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 contrac- 

 tion 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 individual muscle fibre of 

 which the muscle is composed. It seems more probable, however, 

 that, when a minimal or subminimal response is obtained, not 

 all the fibres making up the muscle are contracting. A mini- 

 mal contraction 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 con- 

 cerns each individual muscle fibre every contraction is a maxi- 

 mal contraction. The fibre either contracts to its utmost or it 

 does not contract at all. The rule of ' all or none ' which was ^ Q 



first enunciated for heart-muscle is probably true for every con- FIG. 65. 

 tractile element. The difference between skeletal and heart 

 muscle lies in the fact that in the former the excitatory process 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 ventricu- 

 lar or auricular muscle of the frog or tortoise were treated in the same way, 

 a stimulus applied at A would cause a contraction which would travel across 



the bridge at the upper end and extend to B. 



205 



