Postjunctional membrane specialization at cholinergic myoneural junctions in the leech

Myoneural junctions are described in the body musculature of the leech. Although similar to cholinergic junctions elsewhere, these exhibit a distinctive postjunctional membrane specialization consisting of discrete, concave patches, occupying a small proportion of the total postjunctional area, at which both the inner and outer surfaces of the membrane appear to be coated. The outer coating consists of a hexagonal array of projections which arise from the outer dense lamina of the plasma membrane and extend into the junctional cleft. The projections are spaced at intervals of ∼ 150 Å and are thickened at their tips. The inner dense lamina of the membrane is coated by an amorphous cytoplasmic material. Details of this specialization show up well in specimens that have not been post‐osmicated but are stained (in sections) with permanganate and uranyl acetate. It is hypothesized that the morphologically specialized patches are composed of units containing the acetylcholine receptors.