232 THE INTELLIGENCE SERVICE 



Consider a^ain Fig, 58. Suppose that at a little to the right of d 

 the nerve entered a muscle. With the key of circuit P open, 

 stimulation of the nerve, say, at 6 by a drop of strong saline would 

 produce convulsive twitchings of the muscle. The closure of the 

 key completing the circuit P produces a cessation of the muscle 

 action. The polarising current has blocked the passage of the 

 nerve impulse. Careful experiment shows that this is due to a 

 depression centred at x, an anodic depression. In a similar way, it 

 can be shown that at y the nerve is more easily stimulated when the 

 polarising current is running than otherwise. There is cathodic 

 sensitisation. A drop of saline which, when placed at h or at c, 

 would produce only a series of muscle twitches with the polarising 

 current off, would cause complete tetanus at c only when the 

 polarising current was running. 



To explain this we must consider again the series of little 

 condensers (Fig. 59). When the drop of saline acts on the nerve 

 it produces a brief current of action, i.e. it tends to cause a tempo- 

 rary accumulation of electrons at the stimulated point and the 

 propagation of a current of action along the nerve. When the 

 polarising current is flowing, the condensers near x (Fig. 58) 

 would be positively and those at y negatively charged. The action 

 potential developed by the saline (or other stimulating cause) 

 would, when applied in the neighbourhood of x, merely discharge 

 the condensers, while applied at y, it would augment the charge 

 already present leading to a more complete excitation of the 

 structure, motor organ or sensory area supplied by the nerve. 



Under experimental conditions it may be shown that the nervous 

 impulse may be propagated in any direction in a nerve, but the 

 nervous system as a whole conducts only in one direction. The 

 unidirectional mechanism lies in the synapse, where a non-nervous 

 substance forms the connection between neuron and neuron. 



Further Eeading 

 Adrian. " The Basis of Sensation." Christophers. 



