EXCITATION AND INHIBITION 401 



values of the current durations at which the current strength reaches its smallest 

 values, that is, the optimal rate of incidence of stimulus : we have : 



Substance ft - - 0-0009 second 



Nerve fibre 0-003 



Muscle fibre of sartorius 0-02 



Ventricular muscle 2-00 seconds 



Some interesting conclusions are drawn by Keith Lucas from these figures. If the ions 

 concerned in the excitatory process were the simple inorganic ones, K.% Ca", Cl', etc., the 

 variations in rate could not exceed 10 to 1, whereas between substance /3 and ventricular muscle 

 there is a ratio of 4,000 to 1. The temperature coefficient is also higher than that for a simple 

 ionic velocity. Similarly, when the calcium of the Ringer's solution is replaced by an isotonic 

 amount of sodium, the value representing the rate of movement of the ions in excitation (k) 

 decreases ten times, whereas the ratio of the velocities of Na - to Ca" is as 44 to 53 only. There 

 is evidently some factor not as yet completely accounted for, and the question requires further 

 investigation. 



Lapicque, who has worked out the theory of electrical excitation and come 

 to certain conclusions similar to those of Lucas, but without arriving at a 

 mathematical form of his hypothesis, has made a hydrodynamic model (1909) on 

 which many of the facts can be demonstrated ; the movement of ions is imitated 

 by that of water. Lapicque has introduced a constant which he calls " chronaxie," 

 relating to the rate of movement of the ions concerned in excitation, which 

 constant is of similar significance to Waller's " characteristic " and the logarithm 

 of the constant 6 of Hill's modified Nernst formula. Lapicque and Legendre (1913) 

 find that there is a relationship between this rate of movement and the diameter 

 of the various nerve fibres in the same animal. The larger the fibre, the faster 

 the movement. Thus the clironaxie, which is the reciprocal of the rate, is 0-0003 

 second in the case of the motor nerve to the gastrocnemius, whose fibres have a 

 diameter of 0-02 mm. and 0-02 second in the motor fibres to the stomach, with 

 a diameter of 0-002 mm. It might, perhaps, be expected that the rate of 

 movement would be greater in a large fibre, if the ions in the middle have to 

 reach the membrane on the outside of the axis cylinder in the same time as those 

 in a small fibre. But this is purely hypothetical. W. W. Waller (1914) finds 

 that the optimal time value (Lapicque's " chronaxie ") for excitation of sudo-motor 

 nerves is ten times that for motor nerves to muscle. 



Mode of Connection between Nerve and Muscle. The balance of evidence is 

 decidedly in favour of the view that there is merely close contact at the end-plate 

 of the nerve ; there is, apparently, no continuity of cell substance, but a membrane 

 is interposed. When a nerve fibre degenerates, the process does not proceed 

 beyond the end-plate. There is, no doubt, in excitation, a change at this 

 " synaptic " membrane, by which the transfer takes place. The receptive substance 

 of Langley must be supposed to lie on the muscle side of the membrane and be 

 confined to the region in the neighbourhood of the connection with the nerve 

 fibre. It is not necessary to regard this substance as a constituent radicle of the 

 protoplasmic " molecule " of the muscle cell. 



As we shall see later, a similar problem arises as to the transmission of 

 excitation from the branches of one nerve cell, or neurone, to the cell body of 

 another. We shall find that Sherrington's conception of a "synaptic membrane " 

 is most in accordance with experimental facts. 



INHIBITION 



The name is applied to any process by which an action in progress is stopped 

 by the application of some influence from without. It is not used when a process, 

 such as a nerve impulse or a muscular Contraction, excited by a momentary 

 stimulus, runs a definite time course and then ceases spontaneously. 



The result itself can evidently be brought about in different ways, according 

 to the particular mechanisms involved ; so that it does not seem correct to speak 

 of a general theory of inhibition. Consider a smooth muscle cell in a state of 

 natural tonus : excitation of a certain nerve fibre, connected to this cell, puts 



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