Parallel Inhibition of the Motor Giant, Segmental Giant and Fast Flexor Neurones of the Hermit Crab

Previous work has shown that the giant fibres (GFs) of the hermit crab make excitatory electrical connections with the motor giant (MoG) and segmental giant (SG) neurones, and that the SGs in turn make connections to fast flexor (FF) motor neurones. In this paper we show that synchronous or almost-synchronous IPSPs can be elicited in all three classes of neurone by electrical stimulation of the connectives or roots. These IPSPs are depolarizing in the MoG and SG, and hyperpolarizing in the FFs. The IPSPs can functionally disconnect the MoG, SG and FF neurones from the GF command. Several interneurones have been found which initiate the IPSPs when driven with injected current. These are referred to collectively as inhibitory driver neurones (IDNs). In some cases IPSPs follow IDN spikes 1:1; in others more than one IDN spike is required to produce a single IPSP.