Reflexes mediated by non-impulsive afferent neurones of thoracic-coxal muscle receptor organs in the crab,Carcinus maenas

Summary

In the crab's thoracic-coxal muscle receptor (T-C MRO) system, receptor motor (Rm) input to the receptor muscle (RM) results in a variety of effects on the sensory responsiveness, and hence on the reflex output of the system.

• 1. Repetitive stimulation of the Rm nerve with the RM under isometric conditions results in T sensory fibre depolarisation, to levels which can exceed threshold for reflex activation of at least four promotor motoneurones. Both the rate and amplitude of T fibre depolarisation, and the resulting reflex frequencies, increase nearly linearly with stimulus frequency at a given RM length (Figs. 1, 2A).

• 2. A constant frequency Rm input at increasing RM lengths results in an increase in both the rate and the level of T fibre depolarisation: thus, promoter reflex latencies are correspondingly decreased and discharge frequencies enhanced (Figs. 2B, 5A, 6, 7).

• 3. Rm stimulation and applied RM stretch are qualitatively additive with respect to both T fibre depolarisation and reflex promotor activation (Fig. 3).

• 4. The Rm induced reflex can be quantitatively comparable to that evoked by ramp-function stretch, since equivalent amplitude T fibre depolarisations can be effected via these two distinct routes of reflex activation (Figs. 4, 5).

• 5. High frequency Rm input under isometric conditions can also depolarise the in-parallel S sensory fibre, possibly due to non-uniform contractility of the RM along its length. In terms of a contribution to the promotor reflex, however, the relatively small amplitude S fibre depolarisation thus evoked can probably be discounted (Fig. 4).

• 6. A T fibre negative dynamic (hyperpolarising) response to ramp relaxation, often small or absent in de-efferented preparations, is greatly enhanced during concurrent Rm input, assuming a mirror image of the velocity sensitive positive dynamic response. During this hyperpolarisation, the promotor reflex response to the Rm input is partially or completely suppressed (Fig. 3).

• 7. Rm activity increases sensitivity to imposed stretch in a dramatic and long-lasting manner. Brief trains of Rm stimuli can substantially lengthen the dynamic components of the sensory and reflex responses to a later stretch stimulus. The effect can be seen even in the absence of any overt sensory response to the conditioning trains, and is dependent on train frequency and duration. An optimal effect requires a minimal 1 s delay between conditioning train and test stretch (Figs. 9, 10).

• 8. Concurrent, spontaneous bouts of activity are observed in promotor and Rm motoneurones, indicating a common central drive (Fig. 8).