Exercise XIX 



MUSCLE 103 



into the cells, so depolarizing them and causing 

 them to fire impulses regularly. Acetylcholine 

 exerts its effect only on this area, apparently by 

 increasing the permeability of the cells to K+, so 

 allowing some K+ to leak out. This hyper- 

 polarizes the cells, and lengthens the time re- 

 quired for the sodium leak to depolarize them 

 to the point of firing. 



Epinephrine exerts its effect both at the SV 

 node and over the entire heart, by increasing the 

 cell permeability to Na+, thereby tending to de- 

 polarize the membranes, and facilitating the 

 conduction of impulses. Epinephrine also seems 

 to have some effect on the contractile process 

 itself, since it increases the amplitude of con- 

 traction of the heart fibers. Did you see this 

 effect? 



Atropine is thought to exert its effect by com- 

 bining with the same sites on the membrane that 



acetylcholine would otherwise combine with, 

 much as an inhibitor, by combining with an 

 enzyme, blocks the substrate. 



Finally, show that the ventricular beat origin- 

 ates in the atria by tying a suture (a thread) 

 around the heart between the atria and ventri- 

 cle. This pinches the specialized connecting 

 cells which transmit the excitation from the atria 

 to the ventricle. These cells normally delay the 

 impulse long enough for the atria to complete 

 their contractions before the ventricle begins to 

 contract. When pinched by the thread, they may 

 not conduct at all, or may conduct so slowly 

 that they transmit only one impulse for every 

 two or three contractions of the atria. This is 

 called a heart block. 



After you have caused a heart block, stimulate 

 the ventricle electrically with a stimulator. Does 

 the ventricle contract? What do you conclude? 



EQUIPMENT 



Per student 



dissecting microscope 



compound microscope 



microscope slides 



prepared slide of striated, cardiac, and smooth 



muscle 



petri dish 



fine forceps 



plastic ruler 



Per 4 students 



solution of 0.25% ATP in 0.05-M KCl + 0.001-M 



MgCb 



solution of 25% ATP 



solution of 0.05-M KCI + 0.001 -M MgCb (above 



three solutions in 10-ml dropping bottles) 



15% glycerol-water mixture (25 ml) 



strip of glycerinated rabbit psoas muscle (soaked in 



50% glycerol and water mixture at 0° for 2 weeks; 



changed to cold 15% glycerol and water mixture 



1 hour before using) (preparation of muscle fibers 

 described in text; see also A. Szent-Gyorgyi's book: 

 Chemistry of Muscular Contraction, Academic 

 Press, New York, 1951) 



Per 2 students 



kymograph and smoked paper 

 frog board 

 kymograph lever 

 ring stand 

 needle (small) 

 thread 



medicine droppers 



acetylcholine solution (2 X lO""* M) 

 epinephrine solution (2 X 10"'* M) 

 atropine solution (5 X 10~* M) (above three solu- 

 tions in 10-ml dropping bottles) 

 frog ringer (50 ml, in dropping bottle) 

 plasticene 

 frogs (pith one hour before using) 



