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S H E E T
LAB TIME/DATE _______________________
Special Electrical Properties of Cardiac Muscle:
Automaticity and Rhythmicity
1. Define the following terms. automaticity: Ability to depolarize spontaneously in the absence of external stimulation. rhythmicity: Depolarization/repolarization events occur in a regular and continuous manner.
2. Discuss the anatomical differences between frog and human hearts. The frog heart has a single ventricle and two atria.
Dorsally there is an expanded area called the sinus venosus. The human heart has two atria and two ventricles. No sinus venosus is present. 3. Which region of the dissected frog heart had the highest intrinsic rate of contraction? Sinus venosus
The greatest automaticity? Sinus venosus
The greatest regularity or rhythmicity? Sinus venosus
How do these properties correlate with the
duties of a pacemaker? The human pacemaker (SA node) has automaticity, rhythmicity, and the highest depolarization rate in the heart. Is this region the pacemaker of the frog heart? Yes
Which region had the lowest intrinsic rate of contraction? Ventricle
Investigating the Refractory Period of Cardiac Muscle
4. Define extrasystole. An extra beat occurring before the time a normal contraction would occur.
5. Respond to the following questions if you used a physiograph.
What was the effect of stimulation of the heart during ventricular contraction? No effect.
During ventricular relaxation (first portion)? Extrasystole.
During the pause interval? No effect.
What does this indicate about the refractory period of cardiac muscle? Much longer than that of skeletal muscle.
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Assessing Physical and Chemical Modifiers of Heart Rate
6. Describe the effect of thermal factors on the frog