Mechanical and morphological properties of chronically inactive cat tibialis anterior motor units

1. The lumbar spinal cord was functionally isolated in ten cats by cord transection at the junctions of segments T12-T13 and L7-S1 and cutting bilaterally all dorsal roots between the two transections. Two 24 h EMG recording sessions were used to verify that muscles in the lower limb were virtually electrically silent. The cats were maintained in excellent health for 6 months. 2. Six months after spinal cord isolation, an acute experiment was performed to isolate a single motor unit from the tibialis anterior of each hindlimb using ventral root splitting techniques. Each motor unit was characterized physiologically as either fast fatigable (FF, n = 11), fast fatigue resistant (FR, n = 4), fast intermediate (FI, n = 2), or slow (S, n = 1), and repetitively stimulated to deplete the motor unit of its glycogen. 3. Maximum tensions of the fast motor units were lower than mean maximum tensions of control, whereas the S motor unit remained within the range observed in controls. In general, the isometric contractile properties, as well as fatigability, were within the ranges for each of the motor unit types in control cats. The mean fibre cross-sectional areas of the fibres within the FR and FF motor units were approximately 40 and 50% smaller than control, while the mean fibre size of the fibers within the S motor unit was similar to control. 4. Innervation ratios and specific tensions for all experimental motor units were within the ranges of those reported for tibialis anterior motor units in control cats. Thus, it appears that the decrease in maximum tension of the fast motor units was primarily related to a reduction in fibre size. 5. The spatial distribution of the fibres within fast motor units of a spinally isolated cat, as measured by interfibre distances of the motor unit fibres, was similar to that reported for control tibialis anterior motor units. 6. These data suggest that factors independent of activity play a prominent, if not dominant, role in maintaining the complement of motor unit types typical of adult cat muscles. In addition, normal innervation patterns appear to be maintained in the absence of activity.