596 



Comparative Animal Physiology 



smaller (5 /x) nerve fibers elicits local graded potentials, called small-fiber 

 junctional potentials, which summate on repetitive stimulation and may be 

 accompanied by local low-tension contractions, 10-15 per cent of twitch 

 tension in magnitude (Fig. 229). The slow contractions may relax very 

 slowly, but interposition of a twitch response, when repetitive stimulation 

 of the slow nerve fiber has stopped, results in rapid relaxation. The slow 

 contraction develops tension faster with high frequency than with low fre- 

 quency stimulation. Whether one muscle fiber receives both types of in- 

 nervation is not yet certain, but the net effect of the two types of fiber is 

 peripheral gradation of response, the slow response being "tonic" in character. 

 In mammalian striated muscle, small motor nerve fibers activate intrafusal 

 muscle fibers and so regulate the afferent discharge originating in sensory 

 spindles (Ch. 14). 



The most studied examples of multiple motor innervation are in arthro- 

 pods. Early histological studies^"'"' ^-^ showed that whole muscles in many 

 crustaceans and insects are innervated by only two (or a few) axons and 

 that each muscle fiber has multiple innervation; i.e., each muscle fiber re- 



Fig. 229. Response of frog muscle to stimulation of large and small fibers. Action 

 potential from gastrocnemius, progressive pressure block to sciatic: 1, partial block, 

 threshold nerve stimulation, only spike response; 2, increased strength, spike followed 

 by small junction potential; 3, pressure increased, stimulus as in 2, only small junction 

 potential remains. From Kuffler and Gerard. ^^" 



ceives branches from two or more nerve fibers, often from each of the 

 motor fibers to the muscle. Triple innervation of claw muscles is common 

 among crustaceans,' '' and even quintuple innervation has been seen in a 

 muscle of the leg of Panulinis}'^^ It is likely that multiple innervation of 

 single fibers and innervation of all the fibers of a muscle by the same axon 

 is a general rule among higher crustaceans and insects. 



Crustacean neuromuscular systems have several properties which are 

 related to their multiple innervation. First, a single strong shock applied to 

 a crustacean nerve sometimes elicits repetitive discharges in the nerve.^" 

 This repetitive response depends on the calcium in the medium bathing the 

 nerve. '^^ Second, neuromuscular facilitation is marked with some motor 

 fibers, the contraction increasing in amplitude and rate of rise of tension 

 with increasing frequency (Fig. 226). In addition, stimulation of some 

 motor axons elicits a large fast contraction, whereas other motor axons elicit 



