Dual reflex motor control of non-spiking crab muscle receptor: II. Reinforcement of Rm1 mediated positive feedback by dual afferent excitation of Rm2

Stretching the thoracic-coxal muscle receptor organ (T-C MRO) of the crab, Carcinus maenas , depolarises its two non-spiking afferents (the S and T fibres), and reflexly excites its own two receptor motoneurones, Rm1 and Rm2. This "autogenic" reflex of the latter, thinner efferent axon, which has a smaller contractile effect than Rm1 and, unlike Rm1, also innervates the ("extrafusal") promotor muscle (and is thus analogous to mammalian beta motoneurones), is analysed in this paper and compared with that of Rm1. Whereas Rm1 almost invariably responded to receptor muscle (RM) stretching, Rm2 was reflexly activated in only 32% of preparations in this study. Generally this Rm2 reflex was limited to the positive dynamic (i.e. lengthening) phase of a 1.0 mm "trapezoidal" stimulus (constant velocity stretch, hold, release). This dynamic response of Rm2 increases with RM length above a variable threshold, in marked contrast to the relatively constant dynamic reflex of Rm1 throughout the anatomical length range (Fig. 1). In exceptional cases Rm2 reflex activity continues into the static phase of the stretch stimulus, high dynamic frequencies (> 100 impulses s super(-1)) being attained at the more extended RM lengths.