EXCITATION AND INHIBITION 433 



the way in which the nerve responds to stimulation. There is no heat produced, 

 and the evolution of carbon dioxide is questionable. 



The various practical methods of setting up a propagated disturbance in 

 excitable tissues are described in the text. 



There are no differences of degree in the state of excitation of a nerve or 

 muscle cell in a given state ; a stimulus either produces the maximal effect that 

 the tissue is capable of in this condition, or no response at all, in the way of a 

 propagated disturbance, Nevertheless, a stimulus too weak to do this leaves 

 behind a local change at the point of application. Degrees of contraction, produced 

 in a muscle by different intensities of stimulation, are due to the activity of a 

 larger or smaller number of fibres in the nerve or muscle. 



A narcotised region of nerve reduces the intensity of a propagated disturbance 

 as it passes along it, so that, if long enough or sufficiently deeply narcotised, it 

 abolishes the disturbance altogether. But, if any state of excitation is left at 

 all, the disturbance returns to its original magnitude when it enters a normal 

 region again. 



It appears, then, that the degree of activity of a tissue supplied by nerves 

 depends only on the number of tissue cells acted upon, and on the state of these 

 particular cells ; not on any difference of degree in the stimuli reaching a given 

 cell. It should be mentioned that this view is not accepted by all investigators so 

 far as it applies to reflexes from the central nervous system. 



All excitable tissues are incapable of response to a second stimulus applied at 

 a short interval of time, differing in different tissues, after a previous one. This 

 is the " refractory period," and consists of a first part, where no strength of stimulus 

 whatever will excite ("absolute refractory state"), and of a second part ("relative 

 refractory state "), where stimuli stronger than normal are required. The 

 disturbances set up in this latter period are smaller than normal, but cannot be 

 made greater than those set up by a stimulus just sufficient to excite, however the 

 stimulus is increased. Their magnitude increases progressively up to the end of 

 the refractory period. 



The refractory state is not only local, but follows the propagated disturbance as 

 it passes along the nerve fibre. 



The question of fatigue of nerve fibres is somewhat disputed. There is some 

 evidence, not altogether convincing, that oxygen is necessary for the continuance 

 of the excitability of a nerve fibre. 



The electrical negativity associated with the passage of an impulse is followed 

 by a state of increased positivity, the explanation of which is not yet clear. 



The passage of the nerve impulse takes time, the rate being increased by rise of 

 temperature.. The temperature coefficient is 1*79 for 10 C. 



There is evidence that the state of excitation is accompanied by increased 

 permeability of the cell membrane. If the membrane be impermeable, at rest, 

 to one only of the ions of an electrolyte within the cell, the membrane is 

 " polarised," and the " current of rest," " injury current," or " demarcation 

 current," is accounted for. If this semipermeability is abolished in excitation, 

 the "negative variation" can be accounted for, and also the diminished polaris- 

 ability in this state. 



A certain formula was put forward by Nernst to express stimulation by an 

 electrical current. The basis of this expression is a movement of ions to or from 

 a semipermeable membrane and is suggested as an approximation only. Taking 

 further known facts into consideration, A. V. Hill modified the formula in a 

 way which was found by Keith Lucas to satisfy most cases of experimental test. 

 The factors playing a part are the number of ions and their charge, the distance 

 between the membranes, and the distance of the place where the concentration 

 takes place from the membrane under consideration, the rate -of movement of the 

 ions, and a factor expressing rate of " recombination " of ions. This last factor 

 is probably an adsorption in the sense of Macdonald's theory. 



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