Role of long latency responses to stretch during control of posture and prior to movement onset

Stretch reflex constitutes one of the simplest responses of the nervous system. The stretch reflex is thought to consist of a monosynaptic, Group Ia mediated short latency response and a long latency response of more complex origin. Commonly, stretch reflexes (especially the long latency responses) are thought to contribute to limb stability. To this end, we see excitation of a lengthened muscle and inhibition of a shortened muscle. However, this behavior is not universal. During maintenance of arm posture, perturbations may elicit excitation in a shortened muscle (shortening reaction) and inhibition in a lengthened muscle (lengthening reaction). Before voluntary movement onset, the long latency response to perturbation assists the movement rather than opposes the imposed perturbation. Both these instances involve long latency reflexes that would appear incongruent with a stabilizing response.;Our first study was designed to determine the prevalence of the shortening and lengthening responses following multijoint perturbation of arm posture. Surface electromyograms were used to record activity from eight muscles. Joint angles tracked using an optical system were used in a biomechanical model to determine the corresponding muscle length changes. Of all responses, 20% were shortening responses. Also, the shortening response amplitudes increased with the increase in the voluntary bias requiring antagonist activity. The results suggest that Group Ib pathways might mediate these responses.;The second study was designed to determine if the movement-dependent modulation of the long latency reflex can be explained by an early release of voluntary motor program, akin to the early release evoked by a startling acoustic stimulus. This was tested in the triceps brachii, and the results confirmed our hypothesis. The brainstem is thought to be the origin of this early release response, based on similarity with the acoustic startle response. The last study examined relative contributions from movement- and perturbation-dependent components to the long latency reflex during the transition from posture to movement. The results indicate that they both contribute, but contributions of the movement-dependent component increases going from posture towards movement.;Our results show that the response to perturbation evokes contributions from multiple neural pathways that express task and instruction dependent modulation.