The motor-unit population of the cat tenuissimus muscle.

1. We studied the organization of motor units in the tenuissimus (TEN) muscle of pentobarbital-anesthetized cats. The cat TEN is a long, delicate straplike muscle that spans hip and knee, which has a very flat length-tension curve through 22 mm of length change. 2. The TEN motor nucleus, labeled by retrograde transport of several forms of horseradish peroxidase, was composed of 8-31 cells in different cats, of which about half were, on average, in the size range of alpha-motoneurons. TEN motoneurons were scattered through the ventrolateral portion of lamina IX, over a rostrocaudal distance of up to 6.5 mm, making it relatively easy to isolate individual TEN motor axons for single motor-unit stimulation. 3. Individual TEN muscle units were classified into four groups [fast-twitch, fatigable (FF), intermediate, fatigue-resistant (Fint), fast-twitch, fatigue-resistant (FR), and slow-twitch, fatigue resistant (S)] on the basis of "sag" and fatigue index mechanical properties, as in other cat hindlimb muscles. There was a relatively large proportion of Fint units (28%) in the TEN sample, and the range of tetanic tension (approximately 19-fold) was much smaller than found in other cat hindlimb muscles. 4. A majority of TEN muscle fibers could be classified into the three major histochemical types (IIB, IIA, and I) found in other cat muscles, but a substantial minority remained "unclassified." A single type Fint muscle unit was successfully depleted of glycogen for histochemical study. It exhibited a typical type IIB histochemical profile. 5. Despite its unusual morphology, the cat TEN contains the same types of motor units found in larger, more "typical" limb muscles.