MECHANORECEPTORS AND BEHAVIOR 335 



Scyliorhinus) showed them to discharge to mechanical pressure, skin stretch, 

 and even muscular contraction if this resulted in the skin being pulled, but 

 the natural stimulus appears to be mechanical distortion of the capsule 

 which would be produced by bending movements made by the body during 

 locomotion; i.e., the receptor is to be regarded primarily as a proprioceptor. 

 An attempt to determine the probable behaviour of the corpuscles during 

 locomotion was made (Roberts 1969c) by providing stimuli that simulated 

 body movement. These experiments showed that the receptor was inactive 

 when the body was not bent but that it discharged to a bending movement 

 (Figure Id), responding to the velocity of the movement and sustaining a 

 slowly adapting tonic discharge, the frequency of which was proportional to 

 the amount of bending (Figure lc). Apparently the sense organs are not direc- 

 tionally sensitive, as they respond to equal movements of opposite direction. 

 Mechanoreceptors reported in several elasmobranch electrophysiological 

 studies may be considered Wunderer corpuscles because of their location and 

 discharge properties, although the discharge has not been positively as- 

 sociated with any specific end organ. For example there are endings in shark 

 gills that respond to water flow (Satchell and Way 1962) and to blood 

 pressure (Irving, Solandt and Solandt 1935), and in the jaws are receptors 

 that discharge to pressure on the gums and teeth (Roberts and Witkovsky 

 1975). Szabo (1962) described a receptor in the caudal fin of Torpedo that 

 differs from the usual behaviour of the Wunderer corpuscle in displaying a 

 low spontaneous discharge and in responding to bending of the fin with a 

 discharge of only slightly higher frequency. 



The Poloumordwinoff Ending— The intermuscular endings of the fins 

 of Torpedo, described by Poloumordwinoff in 1898, have since been found 

 in the pectoral fins of other rays but not as yet in any shark (Cavalie 1902; 

 Barets 1956). These endings consist of many beaded thin nerve fibres that 

 are derived from a single myelinated axon and lie among the muscle fibres of 

 the fins (Figure 2). Unlike the muscle spindle, the simplest form of which is 

 in Urodele amphibia where the nerve fibre swellings are embedded in ad- 

 jacent muscle fibres, the endings in ray fins are surrounded by Schwann cells 

 that connect only loosely to the muscle fibres; the Poloumordwinoff ending 

 is the simplest vertebrate stretch receptor and may perhaps be regarded as 

 the prototypal muscle spindle (Bone and Chubb 1975). 



A brief description of some of the properties of this receptor, in the pelvic 

 fin of Raja, has been provided by Fessard and Sand (1937), who reported 

 the presence of a low resting discharge, the increased firing (up to 100 

 impulses/s) in response to stretch, and the slow adaptation. 



In a more recent study Ridge (1977) has confirmed that these receptors 

 are slowly adapting and length sensitive, and he demonstrated that they are 

 also sensitive to the velocity of stretch. The firing frequency of these endings 

 could be affected in various ways by activity of the adjacent muscle fibres 

 and Ridge discusses the possible implications of this in relation to efferent 

 control, which is known to be important in muscle spindle function. 



