Spindle representation relative to distribution of muscle fiber types in the cat capsularis muscle

The spatial representation of muscle spindles (Sps) in the small (approximately 0.2 g), simply structured capsularis muscle that crosses anterior to the cat's hip joint was compared with the distribution of the slow oxidative (SO) and few (< 10%) fast oxidative-glycolytic (FOG) fibers of which it is composed to see if their distributions were consistent with a hypothesis that sensory input from Sps influences the incidence of extrafusal fiber types. In frozen sections from 4 muscles, FOG fibers were enumerated along 1-mm strips across the muscle's maximum width, and between the 'superficial' surface and the 'deep' one that contacts the joint. The locations of Sps in complete serial sections of 2 paraffin-embedded muscles, one perfused with the hip joint flexed and the other with it extended, were plotted on an outline of each muscle at its midlength, and their numbers and density in horizontal and sagittal 'strata' determined. In general, the incidence of Sps increased down the superficial-to-deep axis, while FOG fibers became fewer, as is consistent with support of SO status by Sp input. Along the craniocaudal axis, i.e. width, the numbers of FOG fibers rose toward the hip joint, but this was not associated with a monomodal gradient of Sps. In the extended muscle, however, the lengths of the axial bundle and capsular space of Sps in the half of the muscle next to the joint exceeded those in the longer, cranial half, implying that under stretch the input from Sps became higher toward the joint. In the non-extended muscle these lengths did not differ, although the lengths of extrafusal fibers isolated from 2 macerated muscles and normalized according to sarcomere length decreased linearily by approximately 50% along craniocaudal axis. It is explained that if elastic resistance of a Sp's sensory region exceeded that of an equivalent length of septal tissue in-series, the progressive shift in the ratio of compliances across this trapezoidally-shaped muscle should result in relatively greater lengthening of Sps at the shorter border as the muscle was extended. Levels of discharge conducive to transition of some motor units to the FOG type might be attained. Thus, gradients in the discharge of Sps (but not necessarily incidence) along both transverse and superficial-to-deep axes may be consistent with Sp sensory input influencing the distribution of at least these types of motor units.