The role of muscle receptors in the detection of movements

This review discusses the role of muscle receptors, in particular. that of muscle spindles, in the detection of movements, both passive and active, Em phasis is placed on the importance of conditioning the muscles acting at a joint before making measurements of thresholds to passive movements, to tak e into account muscle's thixotropic property. The detection threshold:movem ent velocity relation is discussed and described for a number of different joints. Implications for muscle spindles are considered from the generalisa tion that, when expressed in terms of proportion of muscle fascicle length change, detection thresholds are about the same at different joints. It is concluded that the available data supports the view that muscle spindles li e in parallel with only a portion of a muscle fascicle and not the whole fa scicle. At the elbow joint, where it has been tested, movement detection th reshold is lower juring passive movements than during contraction of elbow muscles. Both peripheral mechanisms and mechanisms operating within the cen tral nervous system may be responsible for the rise in threshold. The signa lling of movements by spindles during a contraction raises the question of how the central nervous system is able to extract the length signal under s uch circumstances, given that there is likely to be co-activation of alpha and gamma motoneurones. The evidence for a central subtraction of fusimotor -evoked impulses and some recent experiments relevant to this idea are desc ribed. In conclusion, a number of points of uncertainly have been revealed in this area and these should be the subject of future experiments. (C) 199 9 Elsevier Science Ltd. All rights reserved.