Two identified afferent neurones entrain a central locomotor rhythm generator

Sensory feedback can modulate the intensity and timing of the central rhythms underlying locomotion and adjust them to compensate for natural or experimental perturbations1-4. However, because of the complexity of the neural systems involved, the role of specific sense organs and the function of individual afferent neurones are poorly understood. The thoracic-coxal muscle receptor organ (TCMRO), a proprioceptor of the crayfish walking leg, has only two afferent neurones, the non-spiking S and T fibres. Their receptor potentials encode, respectively, the magnitude and velocity of receptor muscle stretch, which occurs during limb remotion (retraction). Rhythmically stretching the TCMRO entrains a central locomotor rhythm, produced by the thoracic ganglia, in which remotor and promotor motoneurones of the leg discharge in alternation. Intracellular stimulation of the S and T fibres can trigger promotor and remotor bursts, respectively. Here we propose a mechanism for proprioceptive entrainment in terms of the opposite feedback effects of these two afferents. The possible role of these effects in the feedback control of locomotion is also discussed.