Membrane specialisation of the extra-junctional sarcolemma of normal and denervated insect muscle.

Corrugated areas of sarcolemma were observed in serial transverse sections. Complexes of intramembranous particles that appear after freeze-fracture replication were concluded to represent the same specialisation. These specialisations, up to 14 microns x 2 microns, are orientated parallel to the long axis of the muscle. Intramembranous particles are concentrated along the peaks of the corrugations, and are associated with the P-face. Corresponding pits are found in the E-face. Fourteen and thirty days after sectioning the excitatory motor-nerve supply to the muscle, corrugated areas 0.5-1 microns x 0.5-1 microns are found. Occurring singly or in groups, their orientation with respect to the long axis of the muscle is more variable than those of control muscles. Thin sections reveal no complementary areas on adjacent fibres or intracellular submembrane attachments or specialisations. A structural role is therefore unlikely. Mitochondria are frequently found in close association with these specialisations. Their possible role as receptors or transmembrane transport systems is discussed.