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Immobilization leads to reduced stretch reflexes but increased central reflex gain in the rat.

Plastic adaptations are known to take place in muscles, tendons, joints and the nervous system in response to changes in muscle activity. However, few studies have addressed how these plastic adaptations are related. Thus, this study focuses on changes in the mechanical properties of the ankle plantarflexor muscle-tendon unit, stretch reflex activity and spinal neuronal pathways in relation to cast immobilization. The left rat hindlimb from toes to hip was immobilized with a plaster cast for 1, 2, 4 or 8 weeks followed by acute electrophysiological recordings to investigate muscle resistance and stretch reflex torque. Moreover, additional acute experiments were performed after 4 weeks of immobilization to investigate changes in the central gain of the stretch reflex. Monosynaptic reflexes (MSR) were recorded from the L4 and L5 ventral roots following stimulation of the corresponding dorsal roots. Rats developed reduced range of movement in the ankle joint 2 weeks after immobilization. This was accompanied by significant increases in the stiffness of the muscle-tendon complex at 4 and 8 weeks following immobilization. Stretch reflexes were absent in all rats at 4-8 weeks following immobilization. This was associated with increased central gain of the stretch reflex. These data show that numerous inter-related plastic changes occur in muscles, connective tissue and the central nervous system in response to changes in muscle use. The findings provide understanding of coordinated adaptations in multiple tissues and have important implications for prevention and treatment of the negative consequences of immobilization following injuries of the nervous and musculoskeletal systems.