Muscle and Electric Organs 



591 



muscle cell as in striated muscle, the muscle cells being innervated as motor 

 units (nictitating membrane, blood vessels, pilomotors),^"- -**- (2) from 

 muscle cell to muscle cell by protoplasmic bridges (visceral muscles),'*^ 

 (3) through the muscle sheet by a nerve plexus/ (4) by diffusion of a 

 mediator (sympathin) from cell to cell.-'^'' Evidence for diffusion comes 

 from the dependence of size of response on number of impulses; maximum 

 tension in striated muscle depends on the number of functioning motor 

 nerve fibers, whereas in smooth muscle the maximum tension when few 

 fibers are stimulated approaches the maximum which obtains when the 

 whole trunk is stimulated. Similar gradation of response occurs with in- 

 jections of different amounts of adrenalin.-"'' Evidence for the first two 

 hypotheses comes from electrical records. 



i. 



Fig. 222. Action potentials of nictitating membrane of cat: a to d, curves resulting from 

 increasing intensities of stimuli. Eccles and Magladerry."^ 



A B 



Fig. 223. Action potentials from the ureter of the rat: A, diphasic, and B, monophasic 

 record with electrodes 1 mm. apart. Bozler.'^ 



In the nictitating membrane of the cat a nerve volley insufficient to elicit 

 a contraction sets up a slow negative wave (N) which mav last 10 msec. 

 (Fig. 222); threshold volleys elicit larger negative waves which appear to 

 be related causally to contraction (A and B waves of Eccles and Magla- 

 derry,"- I waves of Rosenblueth et al.-°'0- After these waves a large positive 

 deflection is seen, and thereafter slow rhythmic oscillations may occur. In- 

 jected adrenalin elicits rhythmic but less synchronous oscillations than nerve 

 impulses. 1 here is disagreement concerning the parts played by liberation 

 and diffusion of mediator and by muscle cell impulses in this complex elec- 

 trical response. In visceral smooth muscle records obtained with electrodes 

 close together show monophasic or diphasic spikes resembling the impulses 

 conducted along striated fibers (Fig. 223*''^). In addition, a slow non-propa- 

 gated negative wave may precede a spike, and there may be some residual 



