Parallel Reflex and Central Control of Promotor and Receptor Motorneurons in Crayfish

We describe the reflex and central control of an identified motorneuron (rml) to a crayfish muscle receptor, the thoracocoxal muscle receptor organ (TCMRO), and compare it with the in-parallel, ‘extrafusal' promotor motorneurons. Rml is spontaneously active in an isolated preparation. This activity is modulated in phase with centrally driven promotor nerve activity, suggesting coactivation of promotor and receptor–motor motorneurons. Rml is autogenetically modulated, in a phase-dependent manner, by stretching the TCMRO : during promotor bursts rml is excited by dynamic stretch, but during remotor bursts it is inhibited by the same stimulus. This effect is mediated by a single, identified TCMRO afferent, the dynamically sensitive T-fibre. At or near maximally stretched lengths of the TCMRO a tonic inhibition of rml is revealed. This effect is mediated by another identified TCMRO afferent, the statically sensitive S-fibre. The thoracocoxal chordotonal organ is a non-muscular receptor spanning the same joint but signalling the opposite direction of movement. Dynamic movement stimulation of this receptor also excites rml, a reflex that could counteract TCMRO slackening. These results demonstrate a complex central and reflex control of the TCMRO, which could regulate reflex gain throughout the step cycle during walking in the intact animal.