PROPERTIES OF FAST AND SLOW MOTOR UNITS IN HIND LIMB AND TAIL MUSCLES OF THE RAT

Abstract

The nerve conduction velocities and motor unit contraction properties of forty‐one hind limb motor units (soleus, 37; extensor digitorum longus, 4) and twenty‐nine motor units from the mid‐tail lateral segmental muscles of the rat were measured. The motor units of the soleus were found to form an homogeneous group as judged by contraction time with just one particularly slowly contracting unit. In the soleus muscle a negative correlation between conduction velocity of the nerve fibre and contraction time of the motor unit was found but no significant correlation between conduction velocity of the nerve fibre and the tetanic tension of the motor unit. The motor units of the tail muscleswere found to form two distinct groups, one of fast units and the other of slow units. In the tail muscles, the slowly conducting motor nerves innervated the slowly contracting units and the fast conducting nerves the fast contracting units. A relationship between the tetanic tension of the tail muscle motor units and nerve conduction velocity was demonstrated. © 1972 The Physiological Society