In this article we will discuss about the invertebrate nervous system. This will also help you to draw the structure and diagram of invertebrate nervous system.
Beginning from a primitive nerve net system, the evolution of nervous system has progressed towards aggregation of ganglia (cephalization) giving rise to a controlling and coordinating centre, the brain, towards the anterior end of the body, which is first to meet the environment and also is the site where important organs are concentrated.
Along with this, different types of neurons, transmitting waves of excitement in one direction and transferring the same from one to the other neurons have developed, which has culminated in cephalopods, the most active invertebrate.
Nerve net system:
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A nervous system is first met with in the cnidarians. The nervous system of cnidarians and some flatworms is a network formed by ganglia and their processes. Such a type is called nerve net system. The ganglia are constituted by cell bodies and the connections by the processes. Sensory, motor and interneurons are present, but they cannot be differentiated.
Usually the processes are connected through interneurons, though connection between sensor and motor neurons through synapses are not wanting. Impulses are conducted across in either direction. A controlling system is lacking. The conduction is diffuse and tends to spread in all directions from the starting point.
Cephalic dominance system:
For the first time the nerve ganglia are formed by the aggregation of cell bodies in the flatworms. The ganglia are close to one another at the anterior end of the body and form the brain, the site for sensory integration and motor command.
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The nerve cords are solid and run backward up to the posterior end of the body. With the progress of evolution the number of nerve cords reduced to two as in turbellaria (Fig. 13.2) and finally one solid, double ventral nerve cord in annelids.
A pair of ganglia in each segment of the body help the segments to act in a coordinated way.
The motor centre is present in the sub-oesophageal ganglion and the brain has an inhibitory control over it. In the arthropod brain protocerebrum associated with vision centre and associated area, deuterocerebrum with olfactory and antennal centres and tritocerebrum innervating the mouth and alimentary canal are distinguishable.
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The motor activity is controlled by the sub-oesophageal ganglion. In molluscs the nerve ganglia are paired like other invertebrates, except the visceral ganglion. They are located at different points along the nerve cords but they tend to aggregate at one point, which is first found in pulmonate.
Extreme condensation of the nerve ganglia forming a central brain, associated with the appearance of a cartilaginous cranium is present in cephalopods.