536 UNEQUAL EXCITABILITY. 



applied to a motor nerve, the greatest effect is produced when the current is made 

 or closed [closing or make contraction], and when it is broken or opened [opening 

 or break contraction]. But while the current is passing, the stimulation does not 

 cease completely, for, with a certain strength of stimulus, the muscle remains in a 

 state of 'tenanus (galvanotonns or " closing tetanus ") (Pfliiger). For the same 

 effect on muscles, see p. 482. With strong currents this tetanus does not appear, 

 chiefly because the current diminishes the excitability of the nerves, and thus 

 develops resistance, which prevents the stimulus from reaching the muscle. 

 According to Hermann, a descending current applied to the nerve, at a distance 

 from the muscles, causes this tetanus more readily, while an ascending current 

 causes it more readily when the current is closed near the muscle. The constant 

 current is said by Griitzner to have no effect on vaso-motor and secretory fibres. 



Over-maximal Contraction. By gradually increasing the strength of the electrical stimulus 

 applied to a motor nerve, Fick observed that the muscular contractions (height of the lift) at 

 first increased proportionally to the increase of the stimulus, until a maximal contraction was 

 obtained. If the strength of the stimulus be increased still further, another increase of the 

 contraction above the first readied maximum is obtained. This is called an "over-maximal 

 contraction." Occasionally between the first maximum and the second there is a diminution, or 

 indeed absence of, or gap or hiatus, in the contractions. The cause of this lies in the positive 

 pole, which with a certain strength of current is sufficient to prevent the further transmission 

 of the excitement ( 335). On continuing to increase the induction current, ultimately a stage 

 is reached where the stimulation at the negative pole again becomes stronger than the inhibition 

 at the positive, and this overcomes the latter. The contractions before the gap are caused by 

 the occurrence of the induction current (their latent period is short) ; the contractions (long 

 latent period, like that after all opening shocks Waller), after the gap, are caused by the 

 disappearance of the induction current, i.e., by polarisation ; this is added to the stimulation 

 proceeding from the negative pole, which after the gap overcomes the inhibition at the 

 positive pole, and excites the over-maximal contractions (Tigerstedt and Willhard). 



Tetanus. If single shocks of short duration be rapidly applied after each other 

 to a nerve, tetanus in the corresponding muscle is produced ( 298, III.). 



A motor nerve has a greater sjyecijic excitability for electrical stimuli than the 

 muscle-substance. This is proved by the fact that, a feebler stimulus suffices to 

 excite a muscle when applied to the nerve than when it is applied to the muscle 

 directly, as occurs when the terminations of the motor nerves are paralysed by 

 curara (Rosenthal). 



Soltmann found that the excitability of the motor nerves of new-born animals 

 for electrical stimuli is less than in adults. The excitability increases until the 5th 

 to 10th month. 



Unequal Excitability. Under certain circumstances, the nearer the part of 

 the motor nerve stimulated lies to the central nervous system, the greater is the effect 

 produced (contraction) ; [or what is the same thing, the further the point of a nerve 

 which is stimulated is from the muscle, the stimulus being the same, the greater is 

 the contraction. This led Pfliiger to his "avalanche-theory," i.e., that the "nerve- 

 motion " increases in the nerve as it passes towards the muscles. This effect is 

 explained, however, by the unequal excitability of different parts of the same 

 nerve]. According to Fleischl, all parts of the nerve are equally excitable for 

 chemical stimuli. Further, it is said that the higher placed parts of a nerve are 

 more excitable only when the stimulating current passes in a descending direction; 

 the reverse is the case when the current ascends (Hermann). On stimulating a 

 sensory nerve, Rutherford and Hallsteu found that the reflex contraction was 

 greater, the nearer the stimulated point was to the central nervous system. 



Unequal Excitability in the same Nerve. Nerve-fibres, even when functionally 

 the same and included in the same nerve-trunk, are not all equally excitable. Thus, 

 feeble stimulation of the sciatic nerve of a frog causes contraction of the flexor 

 muscles, while it requires a stronger stimulus to produce contraction of the extensors 

 (Ritter, 1805, Rollett). According to Ritter, the nerves for the flexors die first. 



