The nervous  system  integrates  and   controls  the  activities of all  other  systems  of  the body,  hence  it  is  also  termed as ‘master system’ of the body. The functions of nervous system include:

1. Reception of stimuli from within and outside the
2. Integration of sensory
3. Initiation and execution of motor
4. Assimilation of experience, an essential requisite to memory, learning and intelligence.

Due to these functions, an individual is able to maintain the internal environment of the body and react to the changes in the external environment.

All activities of nervous system whether simple or com­ plex involve three neural factors or elements: (a) reception, (b) integration and (c) response.

In human beings, the integrative element shows maxi­ mum degree of development and they are the most intellec­tual animals in the animal kingdom.

ANATOMICAL SUBDIVISION (Fig. 12.1)

1. Central nervous system (CNS): It includes brain and spinal
2. Peripheral nervous system (PNS): It includes all neural structures outside the CNS,
   • Peripheral nerves
     • 12 pairs of cranial nerves
     • 31 pairs of spinal nerves
   • Ganglia (autonomic and sensory)
   • Autonomic nervous system (sympathetic and para­ sympathetic nervous systems).

FUNCTIONAL SUBDIVISION (Fig. 12.2)

1. Afferent (sensory) division: It brings information to the CNS.
2. Efferent (motor) division: It gives appropriate motor command to muscles and glands. The efferent division is further divided into components:
   • Somatic nervous system: It innervates somatic structures like skeletal muscles and responsible for voluntary motor
   • Autonomic nervous system: It innervates visceral structures like cardiac muscle, smooth muscle and glands. The activities of viscera are mostly involun­tary, i.e. not under voluntary control.

The structural organization of the nervous system is very simple as it consists of only two principal types of cells:

(a) neurons and (b) neuroglia. The neurons are the structural and  functional  units  of the nervous system whereas neuroglia provides structural and functional support to the neurons.

Structure of a Neuron (Fig. 12.3)

The neurons are the structural and functional units of the nervous system. The number of neurons in the nervous system is estimated to be in the range of 100 billion. The two main properties of neurons are excitability and conductivity. The neurons are specialized for reception, integration, transforma­ tion, and transmission of impulses. The neurons are highly differentiated cells and have lost their power of division.

A neuron consists of a cell body (perikaryon or soma) and has two processes.

Cell Body

The cell body consists of a mass of cytoplasm surrounded by a plasma membrane. It contains a large vesicular nucleus with prominent nucleolus.

The two main characteristic features of the cytoplasm of the neuron are:

1. Presence of a basophilic Nissl substance (Nissl bodies)
2. Neurofibrils

The Nissl substance is composed of the large aggregations of rough endoplasmic reticulum, which contain ribonucleic acid concerned with protein synthesis, viz. neurotransmit­ters. The neurofibrils are filamentous strands of proteins.

N.B.

There are no centrioles and centrosome in the nerve cell body, which indicates that the highly specialized nerve cell has lost its power of division. Consequently, once the nerve cell is destroyed it is replaced by neuroglia.

Process

The processes are of two types:

1. Numerous short processes called dendrites (dendron = tree).

2. A single long process called axon

The dendrites bring impulses towards the cell body, hence they often branch profusely to increase the reception area of the neuron.

The axon is a single long process. It does not branch except at its termination to form telo-dendria which possess terminal boutons (presynaptic knobs).