ALL-OR-NONE PRINCIPLE 39 



preparation the height of contraction varies with the 

 strength of stimukis. Herein hes an apparent difference 

 between the behaviour of skeletal and of cardiac muscle, 

 for the latter, if it responds at all, responds with the 

 maximum contraction of which it is capable under the 

 circumstances — the all-or-none principle. In the striated 

 muscle which we are considering, a submaximal response 

 might conceivably be due either to the stimulation of some 

 of the fibres and not others, those which respond doing so 

 with a maximum contraction, or to the stimulation of all the 

 fibres to an equal but incomplete contraction. Keith 

 Lucas, using a muscle composed of very few fibres, showed 

 that on increasing the stimulus, the increase in response 

 took place in a number of stages never greater than the 

 number of fibres. The increased contraction at each stage 

 therefore appeared to be due to the imphcation of an 

 increasing number of fibres. It would seem, therefore, 

 that a striated muscle fibre obeys the all-or-none principle. 

 This feature of muscular contraction is more obvious in 

 the heart, because here all the fibres are knit together, the 

 contraction wave being conducted from one to the other. 

 The difference may be expressed in this way. Cardiac 

 muscle as a whole obeys the all-or-none principle because 

 the individual fibres do so, and any contraction involves 

 all the fibres. Striated muscle-fibres also obey the all-or- 

 none principle, but the muscles into which they are bound 

 do not do so, because a variable number of fibres may 

 contract, there being no cell-to-cell propagation of the 

 contracted state. 



4. Frequency of Stimulation : Tetanus. — When a second 

 stimulus is thrown in before the contraction from a previous 

 stimulus has subsided, a second contraction occurs which 

 begins at whatever stage of contraction the muscle is in 

 as the result of the first, the height of the second contraction 

 being greatest if the second stimulus acts at the summit 

 of the first contraction. This phenomenon is known as 

 Summation. 



