Sensory nerve deformation in the stimulated frog muscle spindle

Abstract

Quantitative knowledge was sought for stretch-induced deformation of the nerve endings in frog muscle spindles. The fourth toe extensor muscles were stretched by means of joint positioning, preserved in situ, and processed for transmission electron microscopy. Spindles were serially sectioned and montage photographed. All accessible dimensions of the nerve bulbs and their links were measured in relaxed and maximally stretched spindles. Bulb endings contacting the muscle were analyzed separately from those without such contacts. Both compact and reticular zones of the muscle fiber were taken into account. Measurable shape differences in the nerve bulbs occurred with stretch. Contacting bulbs elongated in the compact zone and decreased in width in both zones. The noncontacting bulbs also elongated in the compact zone but no other shape change could be ascertained. Bulb volumes tended to increase with stretch but significance could only be established for contacting compact zone bulbs. Links between bulbs became thinner with stretch. It was concluded that differential deformational forces exist in the frog spindle system with stretch. Not only are nerve endings deformed differently according to whether or not they contact the muscle fiber, but also according to whether they are localized in compact or reticular sensory zones. The data support the notion that the structural specialization of the intrafusal muscle and the extracellular material influence this sensory transduction process. The shape and volume changes appear to be compatible with those anticipated to accompany excitatory nervous activity in the nerve endings with stretch.