The Ultrastructurc of a Frog Muscle Spindle 



199 



layer 150-300 A thick at the muscle and nerve sur- 

 faces. This layer is the outer dense layer of the 

 muscle and nerve double surface membrane complex. 

 Scattered longitudinally disposed collagen fibrils are 

 present in this perimuscular substance usually near 

 the sheath cell surfaces. 



2. The intrafusal nniscle fibers. — The intrafusal 

 muscle fibers in the spindle region vary in diameter 

 from 5-10 //. Their centrally or slightly eccentri- 

 cally placed nuclei occupy the bulk of the sarcoplasm 

 in the smaller fibers. The sarcoplasm of the larger 

 fibers generally is filled by myofibrils (fig. 3). Mode- 

 rate numbers of mitochondria are present. As the 

 region of maximal innervation ("nuclear bag" region) 

 is approached centrally placed closely spaced nuclei 

 appear and mitochondria increase in relative number. 

 Endoplasmic reticulum and vesicular-appearing bo- 

 dies are sometimes prominent in the sarcoplasm 

 (fig. 4). The diameter of the fiber decreases and the 

 nucleocytoplasmic ratio increases. The number of 

 myofibrils decreases and the scanty sarcoplasm some- 

 times exceeds the myofibrillar area in cross section. 

 However, no section has yet been obtained in which 

 no myofilaments at all can be identified. The surface 

 of the muscle fiber is thrown into numerous folds 

 and ridges which partially envelope the terminal 

 nerve twigs. These folds sometimes extend as finger- 

 like processes passing entirely through the middle 

 of the muscle fiber. These penetrating channels 

 contain the perimuscular substance and sometimes 

 axon or Schwann cell branches. 



3. The juxtaterminal nerve fibers. — The spindle 

 region contains numerous myelinated nerve fibers 

 5-10 // in diameter. These penetrate the outer and 

 inner sheath by passing between the sheath cells. 

 The endoneurial sheath is here replaced by the 



spindle sheath. Indeed, it appears that the spindle 

 sheath may be simply a specialized endoneurial 

 sheath. These fibers branch repeatedly as the inner 

 sheath is penetrated and the myelin sheath is reduced 

 in thickness to 0.1 //. The inner or outer surface- 

 connecting membrane (SCM) is sometimes visible 

 in these fibers (figs. 1 and 4), (ref. 1, 5). 



Inside the inner sheath increased numbers of 

 unmyelinated libers arc seen with rare small mye- 

 linated fibers. The former consists of Schwann cells 

 containing one or several axons each with one SCM. 

 In rare instances intermediate fibers arc seen con- 

 sisting of a Schwann cell with one axon and one 

 elongated SCM which partially surrounds the axon 

 in a spiral fashion ( i ). in a few instances one axon is 

 seen with two SCMs. To the lower right of fig. 4 

 may be seen an axon (Ax) which is only partly 

 covered by its Schwann cell iSc/i). One SCM is 

 clearly shown. The axon presumably is leaving the 

 Schwann cell via the second SCM. 



4. The nerve endings. — The axons and Schwann 

 cells develop an unusual number of mitochondria 

 near their termination and this appears to be more 

 pronounced in the axons. Vesicular- or tubular- 

 appearing bodies (fig. 3 inset) about 250-300 A in 

 diameter are fairly prominent in Schwann cytoplasm 

 and less so in axoplasm. Axoplasmic filaments are 

 not prominent in the terminal axons despite their 

 frequent prominence in the larger myelinated fibers. 

 Endoplasmic reticulum is sometimes prominent in 

 Schwann cytoplasm. The small terminal branches of 

 both axons and Schwann cells appear so similar 

 that sometimes it is difficult to identify them with 

 certainty. Often a structure at first thought to be a 

 terminal axon is proven to be a Schwann cell on 

 deeper sectioning (fig. 3). 



Fig. 1. Transverse section of spindle showing two intrafusal 

 muscle fibers and a part of a third. A portion of the outer 

 spindle sheath io.s.) may be seen to the lower right and the 

 middle right (Ns). Numerous collagen fibrils (r.) are present. 

 Each intrafusal fiber contains a centrally placed nucleus (N,n) 

 surrounded by a thin layer of sarcoplasm containing a few 

 myofibrils. The surfaces of the muscle libers are irregularly 

 convoluted and in contact with terminal nerve branches. 

 Some of the terminal nerve structures lie in the diffusely 

 dense perimuscular substance which is clearly shown about 

 the central muscle fiber. The inner sheath appears as a very 

 thin cytoplasmic layer in which nuclei (/Vs ) appear. A small 

 myelinated nerve fiber appears inside the inner sheath to the 

 upper right. The intrafusal fiber associated with this axon 

 is enlarged in fig. 4. Magnification 3,100. 



Fig. 2. Enlargement of two layers of an outer spindle sheath. 

 Each cytoplasmic layer c^ and c, is bordered above and below 

 by a double surface membrane complex about 300-400 A 

 thick. A double membrane about 250-300 A thick separates 

 the two sheath cells. Tubular-appearing structures partially 

 traverse each cell. These appear to be related to the surface 

 membranes. Magnification 80,000. 



Fig. 3. Single intrafusal fiber stained with phosphotungstic 

 acid. The edge of a sheath nucleus ( As ) appears to the upper 

 left. The delicate inner sheath (/..?.) partially surrounds the 

 muscle fiber. The perimuscular substance is practically absent. 



Several terminal nerve processes are seen in the perimuscular 

 space. One Schwann cell (Sc/i) is seen in contact with the 

 muscle and with an axon (.4.v) which also contacts the muscle. 

 Two fibers which may be axons {? Ax) are in contact with 

 the muscle above. A fiber containing numerous vesicular- 

 appearing bodies is in contact with the muscle to the lower 

 left. This was proven in serial sections to be a Schwann cell. 

 The dotted area is enlarged in the inset to show the double 

 membrane between Schwann cytoplasm and sarcoplasm. 

 Note to the right the tubular extension of the nuclear double 

 membrane into Schwann cytoplasm and its suggestive rela- 

 tionship to the round and oval vesicular-appearing bodies in 

 Schwann cytoplasm. Magnification 14,000. Inset, magni- 

 fication 45,000. 



Fig. 4. Enlargement of the intrafusal muscle fiber to the upper 

 right in fig. I. Myofibrils (nif) are present in the muscle 

 fiber. Numerous Lmmvelinated nerve fibers are present in 

 the scanty perimuscular substance. One myelinated fiber 

 with an outer SCM appears to the right. Note the axon (Ax) 

 which is apparently leaving its Schwann cell (Scli) to the 

 lower right. One SCM is present in this liber. The terminal 

 axon or Schwann cell in contact with the muscle fiber to 

 the lower center is enlarged to the left to show the 200-300 A 

 thick double membrane. Magnification 19,000. Inset, 

 magnification 73,000. 



