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 wk followed by acute electrophysiological recordings to investigate muscle stiffness and stretch reflex torque. Moreover, additional acute experiments were performed after 4 wk 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 wk after immobilization. This was accompanied by significant increases in the stiffness of the muscle-tendon complex as well as an arthrosis at the ankle joint at 4 and 8 wk following immobilization. Stretch reflexes were significantly reduced at 4-8 wk following immobilization. This was associated with increased central gain of the stretch reflex. These data show that numerous interrelated plastic changes occur in muscles, connective tissue, and the central nervous system in response to changes in muscle use. The findings provide an 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.NEW & NOTEWORTHY Immobilization leads to multiple simultaneous adaptive changes in muscle, connective tissue, and central nervous system.