Structure of Human Brain

The below mentioned article provides an overview on the Human Brain.

Location and Protective Coverings of the Brain:

The brain is the anterior most part of the central neural system which is lodged in the cranial cavity (cranium) of the skull. The human brain weighs from 1220 to 1400 grams.

The human neural system has about 100 billion neurons, majority of them occur in the brain. The brain is covered by three membranes or meninges (sing, meninx). The innermost membrane, the piamater is thin, very delicate and vascular and invests the brain closely.

The next is arachnoid membrane (also called arachnoid mater), which is a thin “spi­der webby” structure from which it gets its name. The outermost membrane, the duramater is the tough fibrous mem­brane adhering closely to the inside of the skull.

Between the arachnoid mem­brane and piamater is a space known as sub-arachnoid space. The space which is present between the arachnoid and duramater is called subdural space. The sub-arachnoid space is filled with cerebro­spinal fluid.

This fluid serves as a pad to cushion the central nervous system from shocks. It also provides a medium for exchange of food materials, wastes, respiratory gases and other materials. The subdural space contains a little fluid which is not cerebrospinal fluid. The membranous areas between the cranial bones of the foetal skull are called fontanels.

Structure and Functions of Human Brain:

The human brain is divisible into three parts:

1. Fore brain or Pros encephalon includes olfactory lobes, cerebrum and dien­cephalon.

2. Mid brain or Mesencephalon comprises corpora quadrigemina and crura cerebri.

3. Hind brain or Rhomb encephalon consists of cerebellum, pons varolii and medulla oblongata.

Fore Brain:

(i) Olfactory lobes:

The anterior part of the brain is formed by a pair of short club- shaped structures, the olfactory lobes. Each lobe consists of two parts, an anterior olfactory bulb and a posterior olfactory tract. They are fully covered by the cerebral hemispheres and are, therefore, only visible in the ventral view of the brain. A pair of olfactory nerves arises from the olfactory lobes.

Function:

Olfactory lobes are concerned with the sense of smell.

(ii) Cerebrum:

The cerebrum is the largest and most complex of all the parts of the human brain. It consists of left and right hemispheres connected by a large bundle of myelinated fibres, the corpus callosum and other smaller fibre bundles.

Anteriorly the corpus callosum is folded back to form the genu. Posteriorly the corpus callosum curves ventrally to form rounded splenium which joins a fibrous strip called fornix. The fornix is a paired structure, one of which is present in each hemisphere. Left cerebral hemisphere is smaller than the right.

Cerebral Cortex:

The outer portion of cerebrum is called the cerebral cortex that makes up the grey matter of the cerebrum. The surface of the cortex is greatly folded. The upward folds, or gyri (sing, gyrus), alternate with the downward grooves, or sulci (sing, sulcus).

Beneath the grey matter there are present millions of medullated nerve fibres, connecting the neurons of the cerebral cortex with those located elsewhere in the brain. The large concentration of medullated nerve fibres gives this tissue an opaque white appearance. Hence they are collectively called White matter.

Lobes:

A very deep fissure, the longitudinal fissure, separates the two cerebral hemi­spheres. Each cerebral hemisphere of the cerebrum is divided into four lobes: frontal, parietal, temporal and occipital lobes. The central sulcus separates the frontal lobe from the parietal lobe. The lateral sulcus separates the frontal lobe from the temporal lobe. The parieto-occipital sulcus separates the parietal lobe from the occipital lobe.

Functional Areas of the Cerebrum:

In each cerebral hemisphere, there are present three types of functional areas:

(a) Sensory areas:

They receive impulses from the receptors.

(b) Association areas:

They interpret the input, store the input and initiate a response in light of similar past experience. Thus the associated areas are involved in memory, learning and reasoning,

(c) Motor areas:

They transmit impulses to the effectors.

The pre-central (motor) area lies in the frontal lobe immediately anterior to the central sulcus. The nerve cells are called pyramidal cells which initiate the contraction of voluntary muscles. The post central (sensory) area (= somaesthetic area) lies in the parietal lobe immediately posterior to the central sulcus. It perceives sensations of pain, temperature, pressure and touch.

The sensory speech area is situated in the lower part of the parietal lobe and extends into the temporal lobe. It perceives the spoken word. The auditory (hearing) area lies immediately below the lateral sulcus in the temporal lobe. It is the centre for hearing. Wernicke’s area is usually located in the left temporal lobe that plays a role in understanding speech and writing words.

The visual area lies in the greater part of occipital lobe. It is the centre for sight. The olfactory (smell) area lies deep within the temporal lobe. It receives the impulses from the nose via olfactory nerve and interprets them.

The taste area lies in the parietal lobe above the lateral sulcus in the post central (sensory) area. The nerve impulses from the tongue are interpreted here. The motor speech area (also called Broca’s motor speech area) lies in the frontal lobe.

Other functional areas of the cerebrum include the visual association area in the occipital area, parietal association area in the parietal lobe, the frontal association area in the frontal lobe and temporal association area in the temporal lobe. (An association area is a portion of the cerebral cortex that neither receives direct sensory stimuli nor directly initiates motor impulses; instead, it appears to process and interpret sensory impulses).

Basal Ganglia (= Basal Nulcei):

Basal ganglia are the scattered masses of grey matter, submerged in the subcortical substance of cerebral hemispheres. The corpus striatum, the largest nucleus in the basal ganglia is a mass of grey matter situated at the base of the cerebral hemispheres in close relation to the thalamus. The lenticular nucleus is also a part of basal ganglia.

The main functions of the basal ganglia are:

(i) Control of the movements during voluntary motor activity,

(ii) Control of reflex muscular activity,

(iii) Control of muscle tone,

(iv) Control of automatic associated movements and

(v) Role in arousal mechanism. Parkinson’s disease occurs due to damage of the basal ganglia. Wilson’s disease is due to damage of the lenticular nucleus.

Functions:

Each lobe of cerebral hemisphere performs specific functions:

(a) In the frontal lobe creative ideas occur,

(b) In the temporal lobe sounds are interpreted so that one can understand what is being spoken,

(c) In the parietal lobe feelings about touch, hot and cold and pain are registered. It is this area that allows to accurately follow directions on map, reading a clock or dressing a person,

(d) The occipital lobe is where eyes see, and interpret what is seen.

A summary of major functions of cerebral lobes has been given below.

(iii) Diencephalon:

Its main parts are epithalamus, thalamus and hypothalamus. Epithalamus is thin and not formed of nervous tissue. Its anterior part is vascular and folded to form the anterior choroid plexus.

Just behind the anterior choroid plexus, the epithelium forms a short stalk, the pineal stalk which has a rounded body, the pineal body, at its tip. The pineal body is an endocrine gland and, therefore, secretes a hormone, named melatonin. The thalamus, which lies superior to the mid brain is composed prima­rily of grey matter.

The optic nerves which come from the eyes, form a crossing, the optic chiasma in front of the hypo-thalamus. The hypophysis (pituitary gland) is directly attached to the hypothalamus by a stalk, the infundibulum.

The pituitary gland is an endocrine gland and secretes certain hormones. Behind the infundibulum, a pair of small rounded eminences, the mammillary bodies are present. They are like nipple and hence their name.

Functions of Hypothalamus:

Although hypothalamus is relatively small (4 grams, about 1/300 of the total brain mass) yet it is highly vascular. It integrates and controls the visceral activities. It maintains homeostasis. It provides anatomical connection between the nervous and endocrine systems by its relationship to the pituitary gland. 

Through connections with pituitary gland, it controls growth and sexual behaviour. Hypothalamus is thermoregulatory centre. Hence it is called “thermostat” of the body.

It keeps body temperature at roughly 37°C by means of a complex thermostat system. It is also associated with behavioural activities. Appetite, thirst and satiety (feeling of being satisfied) centres are located in the hypothalamus. It also influences respiration and heart beat.

Mid Brain:

(i) Corpora quadrigemina:

The upper or superior surface of the mid brain has two pairs of rounded protrusions collectively called the corpora quadrigemina; one pair is called superior colliculi and the other pair is called inferior colliculi. The superior and inferior colliculi of each side are termed the corpora bigemina.

Functions:

The superior colliculi are concerned with the sense of sight. However, the inferior colliculi are concerned with hearing.

(ii) Cerebral peduncles (Crura cerebri):

These are two bundles of fibres which lie on the lower or inferior surface of the mid brain.

Function:

They relay impulses back and forth between the cerebrum, cerebellum, pons and medulla.

Hind Brain:

(i) Cerebellum:

The second largest part of the human brain is the cerebellum (means simply “little cerebrum”). It is well developed in human brain. It consists of two lateral cerebellar hemispheres and central worm ­shaped part, the vermis.

Like the cerebrum, the cerebellum has its grey matter on the outside, comprising three layers of cells and fibres. The middle layer contains character­istically large flask shaped Purkinje cells. The Purkinje’s cells rank among the most complex of all neurons. The cerebellum also has Golgi cells, basket cells and granule cells.

A cross section of the cerebellar hemispheres shows a branching tree like arrangement of grey and white matter called the arbor vitae (“tree of life”).

Functions:

The cerebellum controls rapid muscular activities, such as running, typing and even talking. All activities of the cerebellum are involuntary, but may involve learning in their early stages. Alcohol affects the cerebellum. Since, alcohol is a depressant, it inter­feres with the functions of the cerebellum.

(ii) Pons Varolii:

It is situated in front of the cerebellum below the mid brain and above the medulla oblongata. It consists mainly of nerve fibres which form a bridge (pons— bridge) between the two hemispheres of the cerebellum and of fibres which pass between the higher levels of the brain and the spinal cord. Pneumotaxic centre is present in pons varolii.

Functions:

Pons varolii relays impulses between the medulla oblongata and more supe­rior part of the brain, between the hemispheres of the cerebellum and between the cerebrum and cerebellum. The pneumotaxic centre limits inspiration.

(iii) Medulla oblongata:

It extends from the pons varolii above and is continuous with the spinal cord below. Its shape is like a pyramid. The medulla oblongata has a very thin, non-vascular folded structure on its lower side called the posterior choroid plexus.

Functions:

Medulla oblongata receives and integrates signals from spinal cord and sends resulting signals to the cerebellum and thalamus. It contains centres that regulate heart rate, blood pressure, breathing, swallowing, salivation, sneezing, vomiting and coughing and some other involuntary movements.

Brain Stem:

The mid brain, pons varolii and medulla oblongata are collectively called the brain stem, connecting the fore brain and spinal cord.

Limbic System:

Components of Limbic System:

Certain components of the cerebrum and diencephalon constitute the limbic system (limbus L. a border or edge or fringe of a part).

Its main components are the following:

(i) Hippocampus:

Its shape roughly resembles the sea horse. It is located inside the temporal lobe

(ii) Amygdala or Amygdaloid nucleus (L., Gr. amygdale – almond):

It is almond shaped and is located in the tip of the temporal lobe,

(iii) Septal nuclei:

These are located within the septal area formed by the regions under corpus callosum and the paraterminal gyrus (a cerebral gyrus),

(iv) Mamillary bodies:

These are present behind the infundibulum.

(v) Basal ganglia:

They are scattered masses of grey matter.

Functions of limbic system:

(a) It is sometimes called the “emotional brain” because it controls emotional behaviour expressed in the form of joy, sorrow, fear, fight, friendship, liking and disliking,

(b) It controls food habits necessary for survival of the individual,

(c) It also controls sex behaviour necessary for survival of the species.

Ventricles of the Brain:

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 foramen (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 brainstem to the fourth ventricle in the pons and medulla. The fourth ventricle is continuous 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. Ventrolateral wall of each paracoel appears striated and hence called corpus striatum.

Cerebrospinal Fluid (CSF):

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 and in the subarachnoid space around the brain and spinal cord.

The cerebrospinal fluid performs the following functions:

(i) Protection of the Brain and Spinal cord:

CSF protects the delicate brain and spinal cord by providing shock-absorbing medium. It acts as cushion jolts to the central nervous system.

(ii) Buoyancy to the Brain:

Since the brain is immersed in the CSF, the net weight of the brain is reduced from about 1.4 kg to about 0.18 kg. Thus the pressure at the base is reduced.

(iii) Excretion:

CSF carries harmful metabolic wastes, drugs and other substances from the brain to the blood.

(iv) Endocrine Medium for the Brain:

Certain hormones are released into CSF. These hormones are carried to different parts of the brain by CSF where they may act.

Mammalian Characters in Human Brain:

(i) Olfactory lobes are small and solid.

(ii) Cerebral hemispheres are quite large in size and divided into lobes.

(iii) Corpus callosum is also found.

(iv) Optic lobes are solid and further divided into corpora quadrigemina.

(v) Pons varolii is present.

(vi) Cerebellum is very much folded and solid.