RESPONSES OF NUCLEUS Z NEURONS TO VIBRATION OF HINDLIMB EXTENSOR MUSCLES IN DECEREBRATE CAT

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  • 1 January 1975
    • journal article
    • research article
    • Vol. 113  (2) , 150-187
Abstract
Small sinusoidal changes of length, applied longitudinally to the deefferented GS [gastrocnemius-soleus] muscle, were used as a stimulus to activate the muscle spindle receptors in precollicular decerebrate cats. To determine the relative contribution of the primary and secondary endings of muscle spindles to the response of the nucleus z neurons, the effects of muscle vibration on this unit activity were studied under conditions in which the segmental monosynaptic reflexes produced by stimulation of the primary endings of muscle spindles were continuously monitored. Vibration of the GS muscle affected the frequency and pattern of discharge in 50 of 168 units recorded from the lower medulla. Histological controls indicated that these responsive units were located within the nucleus z of Brodal and Pompeiano. Among these units, 6 could be attributed to ascending spinal afferents to nucleus z. These units followed frequencies of vibration up to 200/s, and they responded with a mean latency of 2.53 ms with respect to the beginning of the segmental monosynaptic reflexes induced by the same stimulus. Since the low threshold muscle afferents monosynaptically excite both the spinal motoneurons and the neurons from which the ascending axons to nucleus z originate, the calculated mean latency of 2.53 ms represents the conduction time of the proprioceptive volleys along the ascending pathway from the lumbar cord to the nucleus z (corresponding to a mean conduction velocity of ascending fibers of about 100 m/s). The remaining 44 units were attributed to cell bodies located in nucleus z. These neurons responded to muscle vibration with excitation or with excitation followed by inhibition, while only 1 neuron responded with inhibition. In most of the ascending spinal afferents to nucleus z and in relay neurons, the threshold amplitude of vibration was between 10-35 .mu.. An increase in the amplitude of vibration increased the unit response. The resulting stimulus-response curve closely paralleled the curve of development of the segmental monosynaptic reflex. This effect was attributed to stimulation of the primary muscle spindle endings. In some units the magnitude of the response increased for amplitudes of vibration higher than those responsible formaximum development of the segmental monosynaptic reflex, indicating a contribution of the secondary muscle spindle endings to the unit response. Only in 2 instances were the threshold amplitudes of vibration responsible for these unit discharges supramaximal for the primary endings, indicating that the secondary endings were entirely responsible for the responses. Some of the nucleus z neurons responsive to muscle vibration were also submitted to both static and dynamic stretches of the GS muscle. The units responded with a burst of high frequency discharges to the dynamic stretch of the muscle, but they were not significantly modified during static stretch of the muscle from 0-8 mm of extension. The nucleus z neurons probably can be influenced by synchronous discharges of the primary and/or secondary endings of muscle spindles elicited by vibration and by dynamic stretch of hindlimb extensor muscles.