Heterogeneity and central modulation of feedback reflexes in crayfish motor pool

1. Movement of the crayfish thoracocoxal leg joint is monitored by a muscle receptor organ (TCMRO) and a chordotonal organ (TCCO). Both receptors span the joint in parallel but signal opposite directions of leg movement. The TCMRO is innervated by afferents responsive to lengthening, which corresponds to leg remotion, whereas TCCO afferents are responsive to shortening of the chordotonal strand, which corresponds to leg promotion. 2. When both receptors are stimulated in parallel, in an otherwise isolated preparation, reflex responses of coxal promoter and remotor motor neurons occur on both stretch and release. By comparison with experiments where one or the other of these receptors is stimulated selectively, we conclude that reflexes evoked by stretch of the two receptors are due to the TCMRO and reflexes evoked by release are due to the TCCO. 3. Reflexes mediated by these receptors are both state dependent and phase dependent. In preparations that produce patterns of reciprocal motor activity in promotor and remotor motor neurons (the active state), the reflex effect depends on the phase of this centrally generated activity. In preparations that are quiescent, or that produce only tonic motor output (the inactive state), the reflex effect is stable, corresponding to a typical resistance (negative feedback) reflex for both directions of receptor movement. 4. In the active state, coxal promotor motor neurons are both excited and inhibited in a phase-dependent manner by stretching the TCMRO. A subgroup of promotor motor neurons is excited by shortening the TCCO. One subgroup of the antagonistic coxal remotor motor neurons receives phase-dependent excitation from stretch of the TCMRO, whereas a second subgroup receives phase-dependent excitation from shortening the TCCO. 5. There are, therefore, at least two ways in which reflex effects can be modulated. At the level of a single motor neuron, the reflex response can vary in gain, and in some cases in sign, in a manner depending on centrally generated motor activity. In addition, at the level of a pool of synergistic motor neurons, the reflex effect is not uniform; instead, different subgroups of motor neurons display different reflex effects, so that the relative levels of excitability of different motor neuron reflex subgroups can also determine the net reflex effect. 6. Excitation of promotor motor neurons by TCCO shortening and of remotor motor neurons by TCMRO lengthening are positive feedback reflexes. The subgroups of motor neurons in which positive feedback reflexes can be evoked in both promotor and remotor pools are termed group 1.(ABSTRACT TRUNCATED AT 400 WORDS)