ISSN:
1432-1351
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Medicine
Notes:
Summary 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’) promoter muscle (and is thus analogous to mammalian β motoneurones), is analysed in this paper and compared with that of Rm1 (Cannone and Bush 1981 a–c, 1982). 1. 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 1.0mm ‘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). 2. In exceptional cases Rm2 reflex activity continues into the static phase of the stretch stimulus, high dynamic frequencies (〉 100 impulses s−1) being attained at the more extended RM lengths (Fig. 2). 3. Rm2 is more vulnerable to a time-dependent decline in synaptic efficacy within the reflex arc than is Rm1. This may account for the lack of reflex responsiveness of Rm2 in two thirds of the preparations in this study (Fig. 3). 4. Depolarising, constant current pulses injected into either the T or the S sensory fibre elicit reflex discharge of Rm2, at a frequency which varies directly with stimulus intensity; this establishes the dual afferent excitation of Rm2. In contrast, Rm1 is excited by the T fibre but inhibited by the S fibre, a difference which may underly the comparative immunity of the Rm1 reflex to the temporal decline noted above for Rm2 (Figs. 4, 5). 5. The relative contributions of the T and S fibres to the stretch evoked Rm2 reflex varies in different preparations, and is also dependent upon the parameters of the stimulus within any one preparation (Figs. 6, 7). 6. The reflex response of Rm2 is commonly more sensitive to stretch velocity than is that of Rm1, with Rm2 dynamic frequencies exceeding Rm1's at the higher velocities (5–10 mm s−1). In common with promoter motoneurones, both Rm1 and Rm2 frequencies vary as the logarithm of stretch velocity, reflecting the dynamic response characteristics of the T rather than the S fibre (Fig. 8). 7. In vivo, Rm2 is conceivably tonically active throughout the RM length range. This conclusion is strengthened by the evidence referred to in paragraph 3 above (Fig. 9). The major features of the reflex efferent control of the T-C MRO described in this and previous papers are summarised. Analogies drawn with the mammalian muscle spindle, and a novel role for Rm2, are also discussed.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00612585
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