Neuronal control of leech swimming movements: interactions between cell 60 and previously described oscillator neurons

Summary

A recently discovered member of the neuronal oscillator underlying swimming movements in the medicinal leech,Hirudo medicinalis, is described. This interneuron, named cell 60, exhibits membrane potential oscillations that are phase-locked to the swim oscillations observed in other oscillator neurons (phase angle, approximately 220°) and, when depolarized, acts to shift the phase of the swim oscillations. The soma of cell 60 lies near the posterior-lateral margin on the ventral aspect of most (and possibly all) segmental ganglia. The neurite crosses the midline, then turns anteriorly and projects into the lateral intersegmental connective. Cell 60 is connected to cell 28, a previously described dorsal swim oscillator neuron, via an electrically rectifying junction.

Two interactions link cell 60 with cell 208, a swim oscillator neuron found on the ventral aspect of segmental ganglia: a short-latency, fatiguing inhibitory synapse and a powerful electrical interaction. The electrical interaction acts as a diode, in that current can pass from cell 60 to cell 208, but not in the reverse direction. The coupling coefficient in the forward direction is about 0.5 and is independent of the membrane potential difference between cells 60 and 208 provided that the ‘diode’ connection is forward biased.

The rectifying junction acts as a switch which is ‘off’ during swimming activity because cell 208 oscillations are superimposed on a tonic depolarization of about 10–15 mV. This tonic potential reverse biases the electrical ‘diode’ connection between cell 60 and cell 208, leaving the inhibitory synapse as the only effective interaction between these cells. The diode switch is ‘on’ when cell 208 is hyperpolarized. In this circumstance, the dominant connection is electrical; therefore induced potential oscillations in cell 60 induce in-phase oscillations in cell 208.