REFLEX REGULATION OF PRIMARY (ANNULOSPIRAL) STRETCH RECEPTORS VIA GAMMA MOTONEURONS IN THE CAT

Abstract

Reflex regulation of primary (annulospiral) stretch receptors by gamma motoneurons has been studied by recording the discharges of single end organs during stimulation of peripheral nerves in the ipsilateral hind limb of the cat. Single shocks applied to skin and muscle nerves did not alter the rate of discharge significantly, but repetitive stimulation at 50-100/sec caused prompt acceleration or deceleration of firing. By limiting the intensity of stimulation these effects could usually be elicited without accompanying contraction of the extrafusal muscle fibers, hence they may be attributed to reflex excitation or inhibition of gamma motoneurons. The range of frequencies produced by gamma reflexes alone, unsupported by stretch, was large. By stimulation of the sural nerve or the nerve to the lateral gastrocnemius muscle the rate of discharge of end organs in the medial gastrocnemius muscle was transiently increased to levels equivalent to those produced by 1,000 g. loads. By stimulating the tibial or lateral peroneal nerves, receptors in the same muscle could be almost completely silenced for many seconds. Acceleration of primary endings frequently occurred in conjunction with reflex discharge of alpha motoneurons. Examples of opposite effects upon alpha and gamma motoneurons were observed, however, indi-cating that these two kinds of motoneurons are to some extent served by different spinal mechanisms functioning independently. As a part of the control observations the tensions produced by 13 single motor units in the medial gastrocnemius muscle were measured by direct methods. The maximal tetanic tensions ranged from 0.7 to 55.4 g.