Table 8. Electron Microscopic Classification of Human Extraocular Muscle Neuromuscular Junctions and Corresponding Fiber Morphology


  Neuromuscular Junction Type
Classification 1 2 3 3a 4 5
Number observed 2 10 3 2 3 1
Schwann cell investment + + + or - - + +
Terminal axon Thick Thin Thin 7 7 7
Diameter 3 μm 1 μm        
Terminal varicosity ? ± + + 9 9
Neurofilaments + - - - -  
Synaptic vescicles            
 Clear + + + + + +
 Granulated + +   + +  
 Ellipsoidal   +   +    
Muscle cell at neuromuscular junction            
 Junctional folds + + - - ± ±
 Soleplate specializations* - - + - + ±
 Depression of cell surface + - - - - ±
 Incidence of laminated structures 2/2 2/10 0/3 0/2 2/3 1/2
Muscle cell morphology            
 T systems and triads + to + + + ± + + + to + + + + +
 Sarcoplasmic reticulum + to + + + ± ± to + + + to + + + +
 Glycogen granules + to + + + ± + to + + + to + + ± to +
Mitochondria + to + + + to + + ± + to + + + to + + +
 Z line Straight, slender Irregular Inconspicuous Irregular, thicker Straight, slender Straight, thick
Myofibrils Moderately defined at T band level Irregular Irregular Slightly regular Well-defined at I band level Irregular


*Hill-like elevations or bridgelike process.
Mukuno K: Fine structure of the human extraocular muscles: III. Neuromuscular junctions in the normal human extraocular muscles. Acta Soc Ophthalmol Jpn 72:104, 1968.