THE NEURONE 3 



The Neurone. Each nerve-cell consists of a body, variously 

 shaped and containing a nucleus, a nucleolus, etc., and certain 

 processes. The processes are of two kinds (a) the dendrites, 

 which are usually short and break up into numerous branches, 

 and (b] the axon or axis cylinder, which varies in length and 

 gives off no branches of note prior to its termination. 



Physiologically, so far as we know at present, the axon is 

 of much greater importance than the dendrites, and impulses 

 arising within the cell, or destined for it, are transmitted along 

 the axon. 



The term " NEURONE " includes the nerve-cell and all its 

 processes. Neurones can be divided, broadly, into two 

 groups : (a) Those engaged in carrying impulses from the 

 cerebral cortex to the periphery, efferent neurones ; and (b) 

 those engaged in carrying impulses from the periphery to the 

 cerebral cortex, afferent neurones. 



Efferent stimuli arising in the cortex traverse two or more 

 neurones before they reach their destination, and these 

 neurones must all be physiologically intact before the stimulus 

 can produce its result. The uppermost neurone has its cell 

 situated in the cerebral cortex and the lowermost neurone 

 has its cell in the grey matter of the brain stem or the spinal 

 medulla (spinal cord). In the case of the voluntary muscles, 

 interruption of the upper neurone prevents the stimulus from 

 passing on to its destination and the muscle involved is 

 paralysed, i.e. it is unable to react to cerebral stimuli, 

 although its electrical reactions are not altered. As the 

 lower neurone is not damaged, the trophic influence which 

 the nerve-cells exert on the tissues they supply is not interfered 

 with and the muscle involved will suffer atrophy from disuse 

 only. In addition to originating voluntary stimuli, the upper 

 neurone exerts a subconscious controlling action on the lower 

 neurone and, when this controlling action is removed, the 

 muscle, typically, assumes a spastic contraction. On the 

 other hand, when the lower neurones are interrupted, their 

 axons are cut off from the cell body and undergo degeneration. 



