Functional anatomy of the association between motor units and muscle receptors

Muscle spindles and tendon organs occur in most somatic muscles of the mammal and are particularly concentrated in muscles subserving fine movements, including postural muscles and small muscles of the distal extremities. In those mixed muscles in which the different fiber and motor unit types are "compartmentalized," the spindles, and perhaps tendon organs also, are virtually limited to those compartments predominated by "oxidative" muscle fibers. These morphological observations based on a broad array of muscles in many species, complement electrophysiological studies which have emphasized that (1) the "oxidative" motor units have low reflex thresholds and (2) segmental proprioceptive reflexes may be primarily concerned with the control of finely graded contractions. Consideration of the functional anatomy of the association between motor units and muscle receptors suggests the need for detailed structural-functional analyses of those muscles with specializations in architecture, fiber-type composition and distribution, and in the number and distribution of their muscle spindles and tendon organs. An electrophysiological analysis of the relationship between the spinal cord and such muscles might also reveal certain strategies and mechanisms underlying segmental motor control which are either absent or obscured in the analysis of that select number of "homogenously-mixed" muscles conventionally used in the study of the mammalian segmental motor control system.