200 



J. D. ROBERTSON 



These terminal axons and Schwann cells contact 

 the intrafusal muscle fibers in the nuclear bag region 

 and an undiflFerentiated double membrane about 

 250 300 A thick is formed in each instance (figs. 

 3^). Motor endings have sometimes been observed 

 in the same preparation on nearby extrafusal fibers. 

 Here a five layered membrane complex 500-700 A 

 thick resembling closely the type of membrane com- 

 plex observed previously in rat (3) and lizard (4, 6) 

 motor end plates is seen. In this kind of frog motor 

 ending the junctional folds are largely absent. It 

 appears that the difference between the motor and 

 sensory membranes may be helpful in differentiating 

 motor and sensory neuromuscular junctions if 

 followed in serial sections. This is necessary because a 

 sensory terminal may display the m.otor type of 

 membrane complex as the nerve twig parts from the 

 muscle (fig. 4). 



In the perimuscular substance are numerous ter- 

 minal cytoplasmic profiles about 0.1 0.3 /i in 

 diameter containing small mitochondria (figs. 3^). 

 These profiles often drop out in serial sections. But 

 occasionally they expand in size and become packed 

 with mitochondria or rarely enlarge greatly and 

 develop a nucleus. To date no very reliable criteria 

 other than serial sectioning for the identification of 

 these terminal perimuscular profiles as axons or 

 Schwann cell processes has been applied. 



In summary the following kinds of axon, Schwann 

 cell and muscle relationships have been observed 

 inside the inner sheath: (1) Axon and Schwann cell 

 together in perimuscular substance. (2) Axon and 

 Schwann cell together in contact with the muscle 



surface. (3) Schwann cell alone in contact with 

 muscle. (4) Schwann cell alone in perimuscular 

 substance. (5) Numerous terminating axon or 

 Schwann cell twigs in contact with the muscle surface 

 and in the perimuscular substance. If criteria for 

 positively identifying terminal axon and Schwann 

 cell processes had been applied a coherent picture 

 of a systematic mode of termination could probably 

 be presented. In the absence of these no definitive 

 statements may be made at present. But a tentative 

 working hypothesis involving some unproven as- 

 sumptions might be considered. It appears that the 

 axons and Schwann cells run along the muscle sur- 

 face for some distance with a simple double mem- 

 brane relationship shared between all three kinds of 

 cytoplasm. The axons and Schwann cells separate 

 from each other while maintaining their individual 

 synaptic contact with the muscle fibers. Then each 

 leaves the muscle fiber separately, branches and 

 terminates in the perimuscular substance free of any 

 terminal contact with other cells. While none of 

 the facts contradict this interpretation the assump- 

 tions on which it is based require further investiga- 

 tion. 



References 



1. Geren, B. B., E.xptl. Cell Research 7, 558 (1954). 



2. Gray, E. G., Proc. Roy. Soc. B (in press). 



3. Palade, G. E., Anat. Rec. 118, 335 (1954). 



4. Robertson, J. D., Anat. Rec. 118, 346 (1954). 



5. — /. Biophys. Biochem. Cytol. 1, 271 (1955). 



6. — ibid 2, 381 (1956). 



