The physiological cross-sectional area of motor units in the cat tibialis anterior.

The physiological cross-sectional area (CSA) of a motor unit (MU), taken as the sum of fiber areas measured on a single section through the approximate midlength of the MU, has been compared with the physiological CSA more strictly defined as the sum of the maximal areas to be found anywhere along the length of each of the MU fibers. The CSA at intervals along the fiber length was measured in fibers selected from four glycogen-depleted, isolated MUs in the cat tibialis anterior (TA), and profiles of the summed areas made. In one MU, measurements were also taken on all the MU's fibers at less frequent intervals. The profiles demonstrate that the summed CSA based on each fiber's maximum CSA may exceed that derived from observation on any single section by as much as 20%. As a consequence, values that have been reported for specific tension (force per unit area) of MUs in the TA and probably other muscles may have been overestimated, especially for those MUs of fast type. Estimates were also made of the share of the MU's total force transmitted directly to the tendons of origin and insertion via endings of the blunt musculotendinous type as distinct from tapering intrafascicular endings acting through in-series connective tissue and non-MU fibers. In two MUs of slow type in which most fibers ran from tendon to tendon, "partial tapering" extending over 1 cm of the fiber length accounted for a third of the total physiological CSA, and indicated yet another mode for relay of the MU's force to the tendon.