Dual reflex motor control of non-spiking crab muscle receptor

Abstract

The thoracic-coxal muscle receptor organ (T-C MRO) in crabs mediates concurrent reflex excitation (positive feedback) and inhibition (negative feedback) of the one of its two motoneurones with the greater contractile effect on the receptor muscle (RM), viz. the specific receptor motoneurone, Rm1 (analogous to mammalian γ motoneurones). 1. In most preparations the tonic firing frequency of Rm1 declines progressively with increasing RM length over the physiological range (Figs. 1–3). 2. Selectively ablating the displacement sensitive afferent, the S fibre, or damaging its membrane, reverses this RM length-dependent tonic decline, and enhances the dynamic Rm1 response to receptor stretch (Fig. 4). 3. Conversely, ablation of the tension-sensitive T fibre abolishes, and T fibre membrane damage reduces, the Rm1 reflex response to receptor stretch. 4. Intracellular injection of constant current pulses confirms the excitatory drive of Rm1 by the T fibre and the inhibitory influence of the S fibre. Thus while depolarisation of the T fibre excites Rm1, S fibre depolarisation inhibits it; conversely hyperpolarisation of the T fibre abolishes ongoing tonic activity whereas S fibre hyperpolarisation (at extended RM lengths) enhances tonic activity of Rm1 (Figs. 5, 6). 5. Ramp-function 1 mm stretches of constant velocity applied at different RM lengths over the full in vivo range of 5 mm result in invariant Rm1 dynamic reflex frequencies. The dynamic positive feedback reflex is thus stabilized with respect to RM length (Fig. 8). 6. Experiments involving current-induced S fibre hyperpolarisation concurrent with rampfunction stretch stimuli, demonstrate unequivocally the inhibitory role of the S fibre in effecting this dynamic reflex stabilization (Fig. 9). 7. The small dynamic sensitivity of the S fibre (Table 1) does not significantly affect the reflex expression of T fibre dynamic responsiveness, so that Rm1 reflex output remains a direct function of the logarithm of stretch velocity at any given initial RM length (Figs. 7, 9).