246 PHYSIOLOGY 



effect is observed when voluntary muscle is excited by continually recurring 

 stimuli (v. Fig. 70, p. 209). 



We shall meet with other examples of this summation of stimuli when 

 dealing with the physiology of the central nervous system. It is indeed a 

 fundamental phenomenon in the physiology of excitation. 



CHEMICAL STIMULATION. Strong salt solution excites contractions 

 just as in the case of skeletal muscle. Many drugs, such as physostigmine, 

 ergot, salts of lead and barium, digitalis, may act directly on smooth muscle 

 and cause contraction. As one would expect, however, from the greater 

 ' independence of the smooth muscle, the action of these drugs varies from 

 organ to organ, muscle-fibres, which apparently are histologically identical, 

 reacting diversely according to their origin. 



MECHANICAL STIMULATION. Smooth muscle may react to a local 

 -pinch or blow with a local or a general (propagated) contraction. The most 

 important form of mechanical stimulation is that produced by tension. 

 The effect of increasing the tension on smooth muscle may be twofold : 

 causing in the first^ place relaxation and in the second excitation with in- 

 creased contraction. These two effects may be illustrated by taking the 

 case of the bladder. If this viscus (which is surrounded by a complete 

 coat of smooth muscle) has all its connections with the central nervous 

 system severed, it is when empty in a state of tonic contraction. If fluid 

 be injected into it rapidly there is a great rise of pressure in its cavity, due 

 to the forcible distension. If, however, the fluid be injected slowly the 

 bladder muscle relaxes to make room for it, so that a considerable amount 

 of fluid may be accommodated in the bladder without any great rise of 

 pressure. This process of relaxation has its limit. If the injection of fluid 

 be continued the walls begin to be stretched passively, and this increased 

 tension acts as a stimulus causing marked rhythmic contractions of the 

 whole bladder. 



In the same way the response of a smooth muscle to an electrical stimulus 

 is much increased by previous increase of the tension on the muscle fibres. 



PROPAGATION OF THE EXCITATORY STATE, OR WAVE OF 

 CONTRACTION. On stimulating any part of a voluntary muscle fibre, 

 a wave of contraction is started which travels to each end of the fibre, but 

 no further. There is no propagation from muscle fibre to muscle fibre, 

 the synchronous contraction of the whole muscle being brought about by 

 simultaneous excitation of all its fibres. It is doubtful whether this isolation 

 of the excitatory state is found in smooth muscle. As a rule a stimulus 

 applied to any part of a sheet of smooth fibres may travel all over the sheet 

 just as if it were a single fibre. It seems probable indeed that there is 

 protoplasmic continuity by means of fine bridge-like processes between 

 adjacent muscle cells. And even in the absence of such bridges the propaga- 

 tion of the contraction could be easily accounted for. Although in the case 

 of voluntary muscle the rule is isolated contraction, yet a very small change 

 in the muscle, such as that produced by partial drying or by pressure, is 

 sufficient to cause the contraction to spread from one fibre to another. 



