SECTION IV 



THE CONDITIONS AFFECTING THE MECHANICAL 

 RESPONSE OF A MUSCLE 



STRENGTH OF STIMULUS. If a series of single break-shocks be 

 applied to a muscle or nerve at intervals of not less than five seconds, 

 it will be found that beyond a certain distance of the secondary from 

 the primary coil no effect at all is produced. The shocks 

 are said to be subminimal. On pushing the secondary 

 coil nearer the primary a point will be reached at which 

 a small contraction will be observed. On then pushing in 

 the coil a millimetre at a time the contraction will become 

 greater for the next couple of centimetres (e.g. as the coil 

 is moved from 12 to 10 cm. distance). Further increase 

 of current by approximation of the coils is without effect, 

 although the current actually used may be increased a 

 hundred times in moving the coil from 10 to 0. It was 

 formerly thought that this limited gradation of the 

 muscular response according to strength of stimulus was 

 due to a similar gradation in the response of each indi- 

 vidual muscle fibre of which the muscle is composed. It 

 seems more probable, however, that, when a minimum 

 or subminimal response is obtained, not all the fibres 

 making up the muscle are contracting. A minimal con- 

 traction is in fact a contraction in which some fibres of 

 the whole muscle are stimulated. A maximal contraction 

 is one in which all the fibres are stimulated. So far as 

 ^ D concerns each individual muscle fibre every contraction 



Fiu. (55. is a maximal contraction. The fibre either contracts to 

 its utmost or it does not contract at all. The rule of 

 ' all or none ' which was first enunciated for heart-muscle is probably 

 true for every contractile element. The difference between skeletal 

 and heart muscle lies in the fact that in the former the excitatory pro- 

 cess does not spread from one fibre to its neighbours. If, for instance, 

 we take a curarised sartorius and split its lower end, as in Fig. 65, 

 the stimulus applied to A causes a contraction only of the left-hand 

 side of the muscle, while a stimulus applied to B is in the same way 

 limited to the right-hand side. If a piece of ventricular or auricular 



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