Nervous System and Sense Organs 209 



is, it is self -propagated. The energy for the movement is taken from the 

 nerve fiber over which the impulse travels. The movement is a very 

 rapid, smooth one ; perhaps it could even be said that the impulse "rolls" 

 along the nerve as a wave of negativity, following and preceded by 

 positive charges on the surface membrane. 



If a single stimulus of a very low intensity is applied to a nerve, 

 there will be no response ; however, if a series of stimuli of this same 

 low intensity is applied, the nerve will respond. It appears then that 

 the phenomena known as summation occurs in nerve fibers. Apparently, 

 a small stimulus will cause but a slight depolarization of the nerve fiber, 

 and a series will bring about complete depolarization. 



The speed at which the nerve impulse travels varies with two main 

 factors : the diameter of the nerve fiber and the presence or absence 

 of the myelin sheath. \''arious combinations of these factors are utilized 

 for different functions in the many different animal groups. 



In mammals, for example, the speed of nerve transmission is high- 

 est in the large myelinated motor axons. It is least in the nonmyelinated 

 small fibers of the autonomic system. An ordinary motor nerve has a 

 transmission rate of about 120 meters a second while a nonmyelinated 

 nerve of the autonomic system transmits at only 10 or 20 meters a 

 second. 



The interesting mollusc, the squid, has nerve fibers of varying 

 diameters. The rate of conduction of the nerve impulse varies di- 

 rectly with the diameter of the fiber. The giant fiber, which has only 

 a delicate sheath but with a diameter of nearly 1 mm., conducts at the 

 rate of approximately 22 meters per second. The nerve fibers of the 

 squid that conduct impulses only short distances are of much smaller 

 diameter ; thus the squid has well-coordinated control of the various 

 parts of its body. 



The Reflex Arc. — The functional unit of the nervous system is 

 the reflex arc (Fig. 58) which consists of a receptor, an afferent neuron, 

 an association neuron in the spinal cord or brain, an efferent neuron, 

 and finally an effector. 



The receptor is a structure which is specialized in such a manner 

 that it can respond to certain changes in the environment; thus the 

 receptors of the ear respond to sound changes, those in the eye to 

 changes in light intensity. There are many of these sense organs 

 scattered throughout the body, and they are actually the specialized 

 endings of afferent nerve fibers. The effectors are either muscles or 



