nerve 



SECTION VII 

 THE NEURO-MUSCULAR JUNCTION 



THE excitatory process travelling down a motor nerve has to be transmitted 

 to the muscle by the intermediation of the nerve-ending or end-plate. We 

 have learnt to regard the axis cylinder as the seat of the propagated ex- 

 citatory process. In the end-plate, however, the axis cylinder comes to 

 an end. When stained by methylene blue or by impregnation with chromate 

 of silver or mercury, the axis cylinder, after passing through the sarcolemma 

 of the muscle fibre, is seen to break up into a number 

 of branches (in some cases forming a typical end- 

 arborisation), which lie on or are embedded in a 

 small amount of undifferentiated protoplasm con- 

 taining nuclei (the 'sole plate'). A similar break 

 in structural continuity seems to occur in the central 

 nervous system wherever an impulse is propagated 

 from the axon process of one nerve- cell to the body 

 or dendrites of another nerve- cell. The end-pro- 

 cesses of the axon come in contact with the next 

 member in the chain of neurons, but no anatomical 

 continuity is to be made out, at any rate in the 

 higher animals. In the central nervous system 

 the area of contiguity, where an impulse passes 

 from one neuron to another, is spoken of as a synapse. 

 The presence of the synapse, or end-plate, between 

 muscle and nerve imposes certain new conditions 

 on the conduction of the excitatory impulse. One 

 of the most important of these lies in the fact that the conduction across 

 the end-plate, and probably across the synapse of the central nervous 

 system, is irreciprocal. An excitatory process started in the nerve travels 

 easily across the end-plate to the muscle. On the other hand, an excitatory 

 process started in the muscle does not extend through the end-plate to 

 the nerve fibre. This fact may be shown on the frog's sartorius. If the 

 lower tibial end of the muscle be split, as in Fig. 122, a mechanical stimulus, 

 such as a snip with the scissors, applied to the lower nerve-free end of one 

 of the limbs, e.g. at A, causes a contraction of the corresponding half of 

 the muscle, which does not extend to the other half. On snipping the 

 muscle a little higher up at B, where nerve-endings are present, the resulting 



275 



122. 



