1070 



HANDBOOK OF PHVSIGLGGV 



NEUROPHYSIOLOGY II 



well demonstrated, nor are the physiologically ade- 

 quate stimuli for these endings known. Their stimu- 

 lation probably leads to general flexor withdrawals 

 (25, 174) and pain responses (18, 210), but con- 

 tractions restricted to the homonymous muscle have 

 also been noted (18, 210). 



Afferents from Joints 



Sensory endings in and about joints may be classi- 

 fied as tendon organs, Ruffini endings. Pacinian cor- 

 puscles and an additional heterogeneous category of 

 simpler endings (23, 77, 78, 24B). 



PACINIAN BODIES. Corpusclcs of Pacini are found in 

 mesenteries, interos.seous membranes, occa.sionally 

 in fascial planes of muscle and, importantly from the 

 present standpoint, in clusters located deep to fle.xor 

 tendons of the digits (107). It is noteworthy that 

 neither in the latter location (107) nor about the 

 knee joints of the cat (23, 38, 78, 248) or mouse 

 (77) do Pacinian bodies bear a principal relation 

 to joints; rather they lie in locations favoring deep 

 pressure as an adequate stimulus. The application of 

 a rod or bristle directly to an isolated corpuscle elicits 

 a discharge which rapidly decrements (22, 105, 107) 

 and cannot be made to continue longer than 5 sec. 

 despite continued pressure and deformation (107). 

 These organs, therefore, are suited to signal quick 

 changes in pressure (248) such as occur in the acute 

 strains imposed on plantar surfaces of the digits upon 

 initial contact with the ground as in the extensor 

 thrust reaction. It is not certainly known that Pacin- 

 ian bodies participate in muscle reflexes and, indeed, 

 in mesenteric locations it has been claimed that they 

 mediate vascular reflexes (76). 



RUFFINI ENDINGS. Ill monitoring single units from joint 

 nerves (such as in the knee of the cat), the most pieva- 

 lent pattern of discharge is one in which firing occurs 

 over a limited extent of the flexion range and a maxi- 

 mum frequency is found at some optimal position 

 (23, 38, 107, 248). Flexion-extension or rotation of 

 the joint causes the unit first to respond with momen- 

 tary overfiring or underfiring and then to settle to a 

 new frequency which is characteristic of that position 

 (4, 23). E.ssentially, the same rate is attained whether 

 the final position is approached either from a position 

 of greater flexion or of extension (38). This afferent 

 pattern has been identified fairly definitely with the 

 endings of Ruffini (22, 23, 248). These are strewn 

 widely among the multidirectional strands of the 



fibrous capsule and ligaments, thus accounting for 

 the variation in ranges and optima for firing of in- 

 dividual receptors and providing, theoretically, a 

 series of permutations of receptor responses by which 

 the central nervous system may judge the position of 

 the joint. At no position of the joint are all endings 

 silent (23, 38). 



TENDON ORGANS. Scusory endings similar to the organs 

 of Golgi in tendons are present in cruciate, patellar 

 and collateral ligaments but not within the capsule 

 proper (248). For this rea.son, unlike Ruffini endings, 

 the tendon organs of joints are relatively unaffected 

 by contractions of muscles which insert in the vicinity 

 of joints (248) and could conceivably serve as an even 

 more reliable indicator of joint position. The dis- 

 charge from tendon organs of joints, like that from 

 mu.scles, adapts slowly. 



CENTRAL EFFECTS OF JOINT RECEPTORS. Stimulation 

 of articular nerves in the spinal or decapitate cat 

 elicits polysynaptic discharges in the ventral roots, 

 an inhibition of extensor monosynaptic responses and 

 a facilitation of flexor ones (15, 79). With stronger 

 stimuli, signs of pain with flexion of all limbs and the 

 trunk appear (79, 81 ). Ne\ertheless, in the decere- 

 brate preparation, flexor responses are not in\ariable 

 (79, 248) and perhaps, if more discriminate stimula- 

 tion were emploved, flexor and extensor effects would 

 be found to depend upon the position and nature of 

 the ending. Certainly receptors at some joints (such 

 as the cervical intervertelaral) may facilitate either 

 flexor or extensor effects, depending on the direction 

 of movement. Joint receptors, by reason of their 

 strategic locations and the tonicity of their discharge, 

 might be suspected of influencing postural contrac- 

 tions, and such a role of cer\ical receptors is well 

 known as underlying the attitudinal reflexes. The en- 

 tire role may be a complicated one as joint afferents 

 project to the somatosensory cortical areas (80, 

 248) as well as to the reticular formation (200). 



Cutaneous Receptors 



In general, skin receptors are unsuited to the task 

 of mediating postural contractions by reason of their 

 phasic discharge (69, 106, 108, 192) and flexor cen- 

 tral effects (43); and, indeed, decerebrate standing 

 can still be obtained though the skin be removed from 

 the entire preparation (241). However, the positive 

 supporting or "magnet' reaction is elicitable in the 

 decerebrate animal upon barest contact with the 



