Local inhibitor of the crayfish telson-flexor motor giant neurons: morphology and physiology

Summary

• 1. The motor circuits that control telson flexion in the crayfish (Procambarus clarkii) include a curiously arranged subcircuit: a premotor ‘command’ neuron excites a motor neuron via a trisynaptic pathway, but also inhibits (and prevents firing of) the motor neuron via a shorter latency pathway (Kramer et al. 1981a). The premotor and motor neurons in this circuit have been previously identified (Kramer et al. 1981a; Dumont and Wine 1985a, b; see Fig. 1). We have now identified a local interneuron that inhibits the motor neurons.

• 2. The cell we studied is called the ‘C’ cell because of its distinctive structure (Figs. 2, 3). A single pair of bilaterally homologous C-cells was found in the last (6th) abdominal ganglion. The C-cells are invariably dye coupled to one another following injections of lucifer yellow into either one of them, and are frequently dye coupled to smaller axons in the 2nd, 3rd, and 6th nerves. In addition, some of the extensive branches of the C-cell extend out into the 6th nerve, where they are in close proximity to the axons of the motor neurons they inhibit (Fig. 3).

• 3. Two kinds of evidence established that the C-cell directly inhibits the motor neurons. First, when simultaneous recordings were made from the C-cell and the motor neurons, spikes in the C-cell, no matter how evoked, were invariably followed, within 1.5 ms, by depolarizing IPSPs in the motor neuron (Fig. 6). Second, when the C-cell was hyperpolarized so that it could not fire, that same IPSP in the motor neuron was abolished (Fig. 6).

• 4. The inhibitory pathway to the motor neurons must be fired at short latency in order to prevent firing caused by the trisynaptic excitatory input (Fig. 1). The C-cells were fired at short latency (<3 ms) by impulses in either of the escape command cells (Fig. 4), and at even shorter latency by impulses in the Segmental Giant of the 6th ganglion (SG₆) (Fig. 5). It has been established elsewhere that the SGs are a major output pathway of the escape command cells; our results suggest that they may be the pathway for commandevoked firing of the C-cell.

• 5. The C-cells are also excited by two descending, non-giant, flexion premotor neurons, called I2 and I3 (Fig. 5). The EPSPs from a single I2 or I3 impulse were subthreshold, but temporal and spatial summation of EPSPs from the non-giant pathway sometimes fired the C-cells.

• 6. The IPSP produced by a single C-cell impulse was only a small proportion (<15%) of the total IPSP produced by command cell firing. However, each motor neuron received IPSPs from both C-cells, and the C-cells typically fired multiple spikes. The remainder of the compound IPSP produced by the command cells consisted of summated IPSPs from other inhibitory interneurons, including interganglionic ones (Dumont and Wine 1985a, b).

• 7. The identification of a major local inhibitory neuron should help in the interpretation of the functional significance of this unusual subcircuit.