CHAPTER VIII 



THE MECHANISM OF NERVOUS CONDUCTION AND EXCITATION 



Before attempting to describe the part played by the nervous 

 system as an arrangement for integrating the response which 

 an organism exhibits in everyday Hfe, it is desirable to acquaint 

 ourselves with what is known of the properties of nervous 

 tissue as a conducting system. It is assumed that the reader 

 is already familiar with the elements of nervous histology 

 and the distinction drawn between afferent or sensory and 

 efferent or motor nerves. The recognition of the excited 

 state in nerve depends on the response which it calls forth 

 in an effector organ, either directly in the case of efferent 

 nerves, or indirectly through the C.N.S. in the case of sensory 

 fibres. The stimulation of efferent nerves leads either to 

 initiating or augmenting activity in an effector organ (excitatory 

 action) or to diminishing or abolishing response (inhibitory 

 action). 



Reference has already been made to the phenomenon of 

 inhibition, as illustrated by the effects of stimulating the 

 cardiac branch of the vagus in the vertebrate, or certain nerve- 

 fibres connected with the heart in Crustacea and molluscs. 

 Little is known of peripheral inhibition : the phenomena 

 of excitation, conduction, etc., in nerve have been chiefly 

 elucidated through the study of excitatory motor nerves 

 supplying limb-muscles (nerve-muscles preparation) : that 

 the phenomena are essentially similar in other types of nerve 

 is inferred by certain similarities such as the electrical con- 

 ditions which accompany the propagation of the excited state 

 in all irritable tissues. 



The excitation of a muscle through its nerve involves three 



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