CHAPTER LXVI 

 THE PHYSIOLOGY OF THE NERVE-FIBRE 



NERVE may be stimulated by natural or artificial stimuli. Natural 

 stimuli are applied to the " receptor " mechanisms of the body, such 

 as the nervous elements concerned in sight, smell, taste, hearing, 

 touch, temperature. These are dealt with later. 



A nerve may be artificially stimulated in various ways : mechanic- 

 ally, by pinching ; thermally, by a hot wire ; chemically, by placing on 

 the nerve a few grammes of sodium chloride, or some glycerine; 

 electrically, the induced current is commonly employed in experi- 

 mental work upon nerve. 



As judged by the effects of its stimulation on a muscle, nerve 

 responds to minimal, submaxirnal, and maximal stimulation. The 

 response, if any, is probably always maximal, but as by a weak stimulus 

 only a few nerve-fibres are stimulated, the number of muscles-fibres 

 which contract are correspondingly few; hence the minima,! con- 

 traction evidenced by the lift of the muscle lever. In a mixed nerve, 

 all the fibres are not equally excitable. For example, on gradually 

 increasing the stimulation of the sciatic nerve of a frog, first the flexor 

 muscles, and then the extensor muscles, are excited to contraction. 

 Similarly, in a mixed nerve, the vaso-constrictor fibres respond to 

 a stimulation weaker than that which affects the vaso-dilator fibres. 

 The nerve of a nerve-muscle preparation is not equally excitable in 

 all parts of its course. 



Immediately after making the preparation, the nerve is most 

 excitable at its upper end. After a time this passes off, and the most 

 excitable part progressively descends towards the muscle. A nerve- 

 fibre is also more excitable in the neighbourhood of the main branches 

 severed during its preparation. The increased excitability is probably 

 due to changes in the nerve provoked by injury. 



The excitability of nerve is modified by various factors. It is. 

 increased by slight cooling below room temperature, and decreased 

 by greater cold. It is increased by gentle warmth. Loss of water 

 at first increases and then abolishes excitability. Chemical sub- 

 stances affect nerve in various ways. Carbon dioxide, chloroform, 

 and ether depress the excitability. 



If a constant (polarizing) electric current be passed through a nerve, 

 it increases the excitability around the negative pole the kathode 

 and depresses it around the positive pole the anode. 



The rate of conduction of the nerve-impulse is computed to be 



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