This application proposes structural and functional analyses of the internal organization of the mammalian muscle spindle. Neural and muscular features of spindles will be compared: among cat, monkey and man; among functionally different muscles of monkey; and among single isolated human spindles from which afferent responses have been elicited by stimulation of known fusimotor axons. The proposed research addresses: a) whether efferent and afferent features of the human spindle are similar to those of spindles in cat or monkey; b) whether particular, anatomy-dependent motor tasks (flexion vs. extension) necessitate a particular internal organization of spindle; c) whether 'mixed' afferent fusimotor actions have a neural or muscular origin; d) the nature of factors that determine the neural organization of human spindle; e) how the anatomical features of innervation of individual intrafusal fibers and variability in structure along the length of fibers, translate into functional attributes of afferents in single, isolated human spindles. Motor and sensory innervation of spindles are reconstructed by light microscopy of serial transverse sections of plastic-embedded material. Ultrastructure of intrafusal fibers is analyzed qualitatively and quantitatively from electron micrographs. Structural properties of spindles are correlated directly with afferent responses in single, isolated organs in vitro. The study represents the first comparative analysis of the nerve supply of spindles in different muscles and in different species of mammal, and the first comprehensive anatomical/physiological study of the human muscle spindle. The work is essential for clarifying the role of efferent and afferent nerve supplies of the mammalian spindle in controlling movement and posture.