4 THE NERVOUS SYSTEM 



As a result, the muscle involved is not only paralysed but its 

 electrical reactions become altered. Further, the trophic 

 influence of the lower neurone being removed, the muscle 

 atrophies. At the same time, the controlling "tonic" influence 

 is cut off and the muscle, losing its tonus, becomes flaccid. 



Afferent stimuli have their origin in the periphery, often in 

 special nerve-endings, and they pass along the axons to the 

 spinal medulla. Either in the spinal medulla or in the brain 

 stem the axons end by arborising round nerve-cells and the 

 impulses which they convey are transferred to these upper 

 neurones. After passing through one, two or more relays, the 

 afferent impulse eventually reaches the cortex and, depending 

 on its nature, is interpreted or causes a reacting efferent 

 impulse. 



Under normal conditions, an afferent impulse stimulates only 

 a group of axons and the neurones to which they belong, and 

 then is transmitted to the cerebral cortex ; but, under abnormal 

 conditions, an afferent impulse may spread from the cells for 

 which it was primarily intended and affect the neighbouring 

 nerve-cells. Of the nature of this " overflow " we know as 

 little as we do about the nature of the original impulse, but, 

 apparently, stimulation of a neurone by " overflow " from 

 adjoining neurones produces precisely the same results as 

 stimulation arising at its peripheral part. In this way, impulses 

 ascending along the phrenic nerve (C, 3, 4 and 5) reach nerve- 

 cells in the fourth cervical segment, and, when these impulses 

 are altered, as in diaphragmatic pleurisy, they may overflow 

 and stimulate the adjoining cells, which normally receive 

 peripheral stimuli only from the skin of the neck and shoulder. 

 Such an overflow therefore causes stimuli to reach nerve-cells 

 in the cortex, and these cells interpret all stimuli as pain in the 

 region of the neck and shoulder. Pain of this nature is termed 

 "referred pain." The condition is described more fully in 

 connection with Mackenzie's " Viscero-Sensory " and " Viscero- 

 motor Reflexes " on page 190. 



