The sensory ending of duck muscle spindles

Thin and thick types of muscle spindles have been demonstrated from the extensor pollicis (EP) and the extensor digitorum communis (EDC) wing muscles of the duck. The thin type shows a smaller capsule, innervated by a long and narrow sensory ending; the thick type consists of a larger capsule innervated by a truncated and wide sensory ending. Each spindle receives an afferent nerve fibre within the range of 2.1-8.4 microns diameter and the ratio between fibre and sensory ending is always 1:1. There is no secondary sensory ending in duck spindles. By electron microscopy, sensory terminals of different sizes may be seen to lie in depressions of various depths on intrafusal muscle fibres, usually without disturbing the external contour of the muscle fibre, and are covered externally by a rather thick layer of basement membrane.