144 



Inside the Living Cell 



that it is sufficient. Ttie axon usually branches before it reaches other 

 nerve cells, so that the excitation may reach numerous other cells. 



In the cerebral cortex it is difficult to recognize definite points of 

 contact between the nerve cells. The latter often have fine thread-like 

 appendages called dendrites and the axons from other nerve cells 

 often split into many branches (Fig. 27). It is often seen that the 

 branches of the coming-in axon runs parallel with the branching arms 

 of the dendrites for a distance and stimulation passes from one fibre 

 to the other in this region and thus into a new nerve cell. 



When a neurone has fired a Volley' there is a refractory period. 



R enlorhin. 



FIG. 27. Typical neurones and their branching connections, found in different 

 layers of the cerebral cortex of an ape. (From Dr Lorente de No's article in 

 Fulton's Physiology of the Nervous System. [Oxford University Press]) 



which lasts for about a thousandth of a second, during which it can- 

 not be stimulated again. The neurone can also be inhibited, i.e. a sig- 

 nal from another neurone will bring it into a state in which it cannot 

 fire. 



The neurone thus resembles a relay which repeats a sufficiently 

 strong signal and which remains 'dead' for the 'refractory period' 

 after being fired. To a considerable extent this kind of behaviour can 

 be reproduced by arrangements of thermionic valves. They act as 

 relays and can be arranged so that they will respond only to signals 

 above a certain strength. They also have a recovery period after 

 'firing' and can be prevented from firing at all. 



