Give evolution of invertebrate and vertebrate nervous system

In different groups of animals, two types of nervous systems can be recognized: one is the diffused nervous system and the other is the centralized nervous system. It is correlated with the 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 impulses 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 is involved.

Centralized nervous system

It is characteristic of bilaterally symmetrical animals which have a definite anterior and posterior end. Since the head is the first part of the body that encounters food, dangers, etc. they require sensory organs and a control center of the nervous system where the sensory information from different sensory organs is brought for integration and to develop a coordinated response of the body.

Flatworms such as planaria are among the simplest animals having a 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 the 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 animal’s arms.

Another net serves the muscles between the skin plates called ossicles. The 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 the nervous system is similar in all vertebrates. Bilateral symmetry, a notochord, and a tubular nerve cord characterize the evolution of the 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 is present in all vertebrate embryos, but is reduced or absent in adults. During embryological development in most vertebrates, species, and vertebrae are sexually arranged into a vertebral column to 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 underwent expansion, regional modification, and specialization into the 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 complex and bilaterally symmetrical. For example, paired structures such as eyes and ears developed to better gather information from the outside environment.

The vertebrate nervous system has two main divisions:

  • A central nervous system composed of the brain and spinal cord and is the site of information processing
  • The peripheral nervous system is composed of all nerves of the body outside the brain and spinal cord. Nerves arising from the brain are called cerebral or cranial nerves and the nerves arising from the spinal cord are called spinal nerves. Nerves may be sensory-motor or both. Nerves that carry a message from the brain or spinal cord to other organs of the body are called sensory nerves. Nerves bringing messages from other parts of the body to the brain are called motor nerves. Nerves that perform both sensory and motor messages are called mixed nerves.

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