TOUCH AND KINESTHESIS 



homologue of the cerebral cortex (182). Nor have 

 single unit analysis studies revealed any cells at levels 

 of thalamus or cortex which could he activated by 

 stretch of muscle (181; Mountcastle, V. B. & T, P. S. 

 Powell, manuscript in preparation; and Mount- 

 castle, V. B. & J. E. Rose, unpublished observations). 



Data obtained from experiments in which the 

 system is activated by electrical stimulation of bared 

 muscle nerves are somewhat discordant. Mount- 

 castle et al. (182) reported that when the afferent 

 volley was confined to group I afferent fiijers (which 

 innervate the annulospiral endings and the Golgi 

 tendon organs), no responses were evoked in the post- 

 central homologue in anesthetized animals. Nor were 

 such responses observed when the stimulus strength 

 was increased to activate group II afferents (which 

 innervate the flower spray stretch receptors). They 

 did observe, again in anesthetized animals, that when 

 group III fibers of the muscle nerves were activated 

 cortical responses of long latency appeared. The 

 peripheral endings of group III afferents are thought 

 to lie bare nerve terminals and there is no evidence 

 that they are sensitive to mechanical changes in the 

 muscle; it seems safe to assume that these endings are 

 of no significance for position sense. Perhaps they, 

 together with the C-fiber afferents, mediate the sensa- 

 tions of muscle fatigue and pain. These observations 

 are in agreement with the observation that direct 

 stretch of muscle evokes no detectable response in 

 the cerebral cortex. 



Some workers (84, 86, 171) find, on the other hand, 

 that cortical responses do appear when the afferent 

 volley is thought to contain the group II and possibly 

 group I components. Perhaps these discordant results 

 are due to the different muscle nerves used, for it is 

 known that some fibers from joint receptors, perios- 

 teum and deep fascia, may travel in some muscle 

 nerves and are of group II size (76, 225, 231-233). 

 Whatever the final answer, it is clear that muscle 

 stretch afferents are unlikely to play a role in position 

 sense for they are under control of the gamma efferent 

 loop and may discharge over their full frequency 

 range at any muscle length. 



Innervation of Joints 



The tissues in and aijout the joints clearly receive 

 a rich innervation; [the older literature on this matter 

 has been reviewed by Skoglund (225)]. This innerva- 

 tion, the receptor organs in the ligaments and the 

 joint capsules, and the functional properties of the 



receptors have been intensively studied in recent 

 vears. The articular innervation in a variety of animals 

 and in man has been described by a number of 

 workers (16, 18, 28, 29, 75-81, 215, 216, 225, 279). 

 Afferent fibers from some joints have been shown to 

 travel in both muscle and cutaneous nerves. The 

 myelinated fibers vary in size between 2 and 16 n 

 and according to Gardner (76) the spectruin displays 

 definite peaks between 2 to 5 and 7 to 10 /i, while 

 Skoglund's measurements (225) suggest a unimodal 

 distribution around a peak at about 3 to 6 ju. Articular 

 nerves contain large numbers of unmyelinated fibers, 

 some of sympathetic origin, while others are un- 

 doubtedly afferent dorsal root C fibers. It is clear 

 then that articular nerves resemble in composition 

 purely cutaneous ones. 



Joint Receptors and Their Discharge Patterns 



Some recent histological studies indicate three types 

 of receptor organs in articular tissue (16, 18, 28, 76). 



FIG. 15. Graph of the impulse frequency of a single afferent 

 neuron innervating the capsule of the knee joint of the cat. 

 Graph plots frequency of impulses against time as the joint is 

 moved through 10 degrees of flexion and back again, as in- 

 dicated by the dashed line. Note onset transient during move- 

 ment, adaptation to a more or less steady frequency of discharge 

 during steady joint displacement, rapid drop in frequency when 

 joint moves away from excitatory position, postexcitatory silent 

 period, recovery to resting' frequency of discharge, and almost 

 exact repetition of the pattern of discharge when the move- 

 ment is repeated. [From Boyd & Roberts (29).] 



