Jc Rothwell

The Stretch Reflex: Human Spinal and Long-Loop Reflexes

Publisher Summary When the tendon of a healthy muscle is drawn upon by an antagonistic muscle or by the manipulation of the investigator or by the movement of a joint in response to gravity, the muscle actively resists the extending force. The resistance from a muscle in full connexion with the nervous system is a reflex contraction, the “stretch reflex.” The classical mechanism of the stretch reflex involves excitation of primary endings of muscle spindle stretch receptors whose activity causes, via a monosynaptic spinal pathway, a reflex discharge of motoneurons connected to that muscle and to other synergistic muscles. This chapter discusses the origins and functions of long-latency stretch reflexes that have now been discovered. Long-latency responses to muscle stretch that occur in certain human muscles are truly automatic, reflex, responses that cannot be explained on the basis of known spinal circuitry. However, in certain circumstances, they exert powerful and important collaborative servo assistance to movement in man. The chapter presents the responses of human biceps to stretch in circumstances similar to those used to study the long thumb flexor. The main difference in the responses from biceps compared with those from the long thumb flexor is the presence in the former of a large spinal-latency component that is absent or very small in the long thumb flexor. The peak velocity of biceps stretch, in terms of angular rotation of the joint, is much less than that for the thumb. The difference in the distance between the point of muscle insertion and the center of rotation of the joint for biceps on one hand and flexor pollicis longus on the other hand makes the rate of linear muscle stretch comparable.