Parallel reflex and central control of promotor and receptor motoneurons in crayfish

We describe the reflex and central control of an identified motoneuron (rm 1) to a crayfish muscle receptor, the thoracocoxal muscle receptor organ (TCMRO), and compare it with the in-parallel, 'extrafusal' promotor motorneurons. Rm 1 is spontaneously active in an isolated preparation. This activity is modulated in phase with centrally driven promoter nerve activity, suggesting coactivation of promotor and receptor-motor motoneurons. Rm 1 is autogenetically modulated, in a phase-dependent manner, by stretching the TCMRO: during promotor bursts rm 1 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 rm 1 is revealed. This effect is mediated by another identified TCMRO afferent, the statistically 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 rm 1, 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.