592 Comparative Animal Physiology 



negativity following the impulse (intestine).-*"- ■'- Under some conditions 

 (e.g., in pendular movement) the intestine may show a pre-contraction 

 electrical wave which appears broader when the electrodes are separated; 

 when the electrodes are close together fast spikes conducted in millimeter 

 units are seen.^ In the ureter slow waves predominate over fast ones (Fig. 

 223), but there is much species variation.''" Rhythmic activity of visceral 

 muscles is accompanied by repetitive discharges, the slow components show- 

 ing summation. Extension to smooth muscle of the microtechniques used 

 in studying end-plate potentials in striated muscle should help to clarify 

 the electrical picture. 



Striated muscles show graded local mechanical responses which can sum- 

 mate, graded end-plate potentials, propagated all-or-none spikes, and after- 

 potentials. Just how the "fast" and "slow" electrical waves in smooth muscles 

 compare with the striated muscle pattern is not clear. Apparently different 

 components predominate in different muscles and graded potentials may be 

 of widespread importance. 



EFFECTS OF REPETITIVE EXCITATION 



In the normal physiology of animals, muscles are rarely stimulated to a 

 response by a single nerve impulse. Repetitive stimulation is the rule. Most 

 muscles develop increased tension with increasing frequency of stimulation, 

 but there is much variation in the magnitude of the frequency effect on 

 tension, and several mechanisms contribute to the effect. 



In vertebrate striated muscles a single nerve impulse can elicit an all-or- 

 none contraction. These muscles are non-iterative.. In other muscles (e.g., 

 many smooth muscles) repeated stimulation is required to evoke a measur- 

 able response; such muscles are iterative. A subthreshold stimulus delivered 

 to any excitable tissue evokes an excitatory process which decays at a rate 

 characteristic of the tissue. The excitatory state from one subthreshold stimu- 

 lus can summate within a limited time with the excitatory state left from 

 a preceding stimulus. In non-iterative muscles a single nerve impulse sets 

 up sufficient excitatory state at the neuromuscular junction to set off an 

 impulse propagated over the muscle. In iterative muscles facilitation of 

 nerve impulses is usually needed, and the contraction builds up as more im- 

 pulses are delivered. 



When the stimuli applied via the nerves to non-iterative muscles (such 

 as vertebrate striated) are submaximal, some fibers contract while others 

 are subliminally stimulated, hence repeated stimuli bring into action more 

 and more fibers. When, on the other hand, maximal intensities are used 

 and all the fibers are responding, the tension also increases with increasing 

 frequency but to a less degree than with submaximal shocks. Possibly the 

 contractile elements do more work, the proteins folding more completely, 

 with repeated excitation. More probably part of the work of contraction is 

 required to overcome certain "viscous" resistance, to align the molecules 

 of sheath and connective tissue; when relaxation is incomplete between 

 contractions, less work is necessary to overcome passive resistance, and 

 when the contractions are smoothly fused maximum energy can go into 

 the development of tension. The tetanus-twitch ratio is higher with sub- 



