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 excitatory 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 

 nerve protoplasm containing 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-processes 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 im- 

 pulse 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. 

 123, 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 



310 



FIG. 123. 



