Exercise XIX 



MUSCLE 101 



rabbit had been killed, hastily eviscerated, and 

 the body wall chilled. The psoas muscles lie at 

 both sides over the backbone. They are readily 

 divided into narrow strips, and each such strip 

 was tied at its ends to an applicator stick at the 

 slightly stretched length at which it was found in 

 the body. Then the muscle strips, tied to their 

 sticks, were cut away, and soaked in a half-and- 

 half mixture of glycerol and water at 0°C for two 

 weeks or longer. 



This procedure removes almost all the con- 

 tents of the muscle fibers except for the contrac- 

 tile proteins, actin and myosin. The muscle still 

 retains its striated appearance under the micro- 

 scope, and can still contract when supplied with 

 ATP. Indeed it retains these properties for many 

 months in the cold glycerol solution. 



(The following experiment should be com- 

 pleted during the first hour of the laboratory.) 



One such strip of glycerinated muscle will be 

 given to each group of four students. With 

 scissors cut the muscle just inside the sutures 

 (knotted threads) that hold it to the stick, so as 

 to get as long a piece as possible, and then cut 

 this in half, so that each pair of students gets half 

 the strip. Drop each piece into a small petri dish 

 containing cold glycerol-water mixture. 



With fine forceps and dissecting needles, 

 tease out narrow threads of muscle about 15 mm 

 long and about as thick as silk thread. Place one 

 of these in a drop of cold glycerol-water mixture 

 on a microscope slide, and examine its structure 

 in the microscope. Can you see the striations? 

 (To see them clearly, you may have to separate 

 out a single fiber.) Transfer the slide to the stage 

 of a dissecting microscope, and measure the 

 length of the muscle with a ruler. Add a drop of 

 0.25% ATP solution containing 0.05M KCl and 

 0.001 M MgCl2. Remeasure the length of the 

 muscle, and re-examine its striations in the com- 

 pound microscope. (If the strip curls during con- 

 traction, it is too thick.) Can you distinguish any 

 diff"erences? 



Try this several times, each time measuring 

 the original and final length of the fibers. Record 

 your results. Does a second drop of the ATP- 

 salt solution have any further effect? For con- 



traction to occur, certain ions must be present in 

 specific proportions, in addition to ATP. To a 

 degree, other ions can be substituted for the 

 K+ and Mg++ that we use here, but no other 

 substance seems to substitute for ATP, which 

 seems specific for this process. Convince your- 

 self of the need for the ions and the ATP by add- 

 ing drops of ATP alone, and KCl-MgClo mix- 

 ture alone, to narrow threads of muscle. 



Hormonal control of the frog heart 



By the end of the hour, turn to the next experi- 

 ment, on the effects of acetylcholine and 

 epinephrine (adrenaline) on the frog heart. Work 

 in pairs. While one partner is preparing the ani- 

 mal, the other should become familiar with the 

 operation of the kymograph. This will be 

 demonstrated by the instructor. A revolving 

 drum has a piece of smoked paper wrapped 

 around it. (Be careful not to touch or otherwise 

 smudge the paper!) The speed of the drum is 

 controlled with a knob on the base. 



Expose the heart of a pithed frog, freeing it 

 from the pericardial membranes, and cutting 

 away the body wall over it. Grasp the tip of the 

 ventricle with a pair of fine forceps, and pass a 

 thread through it with a needle and thread. Tic it 

 securely (this is called a suture in surgery), but 

 not so tightly as to cut the tissue. Keep the heart 

 continuously wet with fresh Ringer solution. 

 Place the frog on the frog board, with the heart 

 directly beneath the kymograph lever, and at- 

 tach the other end of the suture to the lever with 

 a bit of modeling clay. The heart should extend 

 well out of the chest cavity and the lever should 

 be about parallel with the table. If it seems to 

 strain the heart unduly to lift the lever, help to 

 balance the lever with a little lump of clay at 

 the end attached to the heart. The lever should 

 now be moving up and down rhythmically with 

 the heartbeat. Move the kymograph so that the 

 tip of the lever just touches the smoked paper, 

 and the lever is tangential to the drum. (See 

 diagram.) Make sure you start your record high 

 up on the drum, so that you can get another 

 record below it. 



