Changes in muscle spindle activity of the chronically de-efferented gastrocnemius of the rat.

Abstract

The response to stretch of chronically de-efferented muscle spindles was studied in the rat gastrocnemius muscle 6 days (short-term) and 48 days (long-term) after ventral rhizotomy respectively. Chronic de-efferentation enhances both the dynamic and static component of the response. There was no difference between the short-term and long-term de-efferentation, i.e. the response to stretch was as high several days after de-efferentation as after 1–2 months, when the muscle had undergone considerable atrophy. The response of both spindle primaries and secondaries was enhanced, but more so in the latter group. This suggests that the probable site of these changes after ventral root section lies in the polar regions. Early adaptation of de-efferented group I endings remained unaltered throughout the whole experimental period. Group II and III afferents, on the other hand, exhibited less adaptation than those from control muscles. Fibrillation activity of intrafusal muscle fibres does not seem to be involved, since quinidine sulphate in doses suppressing fibrillations in extrafusal muscle fibres does not eliminate the enhanced response. Ultrastructural changes at the site of sensory nerve terminals on the receptor cells of the spindle caused by chronic de-efferentation are probably responsible.