EXCITATION AND INHIBITION 427 



these subdivisions being excitatory, the other inhibitory, in respect of the motor 

 neurones. This is similar to the most probable arrangement in the crayfish claw, 

 namely, that each nerve fibre divides into two, one division going to the closing 

 muscle, the other to the opening muscle ; the two parts into which each fibre 

 divides are always one excitatory, the other inhibitory. 



Michailov (1911) describes two different kinds of nerve endings in the muscular tissue of the 

 heart. He regards one of them as sensory, the other as the inhibitory endings of the vagus. It 

 is, of course, also possible that the former might be the terminations of the sympathetic supply. 



We have found evidence that the membranes which are the seat of the 

 excitatory process are of such a nature as to be impermeable to one only of the 

 two oppositely charged ions into which certain electrolytes in the nerve are 

 dissociated. It may possibly happen that, when the impermeable ion is allowed 

 passage by the arrival of the propagated disturbance, it produces chemical or 

 physical changes in the substance of the inhibitory synapse of such a kind as to 

 render excitatory synapses on the same cell incapable of undergoing the normal 

 change of permeability associated with the passage of an excitation. In the 

 present state of knowledge, it is unprofitable to follow such speculations far, and 

 the suggestion is made merely as an indication of a possible mode of explanation. 



REVERSAL EFFECTS 



Although under ordinary conditions, such as change of intensity of stimulus, 

 altered time course, and so on, an inhibitory termination of an afferent arc cannot 

 be made to give any other effect than inhibition, nor an excitatory termination 

 anything but excitation, there is evidence, as Sherrington points out (1906, p. 105), 

 that under special conditions an inhibitory termination can be made into an 

 excitatory one, and vice versa. Certain drugs have effects of this kind. More- 

 over, from the cerebral cortex, antagonistic muscles can be put into action at 

 the same time. We saw above that stimulation of the central ends of various 

 afferent nerves of the hind leg produces inhibition of the contraction of the 

 vasto-crureus in the decerebrate cat. Now, Sherrington found (1905, p 287) that 

 a small dose of strychnine converts this effect into a reflex contraction. I found 

 the same fact in the case of the inhibition of the vaso-constrictor centre by the 

 depressor nerve (1908, 2), and Seeman (1910) in respiratory reflexes. Sherrington 

 also showed that tetanus toxin does the same. It is to be noted that the vasto- 

 crureus is a purely extensor muscle and can be completely isolated. By careful 

 gradation of dose, a stage can be obtained in which the inhibitory effect is 

 diminished, but not replaced by excitation. This fact seems to show that the result 

 is not to be explained by decreased resistance allowing the stimulation to spread to 

 other neurones in the centre, since the resistance at this stage is increased. 

 Another possibility is that the afferent nerve stimulated might contain, along with 

 the fibres causing inhibition, others causing contraction, and that the effect of 

 strychnine might be to paralyse the inhibitory reflex fibres or synapses before the 

 excitatory ones. Now these inhibitory fibres would be associated with those 

 causing contraction of the flexors and no trace of depression in the force of the 

 flexor contractions is to be detected. The antagonist muscles are simultaneously 

 thrown into contraction. I have shown, moreover (1908, 2), that the strychnine 

 reversal of the inhibition of the vasomotor centre is not due to unmasking of 

 excitatory fibres mixed with the inhibitory ones, by paralysis of the latter. The 

 dose of the drug required in the rabbit is a comparatively large one and, at the 

 time that the reversal is most perfect, it is found that the excitatory vaso- 

 constrictor fibres in other afferent nerves are completely paralysed, so that the 

 usual rise of pressure from ordinary sensory nerves is absent. It seems on the 

 whole that the view taken by Sherrington at the first (1906, p. Ill) is the most 

 probable one, namely, that the action of strychnine " is to convert in the spinal 

 cord the process of inhibition whatever that may essentially be into the process 

 of excitation whatever that may essentially be. The reflex nexus was pre- 

 existent, but the effect across it was signalised by a different sign, namely, minus 

 prior to the strychnine or tetanus toxin, instead of plus, as afterwards." 



