The morphological variability of neuromuscular junctions in the rat extraocular muscles: a scanning electron microscopical study.

Six morphologically distinct types of neuromuscular junctions were identified by scanning electron microscopy in the rat extraocular muscles: two diffuse and four focal types. The diffuse junctions, spreading out extensively over the muscle fiber surface, were characterized by two types of varicose swellings (or terminal varicosities) of nerve endings. One type consisted of several ramifying nerve endings and shallow postsynaptic depressions with poorly-developed junctional folds. The other type consisted of a single axon and formed many synaptic contacts along the long axis of a muscle fiber. Round synaptic depressions facing the varicosities contained several junctional folds. The focal junctions, confined to an oval area on the muscle fiber surface, were characterized by the complexity and variability of their subneural apparatuses. Four different types of apparatuses, i.e., focal junctions, were found: 1) an apparatus consisting of labyrinthine gutters with numerous slit-like junctional folds, 2) apparatuses consisting of a large number (more than 20) of cup-like depressions with either a small number of pit-like junctional folds or 3) numerous slit-like ones, and 4) an apparatus consisting of a small number (about 10) of cup-like depressions with a few junctional folds. The findings indicate that the rat extraocular muscles contain six different types of muscle fibers.