Modification by previous afferent discharge of the adaptation of frog muscle spindles following an extension.

Abstract

SUMMARY1. Extension of a frog muscle spindle evoked a discharge of impulses in its sensory axon. The adaptation of the discharge after the dynamic phase of stretching occurred in two phases. At first the impulse train was almost regular and adapted quickly, but later this gave place to a phase of slower adaptation in which the variability of discharge was much increased.2. The discharge of action potentials by the muscle spindle depressed the response of the receptor to a subsequent extension. This was true whether they were elicited antidromically by afferent stimulation or orthodromically by longitudinal vibration. This depression had its most marked effect on the first phase of adaptation where it greatly increased the rate of adaptation. The second, slower, phase of adaptation was little influenced.3. The depression increased with the duration and with the frequency of afferent stimulation. It persisted for about 300 msec. 4. The results show that the properties of the spike generating mechanisms in the muscle spindle are modified by previous discharge and that this modification may influence the overall properties of the receptor.5. The fact that afferent stimulation has different effects on the two phases of adaptation is consistent with the suggestion that the impulse train evoked by extension is derived from more than one spike generator within the muscle spindle.

Motor unit contractions initiating impulses in a tendon organ in the cat.

“…Furthermore the effectiveness of a conditioning train of antidromic impulses increased smoothly with the frequency of the train and there were no sudden discontinuities suggesting impulse blockade (Fig. 7, see also Brokensha & Westbury, 1978). adaptation of the response of a tendon organ to a motor unit contraction.…”

Sustained sensitivity modifications induced by brief length perturbations in the crayfish slowly adapting stretch receptor

“…In muscle spindles a depression of the action potential activity (i.e., what will be termed "down-step") follows lengthening perturbations (Brokensha and Westbury, 1978;Ottoson et al, 1969;Ottoson and Shepherd, 19711, and the rate drop is higher with larger background stretch. In the SAO, a "down-step" follows the abrupt termination of perturbing length 'Ijitter" (Buiio et al, 1978).…”