The brain is divided into three main sections –
(I) Fore brain,
(II) Mid brain,
(III) Hind brain
(I) Fore brain consists of olfactory lobes, the cerebrum and the diencephalon.
Olfactory lobes are paired short club shaped structure present at the anterior part of the brain. Each lobe consists of two parts – an anterior bulb and a posterior olfactory tract. They are fully covered by the cerebral hemispheres and are therefore only visible in the ventral view of brain.
Cavity in olfactory lobe is called rhinocoel. A pair of olfactory nerves arises from the olfactory lobes. They are not well developed in human brains.
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 (Sylvian fissure). The two hemispheres are connected by a bundle of transverse fibres called corpus callosum. 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.
The surface of each cerebral hemisphere shows many convolutions called gyri (singular Gyrus) separated by depressions, called sulci. The gyri increase the surface area of the cortex for accommodating far more nerve cells in it.
The cerebrum consists of two surfaces – cerebral cortex and cerebral medulla. Cerebral cortex is the outer layer of the cerebrum. It is made of grey matter and contains many layers of nerve cells.
The nerve cells of different areas of the cerebral cortex differ in size, shape and functions, for example conical-shaped, pyramidal cells of the motor area of the cortex give rise to different fibres for controlling the skeletal muscle movements. The inner surface is called cerebral medulla which is made up of bundles of myelinated axons (the white matter).
It contains the epithalamus, thalamus and hypothalamus.
Epithalamus is non – nervous part which is fused with piamater to form anterior choroid plexus. Just behind this the epithelium forms a short stalk called pineal stalk which has rounded body called pineal body. Pineal body is endocrine in function and secretes a hormone named melatonin.
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 – Just beneath the thalamus, hypothalamus forms the floor arid the part of the lateral walls of the third ventricle. It is partially protected by the sella turcica of the sphenoid bone.
Its lower middle portion consists of the stalk of pituitary gland and the posterior portion mainly consists of a pair of mammilla bodies, a part of limbic system, containing nuclei. The nuclei of these bodies serve as relay centres for those olfactory fibres that are concerned with olfactory reflexes.
It contains reflex centres linked to the autonomic system. They include –
i. Thermoregulation centre for temperature regulation.
ii. Appetite and satiety centre to control the desire for food.
iii. It is stimulated when the stomach is full and inhibits further feeding.
iv. Controls sleeping patterns and wakefulness.
Hypothalamus also contains nerve centre for emotional reactions – sweating, fatigue etc. Hypothalamus secretes neurohormones which control the secretions of anterior pituitary hormones. It synthesizes the posterior pituitary hormones and control their release into the blood.
The axis of hypothalamus and pituitary gland are important because it is the main link between the nervous system and the endocrine system and its hormones.
(II) Mid Brain is very small constricted portion of brain which is covered by the cerebrum.
It is represented by a pair of longitudinal bands of nervous tissue, the cerebral peduncles or crura cerebri on ventral side, dorsally a pair of small swellings called corpora bigemina on either side.
The four lobes (optic lobe) are collectively called corpora quadrigemina in which superior colliculi are related to optical activity, while the two smaller posterior colliculi to the auditory. These two are collectively known as tectum.
(III) Hind Brain consists of cerebellum, medulla oblongata and pons.
Cerebellum is the second largest part of brain and is located at the back of the skull. It consists of a pair of cerebellar hemispheres and a small median vermis. The surface of cerebellum is also highly grooved. The grey matter of cerebellum is called cerebellar cortex, while the white matter inside formed a branched tree like structure called arbor vitae.
i. It helps control body posture.
ii. Maintenance of muscle tone
iii. Coordinate voluntary muscular activities
iv. Equilibrium of body.
Medulla oblongata is most vital part of the brain. It is a pyramid shaped bulb like structure, follows the cerebellum and is continued into the spinal cord below.
The anterior region has intermixed grey and white matters while the remaining medulla has white matter on the exterior and grey matter internally.
Its roof is thin and non-nervous, and constitutes posterior choroid plexus. Below the plexus, the roof has three openings, the two lateral apertures called foramina of Luschka, and the median foramen of Magendie.
Medulla oblongata has many control centres of some vital physiological processes –
i. Respiratory centre
ii. Cardiac centre for regulating heart beat and the force of contraction
iii. Reflex centres for swallowing, vomiting peristalsis, secretion and activity of alimentary canal, salivation, coughing and sneezing etc.
iv. Vasomotor centre for regulating diameter of blood vessels.
Pons is located anteriorly to the medulla oblongata and superiorly, just inferior and posteriorly to the frontal lobes of the cerebrum.
It consists mainly of nerve fibres which form bridges between the two hemispheres of cerebellum and of fibres which pass between the higher levels of the brain and the spinal cord.
These connections are provided by the middle cerebellar peduncle. Also contained or originating from the pons varolii are the 5th, 6th, 7th, sections of 8th cranial nerves, the pneumotaxic and apneustic area. The last two areas aid in the control of respiration.
The medulla oblongata, pons varolii, middle brain and diencephalon are collectively called brain stem which maintains life support systems. It connects the brain to the spinal cord. The brain stem not only coordinates and integrates all incoming information; it also serves as the place of entry or exit for ten of the twelve cranial nerve.
The ventricles consist of four hollow, fluid filled spaces inside the brain. A lateral ventricle lies inside each hemisphere of the cerebrum. Right cerebral hemisphere contains Ist ventricle whereas left cerebral hemisphere contain IInd ventricle. Both are collectively known as lateral ventricles or paracoel.
Each lateral ventricle is connected to the third ventricle by an interventricular foramen (foramen of Monro). The third ventricle (diocoel) consists of a narrow channel between the hemispheres through the area of the thalamus.
Lamina terminalis is a delicate membrane which forms boundary of third ventricles. It is connected by a narrow canal called cerebral aqueduct or aqueduct of Sylvius in the midbrain portion of the brain stem to the fourth ventricle in the medulla.
The fourth ventricle (metacoel) 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.
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 up to first lumbar vertebra where it tapers to a point called conus medullaris. A fine filament of connective tissue called filum terminates starts from the conus, and runs up to coccygeal region. The spinal cord posesses an anterior and a posterior median fissure running along its length.
It has an H-shaped central area of grey matter surrounded by an outer layer of white matter. Grey matter is composed of nerve cells, bodies, dendrites and synapses, (formed by interneurons which are meant for changing, integrating and routing signals), which surrounds a central canal (= neurocoel, lined with ciliated ependymal epithelium) containing cerebrospinal fluid.
Its wing like lateral parts is extended into relatively longer and narrower dorsolateral and shorter and thicker ventrolateral horns with funiculi in between. In thoracic and lumbar regions, the wings form lateral horn also. Fibres of dorsal lateral and ventrolateral horns continue outside the cord as fibres of dorsal and ventral roots of spinal nerves. White matter contains nerve fibre whose fatty myelin sheaths give its characteristic colour. It is divided into six funiculi, three on each side – lateral funiculus, anterior/ventral funiculus and posterior/ dorsal funiculus.
Spinal cord conducts impulses to and from the brain and controls most of the reflex activities and provides a means of communication between spinal nerves and the brains.