Download Addendum to Lab 9:

Lab 6.1
LAB 7: Cardiac Cycle
Objectives - After completing this lab, you should be able to:
1. explain the initiation of the cardiac impulse by the autorhythmic cells of the heart
2. explain how the cardiac impulse coordinates cardiac contraction
3. describe differences in intrinsic rate among types of autorhythmic cells
4. explain in detail the control mechanisms for heart rate and contractility
5. describe the events of the cardiac cycle
6. explain the effects of inotropes and chronotropes at the cell and organ level
Safety:
1. wear gloves and goggles while handling chemicals and frog
2. wash hands after lab
3. atropine is toxic - keep it away from mucous membranes
Background:
A cardiac cycle includes the events which occur in the heart during one heart
beat. These are separated into electrical events (conduction of an electrical impulse
through the myocardium) and mechanical events (the pumping of blood). The
mechanical events of the heart are always preceded by electrical events.
The heart is a pump with two chambers, the atria and the ventricles. During the
first part of the cardiac cycle, the atria contract and push blood into the ventricles.
Following this, the ventricles contract and push blood through the vascular system.
This cycle is repeated indefinitely at varying rates, depending on the needs of the body
and the influences of the internal environment. Contraction of the atria or ventricles is
called "systole" (SIS toe lee) and relaxation is called "diastole” (die ASS toe lee)
To observe the cardiac cycle, we will expose a frog heart and determine the
effects of several inotropes (factors which affect the strength of cardiac contraction)and
chronotropes. (factors that affect the rate of cardiac contraction). Although the frog
heart is anatomically different from the human heart, the conduction and contraction
characteristics are similar to those of mammalian hearts. The main physiological
differences are that 1) the frog heart will operate at room temperature and 2) the frog
can still get oxygen across its moist skin even after the CNS has been destroyed.
Blood flow in the frog heart begins in the sinus venosus, which receives low
oxygen blood from the body via the vena cava and sends it to the right atrium. At the
same time, the left atrium receives high oxygen blood from pulmonary veins. Both atria
then pump blood into the ventricle, which contracts and pumps blood via the conus
arteriosus to the major arteries.
Lab 6.2
PROCEDURES
A. Dissection and setup
Equipment: Labscribe software/hardware
dissecting kit, frog board, clips
double-pithed frog
Ringers, room temperature
Hook
thread
1. Carefully expose the frog's heart by removing the overlying layers of skin and
muscle from the thorax and upper abdomen. You will have to split or remove
the sternum. Be careful not to cut any major blood vessels.
2. Locate the beating heart in the pericardium. Lift the pericardium away from the
heart with forceps and carefully slit it open.
3. Insert the hook (with thread attached) through the apex (tip) of the ventricle, catching
enough tissue to hold the hook firmly but not enough to puncture the cavity and cause
bleeding. You should brace the heart against a fingertip or forceps before you begin to
insert the hook.
4. Remember to keep exposed tissues moist with Ringers solution at all times.
5. Make sure the transducer is no more than 6” above the table. Tie the free end of
the thread through the hole in the transducer using a square knot. The thread should
be vertical.
6. Open LabScribe, load ahk114, then load Muscle 114.
7. Call the instructor to adjust the tension on the thread.
Lab 6.3
8. Type “normal” in the marks box, click START, then hit ENTER. Record for about 60
sec. Click STOP
Lab 6.4
B. Effect of Extrinsic Factors
Group A
Group B
1st
cold Ringers
warm Ringers
2nd
epinephrine
acetylcholine
3rd
acetylcholine
atropine
Group C
warm Ringers
digitalis
CaCl2
Group D
cold Ringers
atropine
KCl
1. Administer your first chemical from the dropper bottle.
2. Type the name of the treatment (abbreviated) into the marks box. Click START and
hit ENTER. Record until you notice a change in either rate or strength or both (usually
1 to 2 minutes). Click STOP. Save file to desktop.
3. If your first treatment was temperature, just wait another 5 minutes for the heart to
return to room temperature. If your previous treatment was a chemical, rinse the heart
well with Ringers. Remove extra fluid from the body cavity. Wait 3 to 5 minutes. Keep
the heart moist while waiting.
4. Repeat steps 1-3 for your other treatments, saving your data after each treatment.
5. For the normal activity and each treatment, measure and record peak to valley
amplitude (V2-V1) and peak to peak duration of one beat (T2-T1). Then use the
duration of one beat to calculate heart rate.
treatment
T2-T1
calculated mean
HR (bpm)
V2-V1
Lab 6.5
Questions
1. Given an ejection fraction (SV/EDV) of 55% and an SV of 70 ml, calculate (show all
work and units):
a. EDV
b. ESV
2. If you were to place myocardial contractile cells in normal tissue fluid and stimulate
them, they would contract. If you put the same cells in very low calcium tissue fluid,
how would they respond and why?
Lab 6.6
3. Digitalis works on the heart by inhibiting the Na/K pump in cardiac contractile cells.
a. The Na/K pump moves ___Na out of the cell for each ___K it moves inwards.
b. This and the presence of leak channels means that normally Na is higher in the ECF
or ICF? _____
c. and K levels are normally higher in the ECF or ICF? _____
d. digitalis would cause the ECF Na level to increase or decrease? __________
e. digitalis would cause the ICF Na level to increase or decrease? __________
f. the Na concentration gradient between ICF and ECF would increase or decrease?
__________________
Cardiac contractile cells also contain a Na/Ca secondary active transport carrier. It
takes 3 Na in for each Ca it moves out, and its purpose is to help maintain low cytosolic
Ca concentration during diastole (relaxation).
g. the energy to run this carrier normally comes from the ______ gradient
h. digitalis increases or decreases the amount of Ca being removed from the cell
during diastole? __________________
i. this effect on Ca transport is caused by the loss of the _______ gradient
k. digitalis increases or decreases? cardiac contractility ___________________
l. digitalis has this effect because it causes the accumulation of _____ in the cytosol
4. When ACh was applied to the heart in lab, it bound to __________________
cholinergic receptors on the heart cells. When EPI was used, it bound to _____
adrenergic receptors on the heart cells.
5. What effect would a muscarinic agonist have on HR and SV?
6. What effect would an a 1 antagonist have on HR and SV.
Lab 6.7
7. Refer to lecture materials. Explain the physiological basis of the effect of these
chemicals as specified in each box. Include the type of adrenergic or cholinergic
receptor involved
a. atropine at the cellular level
b. Ach at the SA node at the cellular level
c. Ach at the AV node at the cellular level
d. Ach on atrial contractile cells
e. EPI at the SA node at the cellular level
f. EPI on the AV node at the cellular level
g. EPI on contractile cells
Lab 6.8
8. On this diagram, draw a normal pacemaker potential in one color, then superimpose
another line that would show the cardiac pacemaker potential after the heart is exposed
to the chemicals indicated. The dotted line represents the threshold. Label both lines
on each graph. Refer to lecture materials.
a. Ach:
membrane
potential
(mV)
b. EPI
membrane
potential
(mV)