Muscle Spindles in Phasic and Tonic Muscle.

Comparisons were made of muscle spindle structure between phasic and tonic muscles stained by the gold chloride method. The most significant difference was in the ring diameter of the sensory nerve terminals wrapped around the nuclear bags. In tibialis anterior it was largest, in gastrocnemius intermediate, and in soleus smallest. This order corresponds to the contraction time, tibialis anterior being the fastest and soleus the slowest of these muscles. Since the nuclear bag fiber is responsible for the phasic component of muscle spindle discharge, and a large ring diameter of its sensory nerve terminals would be more phasic than a small one, the discharge of spindles in tibialis anterior should be more phasic than that from soleus during an identical rapid stretch.