cgQ Comparative Animal Physiology 



In Crustacea the graded potentials of the neuromuscular junction con- 

 tribute largely to the total action potential of the muscle. Their role in con- 

 traction will be discussed below. 



In long-fibered smooth muscles, such as the anterior retractor of the byssus 

 in Mytilns, impulses appear to be propagated at a rate of 13-20 cm./sec. 

 with a simple action potential of duration 2-3 sec.^o^- 10.5 when the muscle 

 is stimulated repetitively the action potentials increase in height (staircase) 

 and at 5-7 per second can fuse. Either there is marked fiber asynchrony or 

 else the action potential, although conducted along the muscle, is not an 

 all-or-none membrane depolarization but resembles more a graded end-plate 

 potential. 



In the retractor of the buccal mass of Helix the action potential con- 

 sists of two waves, fast and slow (Fig. 221 ).^'^ When stimulated through 

 the nerve the neuromuscular junction is refractory for 10 msec, but thereafter 



I I M I I I I I I I I I I I I I 



B 



Fig. 221. Action potential of fast and slow v<'aves, and mechanical record from buccal 

 retractor of Helix. A, Response to single stimulus. B, Response to repetitive stimulation 

 at l/sec, showing summation of contraction. Time in 1/5 second. Bolzer.*' 



shows supernormal excitability with maximum facilitation at 50 msec; the 

 muscle potentials in response to stimuli at an interval of about 50 msec, 

 summate. With repetitive stimulation at 10/sec. the muscle potential de- 

 clines, and at 100/sec. there may be continued contraction but no demon- 

 strable action potential.^""' The retractor muscles of Thyone also show fast 

 and slow electrical waves."" None of these long-fibered smooth muscles 

 have been examined in regard to conduction in their single fibers. 



Short-fibercd and branched-fibered muscles present a difficult electrical 

 problem. In the conduction wave of the vertebrate heart there are fast and 

 slow components (Ch. 15). Recent observations'*' on single cardiac fibers 

 or small bundles show that depolarization persists for tenths of a second, 

 hence the slowness of the muscle mav depend on its failure to repolarize 

 quickly. Action potentials have not been analyzed from any of the short- 

 fibered spiral striated muscles of invertebrates. 



Smooth muscles of higher vertebrates are innervated by postganglionic 

 autonomic nerves, the sympathetics usually liberating an adrenin-like com- 

 pound, and the parasympathetics liberating acetylcholine. Propagation along a 

 sheet of smooth muscle has been variously claimed to be (1) along each 



