Freeze-fracture of sensory nerve endings in cat muscle spindles

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Abstract
Sensory nerve endings and intrafusal muscle fibres of cat muscle spindles were examined by freeze-fracture electron microscopy. The purpose of the study was to reveal intramembranous specializations that might be associated with the mechanosensory transducer system. Nerve ending membranes were found to contain high concentrations of intramembranous particles (greater than 7500 per μm2) in a range of sizes, but no localized aggregations or non-random arrangements were observed. Nerve ending membranes directly apposed to the surfaces of intrafusal muscle fibres did not appear to be strikingly different from non-interface membranes, although some differences in particle concentrations could be detected. The membranes of endings on nuclear bag and nuclear chain muscle fibres differed slightly in the numbers of particles and their distributions to protoplasmic or external fracture faces. It was concluded that intramembranous elements of the mechanoelectric transducer system may be dispersed over the whole surface of a sensory ending, rather than localized to smaller domains.

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