Abstract
This chapter discusses the early investigations on the depolarization of the central terminals of primary afferent fibres and reviews the recent investigations that establish that this depolarization is responsible for the depressed synaptic effectiveness of the primary afferent fibres. The chapter focuses on the investigations on presynaptic inhibition of the Group Ia afferent fibres from muscle, but it is observed that there is a comparable presynaptic inhibition for Group Ib and for cutaneous afferent fibres. Volleys in Group Ia afferents from flexor muscles are shown to produce a prolonged depression both of the monosynaptic excitatory postsynaptic potential (EPSP) evoked by Ia afferent volleys and of the monosynaptic reflex discharge. Presynaptic inhibitory action on Group Ia afferent fibres is produced almost entirely by Group I volleys from flexor muscles. With Group Ib afferent fibres, the effective inputs are Group Ib and II volleys from all muscle nerves and to a less extent from cutaneous volleys.
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