4IO 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



IMPULSES PER SECOND 



100 I- 



EXCITflTORY STIMULUS 



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FIG. 14. Interacting effects of excitatory and inhibitory 

 peripheral stimuli upon the discharge rate of a single neuron of 

 area i of the postcentral gyrus of the monkey. Graph plots the 

 average frequencies of discharge of the neuron in each succes- 

 sive 400 msec, period. Neuron excited by internal rotation of 

 shoulder joint, indicated by upper bar, and inhibited by pres- 

 sure upon skin of the palm, indicated by lower bar; both contra- 

 lateral. Note that recovery from inhibition duplicates onset of 

 excitation, with rapid onset transient and decline to a less 

 rapid firing level. [From Mountcastle, V. B. & T. P. S. Powell, 

 manuscript in preparation.] 



upon the receptor organs associated with the joints. 

 Activity set up in those receptors by the steady posi- 

 tion or movement of the joints is relayed through the 

 medial lemniscal system, in a topographical pattern 

 at each precortical relay and in the somatic sensory 

 cortex itself which is mutually interlocked with the 

 pattern representing the cutaneous sensory sheet. 

 Since this is contrary to the widely held belief that 

 kinesthesis depends as well upon afferent input from 

 muscle stretch receptors, the evidence for it will be 

 presented in some detail. 



Mustif Sitetiii Receptors and h uiesthests 



Evidence accumulates from recent research that 

 the rate of discharge of the stretch receptors of muscle 

 is not linearly or even constantly related to the length 

 of the muscle per se. Since the classical work of 

 Matthews (169) it has been known that the Golgi 

 tendon organs discharge afferent impulses at a rate 

 related to tension. The tension to which these recep- 

 tors are subjected depends upon the length of the 

 muscle, i.e. upon the joint angle, and upon the force 

 exerted h\ the muscle against its load h\ its active 

 contraction which in turn depends on the activity of 

 the alpha motoneurons. It follows that the number of 



active Golgi organs and their rates of discharge are not 

 variables dependent solely upon the angle of the joint 

 or joints acro.ss which the muscle works; these recep- 

 tors cannot, therefore, inform reliably of joint posi- 

 tion. 



The spindle organ receptors of muscle are subject 

 to even more complex influences. Matthews (169) 

 had shown that these receptors are excited by stretch 

 of the muscle but cease to discharge as the muscle is 

 shortened by alpha motoneuron action. They may be 

 completely silent when tension at the tendon is maxi- 

 mal. The work of Leksell (153) revealed, however, 

 that the smaller efferent fibers of the \entral root, 

 the gamma motoneurons, produce upon discharge an 

 increase in spindle organ activity, even when the 

 muscle shortens. These observations have been ex- 

 tended recently (102, 129-132, 144) and it is now 

 well known that the gamina efferents condition affer- 

 ent input from the spindles and thus play an important 

 role in \oluntary movement and reflex regulation. 

 More recently Granit and his colleagues (62, 103, 104) 

 have ciescriijed the central nervous control of the 

 gamma motoneurons and hence of spindle organ 

 discharge, and Eldred & Hagbarth (63) their reflex 

 regulation bv cutaneous afferents. Further details of 

 the function of the gamma efferent-spindle afferent 

 loop are presented by Eldred in Chapter XLI of this 

 work, and an excellent general review of the subject 

 is provided in the monograph by Granit (loi). The 

 important point in the present consideration is that 

 spindle activitv may varv from zero to ma.ximum 

 independenth' of the length or tension of the muscle; 

 these receptors, like the Golgi tendon organs, cannot 

 signal muscle length or joint angle. 



These facts alone are impressive for the argument 

 that stretch receptors of muscle are not likely to in- 

 form of joint position. Complementary to them is the 

 experimental oiiservation of Lloyd & Mclntyre (160) 

 that the large stretch afferents from muscle do not 

 project upwards in the dorsal cokmins but relay in the 

 column of Clarke-Stilling into a.scending systems 

 terminating in the cerebellum. This observation has 

 been confirmed and extended in an elegant series of 

 studies (128, 150-152, 163) which showed that group 

 I-a afTerents from muscle spindle organs relay into the 

 dorsal spinocerebellar tract. In addition, O.scarsson 

 (190) has reported that group I-b fibers from tendon 

 organs project upon the cells of origin of the ventral 

 spinocerebellar tract. Complementary also are the 

 negative observations that direct stretch of muscle 

 produces no detectable response in the postcentral 



