Freeze-fracture study of the crayfish stretch receptor.

The crayfish slow-adapting stretch receptor was fixed under relaxed or stretched conditions (twice the relaxed length) and then processed for freeze-fracture study. The sensory neuron membrane had evenly distributed intramembrane particles mostly on its P face. The density of these particles was higher in the cell body than in the dendritic tips, which are the terminal portions of the dendrites. The dendritic tips were cylindrical under the relaxed condition and showed deformations with stretch stimuli. When they were fixed under the stretched condition with 1.6% glutaraldehyde in 0.12 M phosphate buffer (the total osmolarity of this fixative is isosmotic with the physiological solution), the dendritic tips showed regional swelling and shrinkage. The intramembrane particle density of the swollen parts decreased and there were particle-free patches of membrane, whereas the particle density of the shrunken parts increased. On the other hand when the receptor was fixed with 1.6% glutaraldehyde in 0.2 M phosphate buffer (the total osmolarity is hyperosmotic but buffer osmolarity is isosmotic), the diameter of the dendritic tips became smaller, and their membrane particle densities were almost the same as that under the relaxed condition. The sheath cells covering the sensory neuron were characterized by their sheet-like profiles, gap junctions, and crater-like protrusions. The receptor muscle membrane had longitudinal foldings, occasional invaginations, peripheral couplings, string-shaped particle aggregates, and band-shaped particle aggregates.