Review: Heart Rate
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Frog Cardiovascular Physiology
O B J E C T I V E S 1. To list the properties of cardiac muscle as automaticity and rhythmicity, and to define each. 2. To explain the statement, “Cardiac muscle has an intrinsic ability to beat.” 3. To compare the relative length of the refractory period of cardiac muscle with that of skeletal muscle, and to explain why it is not possible to tetanize cardiac muscle. 4. To define extrasystole, and to explain at what point in the cardiac cycle (and on an ECG tracing) an extrasystole can be induced. 5. To describe the effect of the following on heart rate: vagal stimulation, cold, heat, pilocarpine, atropine, epinephrine, digitalis, and potassium, sodium, and calcium ions. 6. To define vagal escape and discuss its value. 7. To define ectopic pacemaker.
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nvestigation of human cardiovascular physiology is very interesting, but many areas obviously do not lend themselves to experimentation. It would be tantamount to murder to inject a human subject with various drugs to observe their effects on heart activity or to expose the human heart in order to study the length of its refractory period. However, this type of investigation can be done on frogs or computer simulations and provides valuable data because the physiological mechanisms in these animals, or programmed into the computer simulation, are similar if not identical to those in humans. In this exercise, you will conduct the cardiac investigations just mentioned.
Special Electrical Properties of Cardiac Muscle: Automaticity and Rhythmicity
Cardiac muscle differs from skeletal muscle both functionally and in its fine structure. Skeletal muscle must be electrically stimulated to contract. In contrast, heart muscle can and does depolarize spontaneously in the absence of external stimulation. This property, called automaticity, is due to plasma membranes that have reduced permeability to potassium ions but still allow
E
X
E
R
C
I
S
E
6
Frog Cardiovascular Physiology
O B J E C T I V E S 1. To list the properties of cardiac muscle as automaticity and rhythmicity, and to define each. 2. To explain the statement, “Cardiac muscle has an intrinsic ability to beat.” 3. To compare the relative length of the refractory period of cardiac muscle with that of skeletal muscle, and to explain why it is not possible to tetanize cardiac muscle. 4. To define extrasystole, and to explain at what point in the cardiac cycle (and on an ECG tracing) an extrasystole can be induced. 5. To describe the effect of the following on heart rate: vagal stimulation, cold, heat, pilocarpine, atropine, epinephrine, digitalis, and potassium, sodium, and calcium ions. 6. To define vagal escape and discuss its value. 7. To define ectopic pacemaker.
I
nvestigation of human cardiovascular physiology is very interesting, but many areas obviously do not lend themselves to experimentation. It would be tantamount to murder to inject a human subject with various drugs to observe their effects on heart activity or to expose the human heart in order to study the length of its refractory period. However, this type of investigation can be done on frogs or computer simulations and provides valuable data because the physiological mechanisms in these animals, or programmed into the computer simulation, are similar if not identical to those in humans. In this exercise, you will conduct the cardiac investigations just mentioned.
Special Electrical Properties of Cardiac Muscle: Automaticity and Rhythmicity
Cardiac muscle differs from skeletal muscle both functionally and in its fine structure. Skeletal muscle must be electrically stimulated to contract. In contrast, heart muscle can and does depolarize spontaneously in the absence of external stimulation. This property, called automaticity, is due to plasma membranes that have reduced permeability to potassium ions but still allow