Chapter 13 A Positive Feedback Circuit Involving Muscle Spindle Secondaries and Gamma Motoneurons in the Cat

Abstract

Publisher Summary Many negative feedback neuronal circuits are known in the mammalian nervous system. Negative feedback is represented, for example, by recurrent inhibition of motoneurons through their axon collaterals and Renshaw cells, by recurrent inhibition of several other neurons, or by autogenetic inhibition of motoneurons following contractions of the muscles that they innervate. Examples of a clear-cut positive feedback are more difficult to find; in a recent review on proprioceptive feedback and movement regulation, positive feedback was mentioned only once and only in the context of force feedback. An effective positive feedback would require the connection between muscle spindle afferents and γ-motoneurons to be as direct as possible, preferably to be monosynaptic. Previous studies led to the conclusion that group II afferents excite γ-motoneurons only through di-, tri-, or even polysynaptic pathways because of the rather long latencies of their actions. Latencies of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) evoked in α-motoneurons by electrical stimulation of group II afferents in muscle nerves were similarly classified as polysynaptically evoked. A number of subsequent observations have, however, shown that at least some synaptic actions of group II muscle afferents on α-motoneurons might be evoked di-synaptically or even mono-synaptically.