Discharge Patterns in Human Muscle Spindle Afferents during Isometric Voluntary Contractions

Abstract

Unitary nerve impulses were recorded from the median nerve of waking human subjects with percutaneously inserted tungsten electrodes. Activities from 63 muscle spindle endings were studied during isometric voluntary contractions. Three main types of response patterns were encountered: a sustained increase of the discharge, a sustained decrease, and a weak discharge consisting of a few impulses. It was concluded that the sustained increase of the discharge was accounted for by an increase of the fusimotor activity, whereas the sustained decrease was accounted for by mechanical unloading. The weak discharge might be accounted for by either mechanical or fusimotor mechanisms. Fusimotor activation concomitant with the skeletomotor activity could be demonstrated for 81% of the spindle afferents. One particular response pattern was not characteristic for certain endings but many units responded with different patterns depending upon the intensity and spatial extent of the contraction. Analyses of the unitary responses to the contraction of different muscle portions indicated that there was a close spatial correspondence in the muscle between the skeletomotor activity and the fusimotor activity.