The central control of the dynamic response of muscle spindle receptors

The present experiments were performed to search for differences between the effects of intrafusal muscle fibre contraction on the primary and on the secondary endings of muscle spindles. These two kinds of afferent endings are differently arranged with regard to the intrafusal muscle fibres (Ruffini, 1898; Barker, 1948; Boyd, 1961), but in the situations so far examined the effects of intrafusal fibre contraction upon them have appeared to be similar (Hunt, 1954; Harvey & Matthews, 1961 a, b). The behaviour of single endings was studied in decerebrate cats in the hope that the intrafusal fibre contraction maintained physiologically in this preparation as a result of 'spontaneous' fusimotor activity would reveal differences not hitherto demonstrated by electrical stimulation of efferent fibres. The effects of this physiologically maintained activity can be assessed by comparing the responses of individual endings to a standard extension applied before and after de-efferenting the spindles by cutting the appropriate ventral roots (Eldred, Granit & Merton, 1953). The present paper is concerned solely with the effects of intrafusal fibre contraction on the sensitivity of the endings to the dynamic component of a slowly applied stretch (3 mm/sec). The dynamic sensitivity of an ending may be measured by determining the slowing ofits discharge which occurs when the dynamic phase of stretching is completed and the final extension is maintained. In the absence of intrafusal fibre contraction the dynamic sensitivity of secondary endings is appreciably less than that of primary endings (Harvey & Matthews, 1961 b; P. Bessou & Y. Laporte, personal communication); and this has also been shown for innervated spindles in the decerebrate cat (Cooper, 1959, 1961). The present experiments show that intrafusal fibre contraction has little effect on the small dynamic response of secondary endings. The larger dynamic response of primary endings was usually increased by fusimotor activity, as has also been found for the sensitivity of these endings to vibration (Granit & Henatsch, 1956). But the most important observation was that there was no simple

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