Primitive Nervous Systems
It took a long time to develop the complex human nervous system. In a simple animal like an Amoeba, the whole body is a single cell-mass of protoplasm, possessing a generalized capacity for irritability and contractility. Whenever a stimulus impinges at some spot on such a body there results a direct local response at the point of stimulation, but the news of it travels slowly to other parts of the cell. The fact that an Amoeba in escaping from danger extrudes a pseudopod on the side opposite to the point of stimulation is an indication that some degree of protoplasmic conduction is present even here. It is out of the question in such a case to speak of sense organs or receptors, of communicating nerve fibers, adjusting centers or of reacting muscles, because they have not yet developed.
It has been suggested that the coelenterates show what may have been the first steps in the evolution of the neuromuscular mechanism. In Hydra, for example, the inner ends of some of the surface ectodermal cells develop muscular processes. When these cells are stimulated their inner portions contract without mediation of any sensory receptor. Parker has given the name independent effector to this type of cell which serves as both receptor and effector (Fig. 598).
In most animals, including many coelenterates, these two functions are performed by separate cells. In the simplest arrangement sensory cells (receptors), exposed to the outside, are in direct contact, through their branching inner ends, with underlying contractile muscle cells (effectors). Usually, however, a second nerve cell, a motor cell, is inserted between the sensory and muscular elements. In coelenterates the motor cells unite into a continuous network, nerve net, in which impulses may pass freely in any direction and consequently spread through the net to give a diffuse response to a localized stimulation. Nerve networks are found on vertebrate blood vessels (Fig. 599).
Simple localized reflex arcs including two short neurons persist in the walls of the digestive tube of man. By means of large numbers of these arcs, located in the plexus of Auerbach and the plexus of Meissner, the peristaltic response to the stimulating presence of food is brought about without the intervention of the cord or brain.
The final step is taken by the flatworms in which the synapses between sensory and motor neurons as well as the cell bodies of the latter are located in nerve cords. Also nerve impulses are transmitted in only one direction. Similar simple reflex arcs, involving a central nervous system, are found in most invertebrates (Fig. 597) and in the vertebrates.
Primarily the cell bodies of sensory neurons are located in the surface epithelium, as we have seen in the coelenterates and worms. But frequently they shift deeper into the body and develop dendrites, which may be of considerable length if the cell body lies at some distance from the surface of the animal’s body
Steps in the evolution of motor and sensory neurons are shown in Figures 600 and 601.