- 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)
Forebrain:
- It forms the greater part of the brain which consists of three regions: olfactory lobes, cerebrum and the diencephalon.
- 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.
- 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.
- 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.
Midbrain:
- 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.
Hindbrain:
- 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.
Brainstem:
- 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.