spinal cord continues as filum terminale starts from the conus, and runs upto coccygeal region.

The spinal cord shows two enlargements

(i)    Brachial swelling -from 4th cervical to 1st thoracic vertebrae.

(ii)    Lumbar swelling -from 9th thoracic to 12th thoracic vertebrae.

The spinal cord possesses an anterior and a posterior median fissure running along its length.

The grey matter of spinal cord is internal and present around the central canal.

It is produced into a posterior and anterior pairs of grey columns/roots.

Each dorsal root has a ganglion called dorsal root ganglion.

Dorsal root is sensory and ventral root is motor in nature. Both get combine with each other before coming out of vertebral column through intervertebral foramina.

The white mater is outer, and divided into four funiculi one dorsal, one ventral and two lateral.

Spinal cord conducts impulses to and from the brain.

Dorsal funiculus has ascending nerve tract for conducting sensory impulses towards brain.

Lateral and ventral funiculus conduct motor impulse from brain to spinal cord.

It controls most of the reflex activities.