In different groups of animals, two types of nervous systems can be recognized, one is diffused nervous system and the other is centralized nervous system. It is correlated with life cycle of animals.
Diffused nervous system:
This is found in cnidaria and echinoderms. In Hydra and other cnidarians, because of the radial symmetry there are no anterior or posterior ends. The nervous system consists of a network, the nerve net of neurons which is woven through the tissues of the body. The flow of information through this nerve net is not highly directional, rather it diffuses in all directions from the point of stimulation. In this kind of nerve net the transmission of impulse is slow because of the synapses involved. Thus initially the response is localized. If the stimulus is strong and persistent such as capturing the prey, then the whole body involves.
Centralized nervous system:
It is the characteristic of bilaterally symmetrical animals which have a definite anterior and posterior ends. Since the head is first part of the body that encounters the food, dangers etc so they require sensory organ and a control centre of the nervous system where the sensory information from different sensory organs are brought for integration and to develop a coordinated response of the body.
Flatworms such as planaria are among the simplest animals having centralized nervous system. It consists of an anterior brain connected with sensory organs. From the brain of planaria arise two cords like nerves running longitudinally through the body. Two nerve cords are connected with each other at several points along the entire length of the body through transverse nerves which coordinate the movement of two lateral sides of body. Echinoderms have nerve nets for example sea stars have distinct nerve nets.
One that lies just under the skin has a circumoral ring and five sets of nerve cords running out to the animals’ arms.
Another net serves the muscles between the skin plates called ossicles. Third net connects to the tube feet. This degree of nerve net complexity permits locomotion, a variety of useful reflexes and some degree of central coordination. For example when a sea star is flippered over, it can right itself.
Vertebrate nervous system:
The basic organization of nervous system is similar in all vertebrates. Bilateral symmetry, a notochord and a tubular nerve cord characterize the evolution of vertebrate nervous system.
Notochord is a rod of mesodermally derived tissue encased in a firm sheath that lies ventral to the neural tube. It first appeared in marine chordates and in present in all vertebrate embryos, but is greatly reduced or absent in adults. During embryological development in most vertebrate, species, vertebrae sexually arranged into a vertebral column replace the notochord. This vertebral column led to the development of strong muscles, allowing vertebrates to become fast moving predatory animals. Some of the other bones developed into powerful jaws which facilitated the predatory nature of these animals.
Tubular nerve cord:
It is just above the notochord. During early evolution, the nerve Cord under went expansion, regional modification and specialization into a spinal and brain over time the anterior end thickened variably with nervous tissue and functionally divided into hindbrain, midbrain and forebrain.
In the sensory world of fast moving and powerful vertebrates anterior sensory receptors because more couplex and bilaterally symmetrical. For example paired structures such as eyes and ears developed to better gather the information from outside environment.
Vertebrate nervous system has two main divisions (1) central nervous system composed of brain and spinal cord and is site of information processing (2) Peripheral nervous system composed of all nerve of body outside the brain and spinal cord. Nerves arising from brain are called cerebral or cranial nerves and the nerves arising from spinal cord are called spinal nerves. Nerves may be sensory or motor or both. Nerves which carry message from brain or spinal cord to other organs of body are called sensory nerves. Nerves bringing message from other parts of body to brain are called motor nerves. Nerves which perform both sensory and motor messages are called mixed nerves.
