5.2 HUMAN NERVOUS SYSTEM

The human nervous system comprises of central nervous system and peripheral nervous system. The central nervous system consists of brain which narrows into a long tube called the spinal cord. The peripheral nervous system consists of cranial nerves from the brain and spinal nerves from the spinal cord. The structural and functional unit of nervous system is neuron. An outline of division of human nervous system is given below.

5.2.1 Components of Central Nervous System

Here we will discuss the brain and spinal cord as the components of human nervous system.

Brain

It is enclosed within the cranium (skull). The brain and the spinal cord are covered with three protective membranes called meninges. Between the inner and middle layer there is a fluid, the cerebrospinal fluid, which helps to cushion the brain from shock. Meninges also provide nutrients and oxygen to brain tissue through capillaries. The brain contains hollow spaces called ventricles that are continuous with the central canal of spinal cord. These are filled with cerebrospinal fluid. Brain is divided in to three parts, forebrain, midbrain and hindbrain.

Forebrain

The forebrain consists of cerebrum, thalamus and hypothalamus. Cerebrum is the largest part of the human brain. Cerebrum is the main centre of various sensations e.g., hearing, sight smell, memory voluntary action, intelligence, and reasoning. Cerebrum is divided into two cerebral hemispheres. Each cerebral hemisphere is divided into four major lobes. 1. frontal lobe 2. parietal lobe 3. temporal lobe 4. occipital lobe.

Occipital lobe is responsible for vision. Temporal lobe is concerned with hearing and smelling. Frontal lobe controls movement of skeletal muscles including movement involved in speech. Parietal lobe receives stimuli from skin and provides awareness of body position. The right and left hemispheres control the opposite sides of the body. The two hemispheres are connected by a bundle of nerve fibres called corpus callosum.

Thalamus is below the cerebrum. Thalamus receives nearly all impulses arriving from sensory areas of the body, before passing them to cerebrum. Thalamus is also involved in pain reception and consciousness.

Hypothalamus lies below thalamus. It regulates body temperature, appetite, water balance blood pressure, feeling of mood etc. It links nervous system and endocrine system as it