[Spinal neural mechanisms in voluntary movements]

Spinal circuits are utilized not only in reflex movements but also in voluntary movements. Their functions are many-fold such as reciprocal inhibition, output control of motoneurones, integration of central and peripheral inputs in motoneurones, or increase in selectivity of muscle contraction etc. The brain controls the activity of spinal circuits depending on the movements required, by controlling the activity of interneurones in the reflex pathways. In the case of the soleus, the activity of reciprocal Ia inhibition and Ib inhibition increases during voluntary contraction of the antagonists. The increase in both inhibition takes place during contraction as weak as 1-2% of the maximum, suggesting that the interneurones can be fired by the descending commands alone without any peripheral inputs. Such central control is beneficial in suppressing stretch reflexes in the antagonistic muscles which follows voluntary movements. On the other hand, the activity of recurrent inhibition of the soleus is enhanced during weak contraction of the muscle, but depressed during medium strength contraction and strongly depressed during strong contraction. Such supraspinal control of Renshaw cells improves the balance between the preciseness in controlling movements and the crude muscle power required. Importance of spinal circuitry during voluntary movement should be emphasized.