7. THE NERVOUS SYSTEM

One of the most important characteristics of living substances is their capacity to respond to stimuli. Every living organism receives stimuli from its environment and responds to such stimuli by corresponding reactions which link the organism to the environment. Metabolic processes within the organism itself create a number of stimuli to which the organism must also react. In higher multicellular organisms the area receiving the stimulus and the reacting organ are connected by the nervous system.

The activity of the nervous system is based on the reflex. The basic anatomical element of the nervous system is the nerve cell, which together with all the processes, arising from it, is called the neuron. A long axial cylindrical process, called the axon, arises from the body of the cell in one direction. Short branched processes called den-drites lead in the other direction.

Nervous impulse inside the neuron flows from the dendrites to the cell body and from there to the axon. The axons convey the nervous impulse away from the cell body.

Thus, the nervous system is composed of a complex of neurons which come into contact with one another but never grow together.

The nervous system is divided into central nervous system and peripheral nervous system. The central nervous system is composed of the brain and spinal cord. All other nerves and neurons which are outside the central nervous system are included into the peripheral nervous system. The peripheral nervous system is subdivided into the somatic and vegetative nervous systems. The somatic nervous system controls bodily movements and regulates actions at the conscious level. The autonomic nervous system controls human reaction to stress. It can either increase or decrease heart rate, or blood pressure.