The nervous system comprises highly specialized cells (neurons) whose function is to detect and receive innumerable bits of information (stimuli) from the different sensory organs (receptors). This system codes the sensory information in the form of electrical impulses and transmits them to other cells for their response. The nervous system of higher organisms perform three basic functions receiving sensory input from internal and external environment by nerves to the brain processing the input information by a central processor, the brain; and responding to stimuli transmitting motor commands from the brain to determine the response of the body parts or cells.
Sponges lack neurons. In Hydra, all neurons are similar and linked to one another, forming a nerve net or plexus between the epidermis and the gastrodermis. Planaria has two nerve cords that converge to form a rudimentary brain. In earthworm, a central nervous system consisting of a single ventral nerve cord and paired segmental ganglia (associative interneurons in the form of ganglia) have evolved. The ganglia give rise to the segmental nerves to the tissues.
Structure of a Neuron
In multicellular organisms the various activities of the body are controlled or regulated by means of two different systems. One is the chemical system through hormones and the other is the nervous system which consists of the brain, spinal cord and nerves.
Control or coordination of bodily functions involves two categories of activities -
- Control of the functioning of the internal organs, and
- Receiving and interpreting information from the environment. When we see things, our eyes receive visual signals which are sent to the brain for interpretation. The information received through our sense organs such as eye, ear, etc. determines our reactions.
- It regulates the activities of different organs and of the entire organism. Muscular contraction, glandular secretion, heart action, metabolism and all other processes continuously operating in the body of the animal are controlled and coordinated by the nervous system.
- It helps in maintaining the unity of the organism and its external environment. All outside stimuli are perceived by the nervous system through the sense organs.