Living organisms have the ability to react to changes in the environment. This ability to respond to a stimulus is termed irritability. These stimuli are received and perceived through the nervous system, which also controls their various movements and reactions. Most invertebrates have a simple nervous system with a rudimentary brain as its control centre. Vertebrates, on the other hand, have central nervous systems and highly developed brains. The nervous system is also a means of quick co-ordination between different parts of the body.

Many actions performed by humans are automatic or involuntary ones, such as breathing, heartbeat or blinking the eyes. Humans are of course, also able to perform thousands of voluntary actions, or actions under the control of their will. Let us now enter the human nervous system for a closer look at each of its separate organs.

Nerves

- Nerve cells                  

Our nerve cells make up nervous tissues known as neurones. Each neurone consists of a cell body containing a nucleus and nerve fibres. There is usually one long fibre that transmits impulses away from the body known as the axon. Nerve fibres that conduct impulses towards the cell body are called dendrons. The terminal branches are called dendrites. Many more fibres are enclosed by a layer of fatty substances known as myelin sheath, which serves as an insulating layer. A tin membrane, the neurilemma that nourishes the fibre, also surrounds the myelin sheath. The myelin sheath is separated at intervals by nodes of Ranvier, which speeds up transmission of impulses along the fibre.

The nervous tissue of the central nervous system consists of the grey matter and the white matter. The grey matter consists of mainly of all bodies of the neurones and make up the outer surface of the brain. The white matter consists of nerve fibres and for the control parts of the brain and outer layers of the spinal cord.

Impulses are transmitted from the axon of one neurone to the dendron of another neuron across a minute space called the synapse. Through chemical means, there the end branch of an axon comes into contact with the muscle fibre, a motor end plate is formed. The transmission of impulses across the motor end plate is similar to that across a synapse

Brain

• Fore brain – Cerebrum, hypothalamus and the pituitary gland

The two cerebral hemispheres of the cerebrum of the brain are the largest parts of the mammalian brain. They are concerned with intelligence, memory, learning and overall control of all involuntary actions. In man they are also concerned with emotions. Intelligence and the ability to learn depend on the surface area and the degree of development of these hemispheres. The larger they are, the larger their surface and the more nerve cells they can hold, therefore the more intelligent the animal will be. In the higher vertebrates, like man, the cerebral hemisphere are very well developed and greatly enlarged. The surface are of the man is further increased by the presence of many deep fissures or grooves in the cerebral hemispheres.

The brain is not a completely solid structure. The brain tissue is arranged round a series of fluid-filled cavities or ventricles. Each cerebral hemisphere has its own ventricle. These two ventricles communicate with the third ventricle that has a membranous roof. The lateral walls of the third ventricle form the thalamus while the floor forms the hypothalamus. The hypothalamus is an important region concerned with the regulation of body temperature and blood osmotic pressure, appetite sleep, and emotions. In the first case blood flowing through the brain that activates the hypothalamus. The hypothalamus causes the medulla oblongata to control the activity of the sweat gland and the blood pressures. The pituitary gland is attached to the hypothalamus. The function of the pituitary gland is discussed in the chapter on endocrine system.

• The mid-brain

The mid-brain consists of the optic lobes which are represented in mammals by four small bodies called corpora quadrigemina. These are concerned with visual reflexes, e.g. movements of the eyeball.

• The hind-brain

The cerebellum lies dorsally behind the optic lobes. It is large, and its surface is thrown into many folds. The cerebellum plays an important part in controlling muscular co-ordination, and especially in maintaining body balance.

The medulla oblongata lies below the cerebellum. Its posterior or lower end narrows down gradually into the spinal cord. The medulla oblongata controls involuntary actions such as the heartbeat, peristalsis, the rate of respiratory movements and the contraction and dilation of blood vessels.

A short canal leads from the third ventricle to the fourth ventricle in the medulla. This communicates posteriorly with the central canal of the spinal cord. All the ventricles are filled with cerebro-spinal fluid, a colourless, cell-free fluid. It is derived from blood by diffusion through the thin roofs of the third and fourth ventricles, each of which is richly supplied with blood vessels. The cerebro-spinal fluid brings nutrients to the brain.

Spinal Cord and Spinal Nerves

The spinal cord is a cylindrical mass of nervous tissue extending posteriorly from the medulla oblongata to the end of the vertebral column. It passes through the vertebral canal. It is therefore protected by the vertebral column. Spinal nerves are attached at intervals  along the length of the spinal cord. Each spinal nerve emerges through an opening between two successive vertebrae. There are 31 pairs of spinal nerves in man.

Like the brain, the spinal cord consists of both grey and white matter, but unlike the brain, it has grey matter inside surrounded by white matter on the outside. The grey matter has the shape of "H". A narrow central canal containing cerebrospinal fluid runs through the middle of the spinal cord. This fluid brings nutrients brings nutrients to the spinal cord.

Each spinal nerve divides into two roots just before it joins the spinal cord. The dorsal root joins the dorsal part of the spinal cord and it contains only sensory neurones. The cell bodies of these neurones aggregate in the small swelling known as the dorsal root ganglion. Their axons end in a grey matter of the spinal cord while their dendrons become the sensory fibres in the dorsal root and spinal nerve.

The ventral root which is attached to the ventral part of the spinal cord contains only motor neurones. The cell bodies of the motor neurones lie in the grey matter of the spinal cord while their axons leave the spinal cord to enter the ventral root and spinal nerve. The spinal nerve contains nerve fibres from the dorsal and ventral roots. Since it contains nerve fibres from both the sensory and effector neurones, it is a mixed nerve. The spinal nerve, as it leaves the spinal cord, progressively subdivides into branches supplying nerve fibres to the various parts of the body. The sensory fibres and the motor fibres soon separate, the former going to the receptors while the latter run to the effectors.

Intermediate or relaying neurones lie in the grey matter of the spinal cord. They form synapses with the sensory and the motor neurones. In this way they enable impulses to be transmitted from the sensory neurones to the motor neurones.