MYOGENIC RHYTHMS 109 



rhythmicity is affected by stretch. Pharmacological evidence suggests that 

 the rhythmicity originates in the surface membrane. 



A somewhat different type of myogenic rhythm occurs in the embryonic 

 skeletal muscle fibers of various lower vertebrates ( Harris and Whiting, 

 1954; Harris, 1955). In the embryo dogfish the myotome muscle fibers 

 show rhythmic contractions before motor or sensory nerve fibers reach 

 them. The contractions are regular and synchronous down the whole 

 column of one side, but there is no correlation in the timing of contractions 

 on the two sides. At this stage acetylcholine accelerates the rhythm. When 

 nerve fipers first reach the muscles they have what Harris calls a "neuro- 

 cratic" action ; the frequency of the myogenic rhythm is increased, still 

 without bilateral coordination. Finally true coordinated movements occur 

 at about the time the sensory nerve supply develops. Electrical recordings 

 have not been made from these delicate tissues, but Whiting (personal 

 communication) reports that they are unexpectedly inexcitable by elec- 

 trical stimuli. Were it not for this observation one would be tempted to 

 conclude that here again the rhythmicity comes basically from the cell 

 membranes ; the influence of acetylcholine and the exact synchrony of con- 

 tractions down the whole column of one side are difficult to explain except 

 in terms of an impulse mechanism, propagated from muscle cell to muscle 

 cell ; there is a significant overlap of fibers across the myotome boundaries 

 at the myogenic rhythm stage, which disappears later. 



Is there, then, anywhere in the animal kingdom other than the insects 

 any sign of a rhythmically contractile tissue in which the rhythm does not 

 derive from properties of the cell surface ? Rhythmic movement in the lower 

 animals is usually required for swimming, and Gray (1953) has recently 

 reviewed his work on this type of propulsion. We know virtually nothing 

 about the neuromuscular physiolog}' of these movements in any inverte- 

 brate and it is dangerous to argue too closely by analog}^ from fish and 

 snakes. Gray (1951, 1955) has shown that even in bacteria and sperma- 

 tozoa there is a great similarity in the dynamics of swimming to the move- 

 ments of higher animals, and there is thus probably a strong functionally 

 conditioned isomorphism between all propulsive mechanisms of this type. 

 In bacteria and spermatozoa it is hardly possible to conceive of anything 

 akin to a reflex responsible for rhythmicity or coordination, and here at 

 least there is a strong prima facie case for looking in the mechanism of con- 

 traction for the origin of the rhythm. There is no device here for any sudden 

 release of tension in an active tissue, and if the rhythm is myogenic the 

 effective mechanical stimulus must be a relatively slow one. The possibility 

 of a nonnervous origin for the rhythm of swimming should not auto- 

 matically be excluded even for Metazoa ; a lot could be achieved by a myo- 

 genic rhythm with "neurocratic" control. 



