Responses of Higher Animals: The Nervous System - 449 



t 



.01 



.02 



.03 



//me, seconds 



nitude of an impulse provides a good index 

 of the immediate condition of the transmit- 

 ting fiber. Thus, for example, if one section 

 of a nerve is slightly poisoned (Fig. 25-2), 

 this section transmits weaker impulses so 

 long as the toxic condition persists; but if 

 impulses manage to traverse a damaged re- 

 gion of a nerve, the impulses regain full in- 

 tensity as soon as they reach an undamaged 

 section of the nerve. 



THE NERVE NET OF HYDRA: A PRIMITIVE 

 NERVOUS SYSTEM 



One of the simplest types of nervous sys- 

 tem is found in Hydra and other coelenterate 

 animals. Hydra possesses a nerve net (Fig. 

 25-3) that extends throughout the body and 



INTENSITY OF IMPULSE 



NORMAL 



DEPRESSED 



NORMAL 



Fig. 25-2. The intensity of a nerve impulse traveling 

 from left to right along a nerve fiber is indicated by 

 the height of the action potential outlined above the 

 fiber. The slightly shriveled region of the fiber indi- 

 cates the depressed portion in which the impulse in- 

 tensity is reduced. Note that the action potential, once 

 past the depressed (perhaps drugged) section, regains 

 its full intensity. 



Fig. 25-1. Variation of excitability of 

 a nerve after stimulation. 1, absolute re- 

 fractory period; 2, relative refractory 

 period; 3, supernormal period; 4, normal 

 condition. (Adapted from Keith Lucas.) 



tentacles of the animal. The nerve net lies in 

 close contact with the mesoglea (p. 630), but 

 its branches make contact with the cells of 

 both the ectoderm and endoderm. Some of 

 these branches are equivalent to the sensory 



STIMULUS -» 



A INDEPENDENT EFFECTOR 



STIMULUS -» 



MUSCLE 

 PROCESSES 



MUSCLE 

 PROCESSES 



MUSCLE 

 PROCESSES 



B RECEPTOR -EFFECTOR 



STIMULUS 



STIMULUS ->c 



C RECEPTOR-CONDUCTOR-EFFECTOR 



Fig. 25-3. Responsive mechanisms of Hydra. Note 

 gradations of complexity from the independent effec- 

 tor to distinct receptor, conductor, and effector cells. 



