Science

16.1  The nervous system : The nervous system is the mechanism in which sensation of all kinds are received from the environment and passed to the tissues and organs of the body itself. It deals with the rapid conduction of messages in the form of electrical impulses from one part of the body to another. The functional unit of the nervous system is a neuron or nerve cell.  It is divided into two main subsystems

. They are –

1.       Central nervous system

2.       Peripheral nervous system

16.1.1 The central nervous system ( CNS) – this includes brain and spinal cord, both of them are  centrally located and well protected by bony structures . they are coordinating centres of the nervous system.

A. The brain – the human brain is highly developed and delicate organ which is enclosed in a bony case, the cranium or brain box and lies within the cranial cavity of skull. Three membranes collectively known as meninges cover it. The outer membranes is called duramater, middle arachnoid and inner piamater. The spaces between inner two membranes are filled with cerebero spinal fluid, which protects against external shock and mechanical injuries. The brain is divide into three parts –

I. forebrain ( consisting of cerebrum and diencephalon ) – the cerebrum is the lagest part of the fore brain and constitutes about two thirds of it. The mental capacity of a person appears to be related to the number of convolutions and the number of neurons in the cerebral cortex. Within the cerebral hemispheres are cavities called ventricles containing a nutritive fluid , the cerebrospinal fluid. The cerebrum is the seat of consciousness, intelligence , memory, imagination, and reasoning. Hypothalamus is also known as pituitary body.

II. Mid brain – it is the small middle part of the brain which lies between fore brain and hind brain. It is the centre for visual and auditory reflexes.

III. Hind brain ( consisting of cerebellum , pons varolli and medulla oblongata )-  it consists of cerebellum, pons varolli and medulla oblongata. The cerebellum regulates and coordinates the group movements of muscles as in actions like walking or running. It also maintains equilibrium and controls body posture. The pons varolli is bridge of nerve fibres that connect the lobes of the cerebellum. The medulla oblongata is the posterior part of the brain and is also called brain stem where it merges with spinal cord.

C. the spical cord – it is a long whitish thick walled tubular structure and a downward continuation of the brain. It is covered by the same meninges as the brain and it is situated in the neural canal within two enlargements. The spinal cord conducts impulses to and from the brain , and acts as a reflex centre.

16.1.2  the peripheral nervous system ( PNS ) – This may be divided into sections –

A. cranial and spinal nerves and

B. the autonomic nervous system

Neuron- the functional unit of the nervous system is a neuron. The electrical impulses are transmitted to and fro the higher centres through neurons. A collection of neurons or nerve fibres is called a nerve . dendrites , cell body and axon are the basic components of a neuron.

Those neurons that carry an impulse from  a sense organ to the central nervous systmeare called sensory neurons  or afferent neurons and those that carry an impulse from the central nervous  system to a muscle or a gland are called motor neurons or  efferent neurons. A third category of nerves called mixed nerves consists of sensory and motor neurons.

The cranial nerves are those that emerge from the brain and the spinal nerves are those that emerge from the spinal cord. In human beings, there are 12 pairs if cranial nerves and 31 pairs of spinal nerves.

16.2. Reflex Action – the simplest form of irritability associated with the nervous system is the reflex action. This is a rapid , automatic stereotyped response to a stimulus because it is not under the conscious control of the brain. Hence it is described as an involuntary action. A reflex action is defined as and involuntary action in response to and external  or an internal stimulus. This can be illustrated with the following examples. If you accidentally pick up a very hot object, you drop it immediately. If you see or smell one of your favourite dishes, your salivary glands cause your mouth to water. If someone suddenly moves his hand past your face , you blink your eyes. No deliberate effort on your part is involved in any of these actions. These are some examples of reflex actions.