C Fromm — Page 2 | PubMed Health

PubMed1976

Inhibition of extensor gamma motoneurons by anatagonistic primary and secondary spindle afferents.

About 2/3 of the gamma efferents isolated from the medial gastrocnemius nerve were inhibited by longitudinal high-frequency vibration applied to the tendons of the non-contracting pretibial flexors (decerebrate cats). The inhibition appeared at 15-25 mum amplitude of vibration and increased up to maximum at nearly 100 mum. Increasing the frequency of vibration from 100 to 300 Hz increased the inhibition. The reflex effects elicited by muscle vibration corresponded well in incidence and magnitude with those evoked by tetanization of the deep peroneal nerve at group I stimulus strength. The reflex disappeared when the nerve supply of the vibrated muscles was cut. The sensitivity of some pretibial proprioceptors to vibration was also tested. It is concluded that primary spindle endings of the pretibial flexors inhibit the extensor gamma motoneurons. Some findings hint at a spinal pathway involving Ia inhibitory interneurons. In addition, an inhibitory action of pretibial group II afferents, probably secondary spindle endings, on extensor gamma efferents was demonstrated. The described fusimotor inhibition by antagonistic muscle spindle afferents is a further example of alpha-gamma-linkage.

Animals↗

PubMed1976

Reflex responses of gamma motoneurones to vibration of the muscle they innervate.

1. High frequency vibration was applied to the tendon of the non-contracting triceps surae muscle while recording the background discharges of single gamma fibres only small nerve bundles were cut, leaving most of the nerve supply to the triceps intact. 2. 22% out of a total of sixty-three gamma efferents were tonically inhibited by vibration. The inhibition appeared between 25 and 50mum peak-to-peak amplitude of vibration and increased to a plateau for amplitudes of about 100mum. The dependence of the tonic vibration reflex of alpha-efferents on the amplitude of vibration was found to be similar. Increasing the frequency of vibration from 150 to 300 Hz increased the degree of inhibition. 3. 33% of the fusimotor neurones investigated responded to muscle vibration with an increase in discharge rate. The threshold amplitudes of this reflex ranged from 20 to 50mum. Some features of the reflex, in particular the parallel post-vibratory facilitation found in alpha and gamma efferents, pointed to a polysynaptic pathway organized in an alpha-gamma linkage. 4. All gamma efferents inhibited by vibration showed inhibitory responses to antidromic stimulation of the parent ventral root, and most of them were inhibited by ramp stretch of the triceps. The gamma motoneurones facilitated by vibration, however, were excited by muscle stretch and were less susceptible to antidromic inhibition, some lacking it completely. 5. Cutting the nerves to triceps abolished the inhibitory as well as the excitatory responses of gamma efferents to muscle vibration. Both fusimotor reflexes were preserved after spinal section and subsequent administration of L-DOPA. 6. It is concluded that both of the fusimotor reflex effects of vibration are caused by excitation of primary spindle endings within the triceps. The inhibition of fusimotor neurones is thought to be mediated by Renshaw cells activated during vibration. The significance of positive feed-back on to gamma motoneurones as a result of autogenetic facilitation by Ia afferents is discussed in connexion with stability in the stretch reflex loop.

Action Potentials↗

PubMed1975