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After reading this article you will learn about how nervous system influences behaviour of an individual.
The nervous system is the part of the body most intimately involved in behaviour. All the activities of the living organism involve the nervous system. The more complex the behaviour, the greater is the involvement of the nervous system. As we go higher along the evolutionary ladder from simple organisms to the highly developed, the nervous system assumes increasing importance.
Correspondingly, it is also found that the structure and organisation of the nervous system becomes increasingly complex. In fact, in the lower animals the proportion of the weight of the nervous system, particularly the brain, to the total weight of the body is much less than that in man. On the evolutionary scale man has the highest proportion of brain weight of all animals. This fact alone indicates the pivotal role of the nervous system in complex behaviour.
The nervous system can be broadly divided into the central nervous system and the peripheral nervous system. The former is made up of the following parts – the brain, the spinal cord and all the nerves emerging from these and running to the different parts of the body.
The brain can be further divided into the cerebrum, the cerebellum and the medulla oblongata. Thus, the cerebrum, cerebellum, the medulla oblongata and the spinal cord constitute the major part of the central nervous system (CNS).
The peripheral nervous system (PNS) consists of large number of nerves and nerve ganglia spread over different parts of the body and connecting them. To some extent these two systems function independently. In many kinds of human activities the PNS directly goes into operation independent of the CNS. But most activities are largely under the control of the CNS, particularly the brain.
The nervous system is essentially made up of a large number of nerve cells. The nerve cell is the structural unit of the nervous system. It is known as the neuron. A typical neuron consists of a main region or the cell body in the middle, a few short projections known as dendrites on one side and a long wiry projection called the axon.
In a typical nerve fibre these neurons are arranged in the form of a chain with the axon of one neuron linking up with the dendrites of the next and so on. A number of thin fibres fused together constitute a nerve. Neurons are of three types; sensory, motor and associative neurons.
Sensory neurons are found in the nerves which carry messages from the peripheral organs of the body to the central nervous system while motor neurons are found in the nerves carrying messages from the various centres of the nervous system to the different peripheral parts.
Associative neurons are basically the neurons which connect the sensory and motor nerves. They are predominantly found in the various centres of the central nervous system and are not found in the nerves. This differentiation also indicates the different functions of these neurons.
Sensory neurons carry the messages or the nerve impulses from the receptor organs to the centres, while the motor neurons carry the impulses from the centres to the various organs of the body, the effector organs.
The associative neurons are involved in coordinating and integrating the functions of the sensory neurons and the motor neurons. This is why they are found to be predominant in the nerve centres rather than in the nerves found in the different regions of the body. 
Autonomic Nervous System:
The nervous system is normally divided into two parts, the central nervous system and the peripheral or autonomic nervous system. The latter is so called because, to a large extent, it functions not under the control of the brain but under the control of the medulla, though it is under certain circumstances subject to the control of the brain. This system is made up of a number of loosely distributed nerve fibres and ganglia connecting particularly the different glands.
The autonomic nervous system is further divided into two major divisions, the parasympathetic and the sympathetic systems. The former is mainly concerned with vegetative functions, mainly energy producing and storing functions, while the latter is concerned mainly with action and expenditure of energy.
While the parasympathetic nervous system is concerned with the accumulation and conservation of energy, the sympathetic nervous system is concerned with the energizing of behaviour and the releasing of energy for emergency action. Thus, when an organism is in a state of emergency the sympathetic system is activated resulting in the release of energy for greater and speedier action.
It may, therefore, be seen that emotional behaviour is very closely associated with the arousal and activation of the sympathetic system and the activation of various glands, particularly the endocrine glands. The activation of the endocrine glands, in turn, results in the release of activities calculated to meet emergency situations.
For example, when an individual is suddenly faced with a fear-producing object or stimulus, sympathetic nervous activity results. This is manifested by an increasing rate of breathing, blood pressure and other manifestations calculated to enable the organism to meet the situation.
When this situation comes to an end sympathetic activity gives place to parasympathetic activity which serves to replenish the lost energy and the restoration of equilibrium. Thus, the two systems function in a complementary manner though they appear to be contradictory.