Non-nervous aspects of muscle spindle morphology were examined in 15 forepaw lumbricals of a marsupial, Trichosurus vulpecula, using serial transverse and longitudinal sections. The structure of the spindle was similar to that in other mammals with a few minor differences. One to three nuclear bag type and 2 to 16 nuclear chain type intrafusal fibers were present in all spindles. The fibers were largely distinguishable by differences in length, in diameter at all levels along the length of the spindle, and in the arrangement of their equatorial nuclear aggregations. There was, however, some overlap in that many nuclear chain fibers extended for considerable distances beyond one or both poles of the spindle capsule, and several fibers, otherwise classified as nuclear bag fibers, contained single chains of equatorial nuclei rather than nuclear bags. These findings, together with the observation that all intrafusal fibers were heterogeneous in their distribution of myofibrils, and the evidence of other studies on the innervation and histochemistry of mammalian muscle spindles, suggest that a specific functional separation of intrafusal fibers on strictly morphological grounds may not be entirely valid.
The receptor population in each of nine muscles bore no consistent relation to the total number of extrafusal fibers; this appears to confirm the concept that it is the functional attributes of a muscle, rather than its bulk that determine its spindle content.