Mechano- and Equilibpum-Reception 



511 



area of one nerve initiates impulses in the central end of a fiber of another 

 nerve. ^ The skin area served by one. such sensory fiber varies from 4 to 100 

 sq. mm. When the sense endings are stimulated continuously, as by a jet of 

 air, there is an initial burst of 4 to 12 or more impulses lasting 0.1 to 0.2 

 sec. (Fig. 181, fl); when they are stimulated by a puff 5 msec, or less in 

 duration at a rapidly applied pressure one impulse appears (Fig. 181, fc). 

 In addition to these fast adapting endings there are some which discharge for 

 as long as 10 sec. during continued stimulation.^-^ A different sort of adapta- 

 tion, secondary adaptation, occurs when the ending is stimulated repetitively, 



Fig. 181. Responses in single fiber of dorsal cutaneous nerve of frog (a) to pres- 

 sure suddenly applied and maintained and (b) to pressure applied three times by 

 interrupting air jet (slowly rising baseline indicates duration of sudden blast). From 

 Adrian, Cattell, and Hoagland." 



as by an interrupted air jet. The ending may discharge at a rate as high 

 as 250 to 300 per second initially, and may continue at 150 per sec. for 30 

 to 60 seconds, finally dropping more and more impulses until it discharges 

 only occasionally. Sometimes, when the ending is stimulated at high fre- 

 quency (e.g., 150/sec.), there may be two impulses together for one stim- 

 ulus, the second one smaller than the first, because it falls in the refractory 

 period of the nerve. Continued repetitive stimulation of the skin keeps the 

 endings adapted, even though there may be no nerve impulses. By sending 

 impulses backward (toward the skin by electrical stimulation), it is found 

 that the adaptation does not reside in the nerve. If the stimulus-to-rest ratio 

 is small, adaptation is slowed, and if the pressure is high adaptation is also 

 delayed. Increase in the potassium content of the saline bathing the nerve 

 ending speeds adaptation reversibly, and it has been shown that repeated 

 mechanical beating of frog skin by an air jet increases the potassium con- 

 tent of bathing saline, hence this secondary adaptation may be due to ac- 

 cumulation of potassium lost. from epithelial cells around sensory endings.*'* 

 46, 47, 48 Sm^}^ potassium leakage is probably not the cause of the fast adapta- 

 tion to a single stimulus. 



The encapsulated mechanoreceptors, such as pacinian corpuscles, are inter- 

 mediate in adaptation speed. When pressure is applied to a mammalian 

 tendon containing a pacmian corpuscle, a discharge starts at an initial fre- 

 quency of 50-80 per second, adapting in a few seconds to 10-15 per second.'* 



