594 



Comparative Animal Physiology 



Neuromuscular facilitation is shown by vertebrate striated muscle when 

 it is partially curarized; one end-plate potential is insufficient for a muscle 

 impulse, but by summation an impulse can be started. The muscle is 

 normally non-iterative, but under partial curarization it is iterative. Crus- 

 tacean muscles, as will be shown below, may respond completely to single 

 impulses in a motor nerve fiber of one type but require facilitation when 

 stimulated bv another nerve fiber. The claw closer of Limulus responds 

 weakly to single maximal stimuli, but at frequencies of 30-40 per sec. it de- 

 velops 18-28 times as much tension. 



Fig. 226. Responses of flexor muscle of leg dactylopodite of Carcinus to repetitive 

 stimulation. Frequencies, from highest tension, correspond to intervals of 4.0, 5.4, 7.3, 

 9.7, 10.6, 12.6, 15.0, 19.7, 22.5, and 30.0 sigma. Pantin.'"' 



Fig. 227. Responses of the sphincter of a sea anemone to repetitive stimuli, intervals 

 between separate stimuli given in seconds. Pantin.''" 



Repetitive stimulation is essential to elicit complete responses of short- 

 fibered muscles where the excitation spreads from fiber to fiber or by a 

 nerve network in the muscle. In vertebrate smooth muscles the mechanical 

 response to single shocks may be propagated over many fibers (ureter), or it 

 may be local (intestine). The total response increases as more impulses 

 are delivered. In the nictitating membrane of the cat the contraction rate 

 and height of response increase up to 25-30 per second.^'- In coelenterates 



