Contraction time, histochemical type, and terminal cisternae volume of rat motor units.

Isometric contraction time (Tc) of 19 fast and slow rat motor units in the soleus and the tibialis anterior muscles was measured. The motor unit fibers subsequently marked by glycogen depletion were histochemically fiber typed as well as analyzed ultrastructurally with respect to volume of the sarcoplasmic reticulum terminal cisternae. The volume density of terminal cisternae (VVTC) was inversely related to Tc over the whole range of motor units, irrespective of type of muscle. The continuous variations in VVTC are concluded to match the extrinsic control of time course of contraction, and fusion frequency of the motor unit to match the frequency characteristics of the individual motoneurons. The volume density dependence of terminal cisternae function would result in an indirect coupling between amount and rate of calcium release and rate of calcium recaptured. VVTC and Tc were the same for some tibialis anterior and soleus motor units in spite of the different types of myosin, indicating that the type 1 and type 2 myosins have specific structural differences in fast as compared with slow muscles.