Acetylcholine receptors at the rat neuromuscular junction as revealed by deep etching

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Abstract
Collagenase treatment of rat intercostal muscles yielded single muscle fibers in which the nerve terminals and basal lamina were removed allowing an unimpeded view of the external surface of the postsynaptic membrane. This was revealed by deep etching of freeze-fractured preparations and appeared as a maze of folds separated by deep troughs, showing on the crests of the folds a densely packed population of protrusions .apprx. 8.5 nm in diameter. These densely packed protrusions (.apprx. 9000 .mu.m-2) were mainly confined to the postsynaptic regions of the sarcolemma and presumably represented the acetylcholine receptor molecules, which were highly concentrated in these areas. The protrusions were generally tightly packed without obvious regular arrangement, but in some areas, usually on the tops of the crests, they were arranged into irregular rows normal to the long axis of the folds.