Figure 2. Increase in the response of a primary muscle spindle afferent to sinusoidal stretching during the performance of mental computation.

A, the primary afferent activity (TA afferent) recorded during a whole sequence is illustrated by the instantaneous discharge frequency curve. Note the increase in the firing rate, together with an increase in the variability of the instantaneous discharge frequency, both of which occurred shortly after the instructions were given to the subject. In addition, these stretch-sensitivity enhancing effects lasted throughout the period of computation and continued after the subject had been asked to relax, as shown by the flat electrodermal recording. Inst, instruction; Answ, answer. B, expanded time scale recording corresponding to 6 movement cycles during the computation period (right) compared with the control period (left) showing the frequent occurrence of a third spike at various times from one cycle to another during mental computation. PF, plantar flexion. C, cycle histograms were constructed for the whole period of stretching during which mental computation was performed (right), and for the equivalent part of the control period (93 stretch movements combined) (left). Note the increase in the number of spikes and the enlarged histogram obtained while mental computation was being performed.