Quantitative Studies on Mammalian Muscle Spindles and their Sensory Innervation

It is not surprising, in view of the various functional roles played by skeletal muscles, that each muscle should possess a characteristic proprioceptive innervation. Muscle spindles are relatively easy to count and have been the main subject of quantitative studies. In drawing comparisons between different muscles, most authors have used the number of spindles per gram of adult muscle, or spindle density, as a measure of relative abundance. In both man and cat, where sufficient muscles have been examined, smaller muscles have been found usually to have higher spindle densities than larger muscles (reviewed by Hosokawa, 1961; Voss, 1971; and Barker, 1974). This has frequently led to the suggestion that the higher densities are functionally appropriate to small muscles involved in fine postural adjustment or manipulation, yet it has never been demonstrated that it is justifiable to relate spindle number linearly to muscle mass as a simple density.