414 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



nucleus and in the postcentral honiologue are acti- 

 vated by, and only by, movement of the joints (i8i; 

 and Mountcastle, \'. B. & J. E. Rose, unpublished 

 observations). The great majority of these neurons 

 respond not only to transient rotation of the joint to 



IMPULSES PER SECOND 

 50 



CORTICAL NEURON RESPONDING TO 

 MAINTAINED ROTATION OF SHOULDER JOINT 



FIG. ig. Impulse frequency of a single neuron of area i of the 

 postcentral gyrus of the macaque, plotted continuously as 

 average frequency in each 400 msec, period. Neuron is a deep 

 joint unit. At onset of rotation of the contralateral shoulder 

 joint there is a rapid rise in discharge frequency which declines 

 slightly to a more or less steady rate of discharge during 

 steadily maintained joint rotation (period of stimulation in- 

 dicated by black bar). Note rapid fall of frequency when joint 

 is returned to its neutral position, and the postexcitatory period 

 of low frequency discharge. [From Mountcastle, V. B. & T. P. S. 

 Powell, manuscript in preparation.] 



which they are related but continue to discharge 

 impulses steadily when the joint is held within the 

 excitatory angle, subserved by the unit (.see figs. 19, 

 20). They adapt as a rule very slowly. The onset 

 transient is a function of the degree and rate of joint 

 movement; the subsequent steady state of activity is 

 a function of joint angle only. In the cortex, as in the 

 periphery, excitatory angles for different neurons 

 related to a given joint are different, and some units 

 of the group are active at any joint position. Phasic 

 joint movements may recruit additional neurons, 

 increase the discharge frequency in some neurons 

 already active and decrease it in still others. From 

 this description a generality is once again confirmed; 

 the discharge patterns of central neurons of the lemnis- 

 cal system arc determined by those ot the peripheral 

 receptors to which the)- are linked. 



It is an observation of interest that pairs of closely 

 related cells in the cerebral cortex may be reciprocally 

 related to a given joint. One of the pair is active and 

 the other silent as the joint moves in one direction, and 

 the reverse occurs when the movement alternates 

 (181) (fig. 20). Whether the reciprocity is due to 

 some central reciprocal inhibitory exent or simply to 

 alternate loading and unloading of appropriate groups 

 of receptors on the two sides of the joint is unknown. 

 In any case, it is likely that such a mutual interaction 

 could serve to increase the discriminatory capacity in 

 respect to the rate and extent of joint mo\ement. 



90 - 



o 



■z. 80 

 O 

 a 70 



q: 60 

 uj 



'^ 50 

 tn 



i 30 



- 20 



10 



Reciprocolly Responding Cortical Cells, Recorded Simultaneously 



Driven Respectively by flexiono — o ond extension •--• 

 of Contralateral Elbow 



Peripherol Receptors Within Elbow Joint 



I 



\ 



FIG. 20. Impulse frequency graphs of two neurons of postcentral homologue of the cerebral cortex 

 of the cat. Discharges of the two units observed simultaneously at a single microelcctrodc position. 

 Units responded reciprocally to alternating flexions and extensions of the contralateral elbow. 

 Graphs plot continuously the average frequencies for each consecutive 400 msec, period. Impulse 

 frequency reaches zero for each unit when the joint reaches the position maximally excitatory for the 

 other unit. During fourteenth and fifteenth seconds the joint was held in steady extension, and the 

 extension unit fires steadily, while the flexion unit is almost completely silent. [From Mountcastle 

 (.81).] 



