Properties of synapses made by IVN command-interneurones in the stomatogastric ganglion of the spiny lobster Panulirus interruptus.

1. The IVN command interneurones synapse directly onto 11 identified neurones in the stomatogastric ganglion: the two pyloric dilators (PDs), the anterior burster (AB), ventricular dilator (VD), the four gastric mill neurones (GMs), the two lateral posterior gastric neurones (LPGNs), and Interneurone I (Int 1). 2. The IVN p.s.p.s in PD and AB are biphasic, and consist of a fast depolarizing component followed by a slower hyperpolarizing component. 3. The hyperpolarizing component of this biphasic postsynaptic potential is inhibitory, and appears to be the result of a conductance increase to K+ and Cl-. 4. The IVN p.s.p. in VD is excitatory and can drive VD one-for-one. 5. The IVN p.s.p.s in GM and LPGN are inhibitory. The amplitude of a single p.s.p. is small but, at high frequency, summation of p.s.p.s holds the postsynaptic membrane potential below threshold. 6. The facilitation characteristics of the p.s.p.s in each neurone are described. 7. The functional significance of these synaptic characteristics is discussed in terms of the modification of motor output caused by a burst of the IVN interneurones.