FUSIMOTOR ACTIVITY AND THE TENDON JERK IN THE ANESTHETIZED CAT

This is a study of the tendon jerk reflex elicited by a brief stretch applied to the triceps surae muscle group in the chloralose-anaestheti sed cat. The size of the recorded reflex depended on stretch parameter s (optimum at 300 mum amplitude at a rate of 100 mm/s) and on how the muscle had been conditioned. A reflex elicited after a conditioning co ntraction at the test length was often twice as large as after a contr action carried out at a length longer than the test length. This diffe rence was attributed to the amount of slack introduced in the intrafus al fibres of muscle spindles by conditioning. The question was posed, did ongoing fusimotor activity exert any influence on the size of the tendon jerk? Depolarization indices (DPI) were calculated from respons es of muscle spindles to stretch and correlated with the level of refl ex tension. Values of DPI obtained from afferent responses with and wi thout repetitive stimulation of identified fusimotor fibres suggested that with the stretch parameters used here the main influence of fusim otor activity was that it removed any pre-existing slack in muscle spi ndles and thereby increased reflex tension. In the absence of intrafus al slack, stimulation of static and dynamic fusimotor fibres had littl e additional influence on the size of the reflex. It is concluded that much of the variability typically seen with tendon jerks is due to mu scle history effects. Since in muscles which have not been deliberatel y conditioned there is commonly some slack present in spindles, activi ty in fusimotor fibres is likely to reduce slack and therefore increas e reflex size.