512 Comparative Animal Physiology 



The pacinian corpuscles along blood vessels in the mesentery discharge as the 

 vessels are distended with each pulse.'^'' 



The campaniform sensilla of insects are also intermediate in speed of 

 adaptation. These sensilla respond to strains in the cuticle produced by 

 straight or lateral bending, the leg sensilla being stimulated while the insect 

 stands. The discharge starts at a high frequency ClOO-300/sec.) and falls 

 after 1-2 seconds to low levels.'^'^ 



Tension receptors in muscles adapt very slowly, compared with tactile 

 receptors. The impulses from single sensory spindles in a frog toe muscle 

 under continued stretch or load start at 120-260 per second and decline to a 

 rate of about 20 per second, which is maintained regularly for many min- 

 utes. Responsiveness to a test reload is less as adaptation is prolonged."^- '^'^ 

 Frequency of afferent discharge depends not only on ,the amount but also 

 on rate of development of tension (Fig. 182). If a muscle is stretched and 



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Fig. 182. Response in nerve fiber from a muscle receptor in peroneus longus of cat at 

 different rates of stretching of the muscle to final tension of 25 gm. Action potentials, 

 small deflections from central lines, muscle contraction indicated by lower records. From 

 Matthews."' 



the spindle is firing continuously, contraction is accompanied by a brief 

 interruption in the steady sensory discharge. Matthews'^*' described three 

 types of sensory endings of mammalian muscle: (1) receptors (probably 

 spindles) which are arranged in parallel with the contractile elements, and 

 which respond either on quick stretch or release or in active contraction; 

 (2) receptors of high threshold arranged in series with the contractile ele- 

 ments; and (3) a type occurring in fascia, rapidly adapting and responding 

 only during movement of the muscle. 



Stretch receptors in muscles of the elasmobranchs Raja and Scylliiun 

 adapt slowly and maintain a stcadv discharge longer than do the muscle 

 receptors of amphibians and mammals, firing under constant tension for 

 more than an hour. When the tension is suddenly decreased there is a silent 

 period before the discharge resumes at a new level.-" 



Virtually no adaptation occurs in the pressure receptors of the carotid 

 sinus of mammals. Some of the sensory elements are normally continually 

 active and lor each pulse wave their frequency increases; other elements fire 

 three or four impulses for each pulse. When the carotid artery is perfused at 

 steady pressures there is little stimulation until the pressure rises above 40-50 



