You will receive an anesthetized turtle with the plastron removed. The instructor will show you how to hook the turtle's atria and ventricle to balanced myographs. The myographs will be attached today using a hook with a wire leading to a shielded input cable. This will allow you to measure ECG as you measure the mechanical events (contractions) of the heart. You will also need to attach a sleeve electrode to the vagus nerve on the right side of the turtle's neck. Test the stimulus and make sure that the heart will stop within 10 seconds (10 volts, 2 msec, 30 stimuli/sec). When you turn the record on you will have three channels in use. The atrial contractions should be on channel 1, the ECG should be on channel 2 and the ventricular contractions should be on channel 3.
Part A: Control Record and Vagal Stimulation
For the first record just run about 1 page with the following recording criteria:
Transducer: F-60 Myograph
Paper Speed: 1.0 cm/second
Timer: 1 sec.
Special Equipment: High Gain Coupler (ECG)
Label Avg. CPD for atrial and ventricular contractions (over a 30 second period). Also, label heart rate in beats per minute. Mark the events of the ECG and draw vertical lines that connect with the mechanical event that those electrical events cause.
For the second record, increase the paper speed and record 1 page then stimulate the vagus until the heart stops then record as all the events return.
Transducer: F-60 Myograph
Paper Speed: 5.0 cm/second
Timer: 1 sec.
Stimulus: Electrical
Duration: 2msec
Intensity: 10 Volts
Frequency: 30 stimuli per second
Special Equipment: High Gain Coupler (ECG)
Label force of contraction for atrial and ventricular contractions. Also, label heart rate over the same 30 sec. period. Label order of event return after the heart stopped. Measure the down time in seconds and label Starling's Law if seen.
Part B: Systole, Diastole, and Starling's Law
Procedure: Now you will unhook the atrial myograph and the EKG apparatus. Record 3 ventricular contractions (make sure they are at least 2 cm tall). Measure the length of Systole and Diastole in seconds and average these values for the three contractions.
Paper speed: 5 cm/sec
Timer: 1 sec
For the Starling's Law record decrease tension so that the contractions are 1 cm tall. Record 1/2 page then adjust the myograph stand inner venier knob to increase tension gradually. Stop before the top of the pen range and decrease tension gradually. Measure the force of each contraction from trough to peak. Mark off areas of increasing and decreasing tension.
Paper speed: 0.5 cm/sec
Timer: 1 sec
Special Equipment: Inner venier knob on myograph stand.
Part C: Absolute and Relative Refractory Periods and Attempt at Tetanus in the Heart
Procedure: Attach the stimulator to the ventricle using the special myograph hooks, so that you can stimulate the heart directly. Adjust the tension so that you have large contractions. Administer single stimuli at four different points along the contractile period (the upward pen deflection) and one stimulus along the relaxation period. The stimulus applied during relaxation should cause an extra systole because it was applied when the cells were repolarizing. If you did not see this increase the voltage and try again. Once you do see the extra systole allow the record to run until the contractions begin again. The long downtime you see is called the compensatory pause. You need to measure its length in seconds.
Transducer: F-60 Myograph
Paper Speed: 5.0 cm/second
Timer: 1 sec.
Stimulus: Electrical
Duration: 2msec
Intensity: 10 Volts
Frequency: single
Attempt at Tetanus:
Procedure: Record 1/2 page then stimulate the heart on continuous for 15 seconds. Record the return to normal.
Stimulus: Electrical
Duration: 2msec
Intensity: 10 Volts
Frequency: 30 stimuli per second
Part D: Effect of Temperature on Cardiac Muscle
ReturnProcedure: Balance a thermometer in the pericardial area and record the temperature. Record ventricular contractions for 30 seconds at room temperature. Apply ice to the ventricle to bring the temperature to 17° C then record for 30 sec. Repeat at 12° C, and 7° C. Measure rate and force (average cm of pen deflection) in each section for comparison. Make two graphs: Rate vs. Temperature, and Force vs. Temperature (put Temp. on the X-axis on both graphs).
Paper speed: 0.5 cm/sec
Timer: 10 sec