224 HUMAN PHYSIOLOGY 



The nerve cells are irritable. Their physiological stimu- 

 lation is of two kinds : 



1 . The stimulation originates by processes in the cells 

 themselves automatic activity. The automaticity is either 

 tonic, when the impulse travels continuously through the 

 nerve fibre connected with the cell, or rhythmic, when the 

 impulse proceeds periodically down the nerve fibre. For 

 example, lack of oxygen and accumulation of carbon 

 dioxide stimulate the cells of the respiratory centre ; they 

 are conditions which the cell itself produces by its meta- 

 bolism. The automaticity of the respiratory centre is 

 rhythmic. The vaso-motor centre is also stimulated by 

 lack of oxygen and accumulation of carbon dioxide; its 

 automaticity is tonic. 



2. The stimulation is carried to the cell by a nerve fibre. 

 As the stimulation can be conducted from this cell to its 

 axis-cylinder, there results the conduction of impulses from 

 one nerve fibre to another through the nerve cell. The 

 conduction of the impulse through the cell differs from that 

 through the fibres in the following points: 



(a) The cell is able, independently, to modify the impulse 

 either 



(cr) In intensity: it can increase or decrease the strength 

 of the impulse ; 



(ft) In frequency of the impulse. 



For example, the impulses which in the radiated reflexes 

 (see page 231) affect the muscles are not proportional to 

 the strength and frequency of the sensory stimulation which 

 calls forth the reflex. 



(/;) The conduction is not double, but passes in one 

 direction only. In the spinal cord, for example, the impulse 

 passes, in reflex action, from the sensory nerve through the 

 cell to the motor fibres, but never in the reverse direction. 

 The electrical phenomena characteristic of the impulse can- 

 not be called forth in the sensory nerve by the stimulation 

 of the motor roots. 



