Structure and functions of the human brain

  • The brain is soft, whitish, somewhat flattened organ situated in the cranial cavity of the skull which protects the brain from mechanical injury.
  • The brain remains surrounded by three protective coats of connective tissue besides the bony cranium.
  • There are three such protective coats called meninges which are as follows:
    • Pia Mater: It is the inner meninx which is very thin, highly vascular and is in direct contact with brain.
    • Arachnoid Mater: It is the middle meninx which is thin but non-vascular. There is a narrow space between the pia mater and the arachnoid mater called subarachnoid space which contains CSF.
    • Dura Mater: It is the outer meninx which is thick, tough and lines the cranial cavity. A very narrow space also exists between the dura mater and the arachnoid mater called subdural space containing a fluid which is not CSF
  • The brain forms about 98% of the weight of the whole CNS.
  • The average human brain weighs about 1200-1400 grams and consists of about 100 billion neurons.
  • The brain is divisible into three main regions:
    • Forebrain (Prosencephalon)
    • Midbrain (Mesencephalon)
    • Hindbrain (Rhombencephalon)


  • It forms the greater part of the brain which consists of three regions: olfactory lobes, cerebrum and the diencephalon.
  1. Olfactory lobes:
    • They are a pair of very small, solid, club-shaped bodies widely separated from each other.
    • They are visible only in the ventral view of brain as they are fully covered by the cerebrum.
    • Each olfactory lobe consists of anterior olfactory bulb and posterior olfactory tract associated with the perception of smell.
  2. Cerebrum:
  • It is the largest part of the brain occupying about 80% of the brain.
  • It is a dome shaped part, which is divided into the left and right hemispheres by a deep longitudinal cerebral fissure.
  • At their bottom, the hemispheres are connected by a large, curved nerve tract called corpus callosum.
  • The outer surface of the cerebrum is made up of grey matter containing many layers of nerve cells called the cerebral cortex.
  • The cerebral cortex is highly convoluted (folded) with many elevations (gyri, sing. gyrus) and depressions (sulci, sing. sulcus) to increase the area for accommodating more neurons.
  • Three deep and wide sulci, called fissures, divide each hemisphere into four lobes:
    • Anterior frontal lobe: It is the region for speech, facial and muscular activities, higher mental activities, reasoning and decision making, expression of emotions etc.
    • Middle parietal lobe: It is the region for sensory perceptions of taste, touch, pain, cold, conscious association etc.
    • Lateral temporal lobe: It is the region for decoding and interpretation of sound (hearing), emotions and memory.
    • Posterior occipital lobe: It is the area of for visual perception like shape and color of objects.
  1. Diencephalon:
  • Within a brain is a series of connected cavities called ventricles which are filled with CSF.
  • Diencephalon encloses one of these slit-like cavities called the third ventricle.
  • The thin roof of this cavity is known as the epithalamus, the thick right and left sides are the thalami (sing. thalamus) and the floor being hypothalamus.
  • The pituitary gland is present just below the hypothalamus attached to it with an infundibulum.


  • It is significantly small and consists of two heavy fiber tracts, called cerebral peduncles or crura cerebri on the ventral side.
  • Two swellings called superior and inferior colliculi are present on each side of the dorsal surface.
  • The two colliculi are referred to as corpora bigemina and the both of the sides are called corpora quadrigemina.
  • The cerebral peduncles connect the forebrain with the hindbrain.
  • The mid-brain controls the reflex movements of the head, neck and trunk in response to the visual and the auditory stimuli.
  • It also controls the reflex movements of the eye muscles, change in pupil size and shape of the eye lens.


  • It is the posterior most part of the brain which consists of cerebellum, pons varolli and medulla oblongata.

1. Cerebellum:

  • It is the second largest part of the brain after cerebrum.
  • It lies below the posterior portions of the cerebral hemispheres, above the medulla and behind the pons.
  • It consists of two large lateral cerebellar hemispheres and a small median portion called vermis.
  • The cerebellum maintains our body equilibrium (body balance) and also controls our body posture.
  • It makes our body movements smooth, steady and co-ordinated.
  • It regulates and co-ordinates the contraction of voluntary muscles, i.e. muscular tone

2. Pons varolli:

  • It is an oval mass lying between the midbrain and the medulla oblongata and forms a brainstem on the floor of the hindbrain.
  • It consists of nerve fibers which interconnect the two cerebellar hemispheres.
  • It also joins the medulla with the higher brain centers in the cerebrum thus serving as a relay station or bridge carrying signals from the cerebrum to the cerebellum.

3. Medulla oblongata:

  • It is the lowermost part of the hindbrain which is conical and encloses a cavity, fourth ventricle.
  • This ventricle has a very thin, non-nervous, folded roof called the posterior choroid plexus below which lie lateral and median apertures that permit the exit of CSF into the spaces around the brain.
  • It lies just above the spinal cord and contains vital reflex centers like the cardiac center, centers for coughing, sneezing, vomiting etc.
  • The medulla oblongata controls breathing, blood pressure, heart beat, contraction and relaxation of blood vessels etc.
  • It controls the activities of the digestive tract, peristalsis, secretion of saliva, hormones and enzymes.
  • Any damage to the medulla oblongata causes instant death.


  • The medulla, pons and the midbrain collectively form the brainstem.
  • It is the stalk of the brain and relays information between the spinal cord and the cerebrum.
  • It narrows slightly as it leaves the skull, passing through the foramen magnum to merge with the spinal cord.

Structure and functions of the human brain