Nervous Systems 



111 



Conduction in higher animals is predominantly a function of cell mem- 

 branes, and in neurones the fibrils per se probably have nothing to do with 

 conduction. Taylor ""'^ suggests that the fibrillar system in ciliates actually 

 may have several different functions. Ciliates are highly specialized acellular 

 organisms. Hence, the ciliate fibrillar system is not a precursor of nervous 

 structures. 



In sponges there are no nerve cells, but spindle-shaped muscle cells bring 

 about closure of oscula. These muscle cells have been called independent 

 effectors,'^^^ because they combine sensory and motor functions. There is 

 a slow conduction (about 1 cm./min.) from cell to cell in a sponge, as shown 

 by the response of the osculum to a prick a short distance away.'^^^ In some 

 coelenterate tentacles there may be a few two-celled arcs (sense cell— muscle 

 cell),^^'^ but beginning with the coelenterates coordination between sense 

 organs and effectors is principally nervous. 



4- 



-041- 



1 msec 



Fig. 290. Electrical changes in crab nerve at the stimulating electrodes to shocks of 

 increasing intensity. Electrotonic potential seen at anode (below zero) and in first five 

 records at cathode (above zero), local response only in upper records 6-9, and beginning 

 of action potential spike in upper three responses. Upper six curves at threshold intensity. 

 Insert a, another series at cathode. From Hodgkin.^*" 



NERVOUS CONDUCTION 



The Action Potential Wave. A nerve impulse is the sum total of chemical 

 and physical events in the propagation of a wave of physiological activity 

 along a nerve fiber. The electrical components of a nerve impulse have been 

 measured most precisely and are of interest in comparative physiology. 



A nerve trunk is composed of many nerve fibers, each fiber conducting 

 independently of the others. Each nerve fiber is polarized when at rest by 

 about 60 millivolts, the inside of the cell membrane being negative to the 

 outside. Excitation consists, in part, in the breakdown of this polarized, 



