SKELETAL NEUROMUSCULAR TRANSMISSION 



201 



impulse is a propagated action potential and a twitch. 

 These are the junctions of which both the morphology 

 and physiology have been most intensively studied. 

 There are other junctions, in the amphibian at least, 

 where the normal mechanical response of the muscle 

 fibers is a slow tonic contraction which can only be 

 elicited in appreciable tension by a train of nerve 

 impulses (53, 55). These fibers are innervated by a 

 special class of small diameter nerve fibers which form 

 numerous, widely distributed terminations of the 

 en grappe type on them. 



The twitch muscle fibers are innervated by coarse 

 motor nerve fibers. On issuing from the central 

 nervous system, each nerve fiber branches repeatedly 

 both before and after reaching the muscle it supplies. 

 By this branching the nerve fiber forms junctions on 

 many muscle fibers, the number varying greatly for 

 mu.scles in different parts of the body in a given 

 animal. Conversely, muscle fibers have been found 

 to be supplied each with a few nerve endings at 

 widely separated positions along their length (47, 50). 

 These multiple junctions are in some cases made by 

 separate nerve fibers and in others bv branches of a 

 single fiber. The variations in the distribution of nerve 

 fibers to muscle fibers in different preparations and 

 their probable relation to differences in function have 

 been discussed by Tiegs (73). 



In the morphology of the single junction, the pat- 

 tern made by the nerve fiber in terminating also 

 shows marked differences from species to species and 

 from muscle to muscle. This field was early thoroughly 

 explored Ijy Kiihne (56). Confining consideration to 

 the more familiar objects of investigation, he drew a 

 distinction between the plate type of ending in the 

 mammal and reptile and the bush type in the frog. In 

 both types the nerve comes into contact with the 

 muscle fiber immediately after losing its myelin 

 sheath and branches repeatedly on its surface to form 

 the terminal apparatus. In the former type, the 

 extent of this apparatus is limited to a roughly circular 

 space (the endplate) which has a diameter of 25 to 

 70 /x. Viewed in a section at right angles to the muscle 

 fiber surface this region is marked by a rounded 

 eminence. Within the confines of the endplate the 

 terminal branches cover a large fraction of the in- 

 cluded muscle fiber surface. In the case of the other 

 type of ending (the endbush) the nerve terminal 

 branches range over a much wider area. The terminal 

 apparatus here consists mainly of several large, 

 straight branches 100 to 300 /j. in length, running 

 parallel to the axis of the muscle fiber and connected 



into a continuous system by shorter lengths at right 

 angles. 



As a result of careful cytological examination, it is 

 recognized that there are three sharply defined com- 

 ponents of different cellular origin at the junction 

 (14). The first of these is the terminal apparatus of the 

 nerve. The second is the specialized region of muscle 

 fiber surface contacted by the nerve endings. A char- 

 acteristic of this region is an increased density of 

 muscle nuclei (fundamental nuclei of the junction), 

 the presence of which is suggestive of a higher degree 

 of synthetic activity here than elsewhere in the muscle 

 fiber. The third component is a layer of neuroglia 

 which in this position is referred to as the teloglia 

 and which appear to be continuous with the -Schwann 

 cell envelope of the myelinated fiber. It contributes 

 about half the nuclei seen in the junctional region 

 (the sole nuclei), the remainder being the fundamental 

 nuclei in the muscle. It is dispersed over the entire 

 endplate where it forms the rounded eminence and 

 accompanies the terminal nerve filaments along their 

 extended course in the endbush. In spite of the gross 

 differences that exist between the two types of ending, 

 the detailed relationships between these three cellular 

 components are fundamentally the same. From the use 

 of cytological and histochemical staining methods it 

 appears that the nerve terminal branches lie sunk in 

 grooves in the muscle fiber surface (14, 16). Only a 

 small part of the circumference of the nerve is not in 

 close proximity to the surface of the muscle lining the 

 groove. The sides of the groove appear to be marked 

 with a set of parallel lines 0.3 to i m apart which are 

 oriented more or less normal to the axis of the groove 

 and extend a short distance into the muscle beyond 

 the clearly defined edges of the nerve cylinder. In the 

 case of the endbush, where there are long stretches of 

 unbranched nerve fiber, the ruling is highly regular, 

 the lines running from one edge of the groove to the 

 other without deviating from this geometrical rela- 

 tion. In the endplate where the nerve filaments 

 usually extend for no more than a few diameters 

 before terminating or branching, the arrangement 

 of the lines is less regular, adjacent lines frequently 

 fuse with one another, while their spacing is main- 

 tained relatively constant. Examination of the junc- 

 tion with the electron microscope reveals regularly 

 spaced narrow infoldings of the membrane of the 

 muscle fiber lining the groove (71). These fine junc- 

 tional folds very probably correspond to the lines 

 seen under the light microscope. Figures i and 2 show 

 the relation between nerve and muscle over a wide 

 range of magnification. The width of the folds is 



