Response to stretch of proprioceptors in adult rat muscles de-efferented at birth.

Hind-limb muscles of new-born rats were de-efferented by removing the lumbosacral spinal cord. Spinal ganglia remained intact, together with their peripheral axon. The presence of sensory terminals in limb spindles, induces the full ultrastructural differentiation of muscle spindles, as has been shown previously. In the present paper we have shown by integrating the sensory discharges in the whole nerve from chronically de-efferented muscles that even several months after birth, muscle proprioceptors (probably mostly spindles) still maintain their basic mechanoreceptor properties. Although the limbs were completely immobilized throughout the whole experimental period, spindles from these chronically de-efferented muscles still responded as slowly adapting receptors. The dynamic component was also present in the integrated neurogram response during stretching. It thus appears that basic functional properties of rat muscle proprioceptor persist even when these receptors differentiate and survive without motor innervation and any adequate functional stimuli, i.e. under conditions of permanent disuse.