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While postsynaptic specialization of various kinds has been described at the skeletal neuromuscular junct ion (1, 2) and at nerve synapses (5), no welldefined postsynaptic structures have been established for smooth muscle at autonomic neuromuscular junctions (3, 4) Postsynaptm structures of three kinds have been described, although these have been reported to be inconsistent features not present at all junctions: (a) Aggregations of micropinocytotic vesicles opposed to the nerve varicosities in smooth muscle cells have been described in arterioles of the pancreas (15) and in the muscle coat of the intestine (4, 14). (b) Areas of increased density of postsynaptic membrane (26, 17) or desmosome-like structures (11) at the close-contact area between axons and muscle ceils have been reported. However, these appear to be a rare feature and may represent mechanical at tachment points between axon and muscle ceil rather than structures related to the neurotransmissmn process (11) (c) Subsurface osternae have been observed in tile region of smooth muscle closely opposed (less than 200 A) to some nerve terminals in the vas deferens (20, 16, 4) In the present study, postsynaptic regions at neuromuscular junctions in the guinea pig sphincter puplllae have been examined. This tissue was chosen because it is relatively densely innervated with close (about 200 A) neuromuscular junctions (21), and because the need for rapid neuromuscular transmission seems likely to be greater in this situation than in visceral or vascular systems. Irises from adult albino guinea pig were fixed in 0 05 ~ phosphate-buffered (pH 7.4) 4% glutaraldehyde for 20 rain, washed in phosphate buffer for 2 hr, and postfixed in osmium tetroxide for I hr. They were block-stained in 2 % uranyl acetate solution (27) before dehydration and embedding

POSTSYNAPTIC SPECIALIZATION OF SMOOTH MUSCLE AT CLOSE NEUROMUSCULAR JUNCTIONS IN THE GUINEA PIG SPHINCTER PUPILLAE