The fine structure of the sensory region of cat muscle spindles.

Transverse and longitudinal sections through the sensory region of muscle spindles in the cat tenuissimus has shown no difference in the fine structure of sensory terminals lying on nuclear-bag or on nuclear-chain muscle fibers. Each terminal lies on a shallow trough of the intrafusal muscle fiber and is surrounded on its outer surface by basement membrane. A single terminal may encircle as much as 80 % of the perimeter of an intrafusal muscle fiber. The usual types of organelle such as mitochondria, microvesicles, tubules, and dense bodies are present in varying forms and numbers in the terminals. No cytoplasmic continuity has been observed between sensory terminals and the sarcoplasm of intrafusal muscle fibers. The diameter of nuclear-bag muscle fibers is quite uniform along the length of the nuclear-bag region. There is no contact between these fibers and any of the neighboring intrafusal muscle fibers. In nuclear-chain muscle fibers, however, the diameter may vary by as much as 50 % within the sensory region. Some nuclear-chain fibers converge on one another and establish close contact, the basement membrane being absent along their common border. This convergence will be designated as "pairing." The possibility of cytoplasmic continuity between fibers in such regions has not been excluded. Neighboring nuclear-chain fibers have also been shown to be connected through their junctions with common sensory terminals. These shared terminals are designated "sensory crossterminals." Such cross-terminals have not been observed between nuclear-bag muscle fibers or between nuclear-bag and nuclear-chain muscle fibers. Satellite cells closely related to intrafusal muscle fibers, have been consistently found in all spindles. Additional features of the fine structure of nuclear-bag and nuclear-chain fibers will be described, including some components which are, as yet, unidentified. The possible functional significance of these observations is discussed.