Central Nervous System

Central Nervous System (CNS) is regarded as the "information processor" in the body. It consists of the brain lying under the skull, and the spinal cord contained within the vertebral column.

1. Brain

The brain is a very delicate organ. It is well protected within the bony cranium (brain box). It is further protected by three meninges (membranous coverings) which continue backward over the spinal cord. These meninges are:

  1. Dura mater, the outermost tough fibrous membrane
  2. Arachnoid, the thin delicate middle layer giving a web-like cushion
  3. Pia mater, the innermost highly vascular membrane, richly supplied with blood

The space between the covering membranes is filled with a watery fluid known as cerebrospinal fluid which acts like a cushion to protect the brain from shocks.

The brain consists of three important parts - Cerebrum, Cerebellum and Medulla.

A. Cerebrum

It is the largest portion of the brain, vertically divided into two halves: right and left cerebral hemispheres. The outer portion or the cortex of the cerebrum contains cell bodies of the neurons which is the basic unit of nervous tissue. Being grayish in colour, it is called the gray matter. The inner portion of the cerebrum consists of white matter which mainly contains the axons or nerve fibres of the neurons.

The cerebrum is the seat of intelligence, consciousness and will-power. It controls all voluntary actions. Cerebrum helps us to make well thought out and informed decisions, for example, decisions related to the career choices.

B. Cerebellum

The cerebellum is a much smaller area of the brain located below the cerebrum. It has no convolutions, but has numerous furrows. This also has an outer cortex made-up of gray matter and an inner white matter. The main function of the cerebellum is to maintain the balance of the body and coordinate muscular activity.

The cerebrum and cerebellum work in close coordination. For example, if you stand up and walk, the impulse for this activity arises in the cerebrum. The act of walking involves coordinated working of many muscles. Proper coordination and timing of contraction and relaxation of muscles is the responsibility of the cerebellum.

C. Medulla oblongata

The medulla oblongata is the lowest portion of the brain located at the base of the skull. It is roughly triangular and is continued behind as the spinal cord. Its function is to control the activities of our visceral organs like the alimentary canal, movement, breathing, beating of heart and many other involuntary actions. Injury to the medulla generally results in death as the involuntary and vital functions like breathing and heart beat may be stopped.

2. Spinal Cord

The spinal cord is an integral part of the central nervous system. It extends from the medulla oblongata and continues downward almost throughout the length of the backbone, and lies within the neural canal of the vertebral column or the backbone.

In the spinal cord, the arrangement of the gray and white matter is reversed from that in the brain. The gray matter containing the cell bodies of motor neurons lie on the inner side, while the white matter on the outer side. The white matter contains axons running longitudinally to and from the brain and even crossing from one side to the other. There is a small central canal in the centre which runs through the entire length and continues with the cavities of the brain. It is filled with cerebrospinal fluid which acts as a shock proof cushion and forms a medium for the exchange of food materials, waste products, and respiratory gases with neurons.

Externally, the spinal cord is covered by the same three membranes - dura mater, arachnoid and pia mater in continuation with those of the brain.

Functions of Spinal Cord

The spinal cord is concerned with the following three functions:

  1. It controls the reflexes below the neck.

  2. It conducts sensory impulses from the skin and muscles to the brain.

  3. It conducts motor responses from the brain to muscles of the trunk and limbs.