Disuse enhances synaptic efficacy in spinal mononeurones.

Abstract

Monosynaptic excitatory post-synaptic potentials (EPSP) were recorded from triceps surae motoneurons in the cat following section or chronic conduction block of the medial gestrocnemius (MG) nerve with tetrodotoxin (TTX) or after daily stimulation of the sciatic nerve. The mean maximum amplitudes of homonymous and heteronymous monosynaptic EPSP evoked by stimulation of the MG nerve were reduced significantly between 1 and 2 wk after section of the muscle nerve. The mean amplitudes of monosynaptic EPSP produced in the same motoneurons by afferent volleys from the intact synergists showed no significant alterations. Reduction of the amplitude of monosynaptic EPSP evoked by the sectioned MG afferent volleys was not prevented by daily stimulation of the sciatic nerve. The chronic stimulation of the sciatic nerve did not increase the amplitude of monosynaptic EPSP evoked by stimulation of the intact, lateral gastrocnemius (LG) or soleus nerve. Chronic conduction block of the intact MG nerve with TTX cuffs for 2 wk resulted in a significant increase in the homonymous EPSP amplitude. The amplitude of the heteronymous EPSP evoked in the same MG motoneurons by stimulation of the intact LG or soleus nerve showed no significant changes. Decreased central synaptic transmission following section of the peripheral nerve is not due to elimination of impulse activity (dissue) of the sensory input and that prolonged disuse of the sensory fibers causes an increase, rather than a decrease, in central synaptic efficacy.