| Popis: |
Internode lengths, axon diameters, and numbers of myelin layers were measured in the last four subterminal internodes of sensory nerve fibers in neuromuscular spindles of the cat tenuissimus muscle, and in the last three internodes of extrafusal motor fibers of the same muscle. Principal findings are: (1) All dimensions decrease as a fiber proceeds peripherally, but internode length decreases proportionally more than axon diameter or myelin thickness. (2) Near their endings, spindle primary axons are 50% larger in diameter than the secondaries; and these, in turn, are nearly twice as large as extrafusal motor fibers. (3) Internode lengths of sensory fibers are usually 60–140μm near the terminals, while distal internodes of the motor fibers can be as short as 10–20 μm. (4) Small subterminal fibers (sensory or motor) have much thinner myelin sheaths than would be predicted from nerve trunk data. (5) Near-terminal branching of myelinated axons is much more profuse in the sensory fibers than in motor fibers. It is concluded that the shortness of subterminal internodes in sensory fibers could affect sensory thresholds. In motor fibers, short subterminal internodes could increase the safety factors for conduction of an impulse into the unmyelinated terminal, and increase the amount of transmitter released per impulse. There is some circumstantial evidence that subterminal myelin segments in motor fibers may often be developmentally immature. |
