What is the primary effect of the fivefold decrease in potassium permeability immediately after the onset of the cardiac action potential?
Explanation
This question focuses on the ionic basis of the action potential plateau. A key, unique feature of cardiac muscle is the temporary decrease in potassium permeability, which works in concert with calcium influx to prolong the depolarization.
Other questions
What are the three major types of cardiac muscle identified in the text?
What is the term for the structures that are actually cell membranes separating individual cardiac muscle cells, which appear as dark areas crossing the fibers?
The action potential in a ventricular muscle fiber averages about how many millivolts?
What are the two types of channels responsible for the long action potential and plateau in cardiac muscle?
During which phase of the cardiac muscle action potential do fast sodium channels close, leading to initial repolarization?
What is the velocity of signal conduction in the specialized Purkinje fibers of the heart?
What is the normal refractory period of the ventricle?
In addition to calcium released from the sarcoplasmic reticulum, where does the calcium that activates cardiac muscle contraction also come from?
The strength of contraction of cardiac muscle depends to a great extent on the concentration of which ion in the extracellular fluids?
What is the duration of contraction for atrial muscle and ventricular muscle, respectively?
What are the events that occur from the beginning of one heartbeat to the beginning of the next collectively called?
What is the role of the delay of more than 0.1 second in the passage of the cardiac impulse from the atria into the ventricles?
In the electrocardiogram (ECG), which wave is caused by the spread of depolarization through the atria?
Approximately what percentage of blood flows directly through the atria into the ventricles even before the atria contract?
In the atrial pressure curve, which wave is caused by the bulging of the A-V valves backward toward the atria during ventricular contraction?
During the cardiac cycle, what is the period called when contraction is occurring in the ventricles but no emptying occurs because the ventricular pressure has not yet risen enough to open the semilunar valves?
What is the term for the volume of blood in each ventricle at the end of diastole, which is typically about 110 to 120 ml?
What is the primary function of the papillary muscles?
The incisura in the aortic pressure curve is caused by what event?
On the volume-pressure diagram, what does the area labeled 'EW' represent?
In the context of cardiac contraction, what is considered to be the 'preload'?
Approximately what percentage of the chemical energy used by the heart is normally derived from the oxidative metabolism of fatty acids?
What is the maximum efficiency of the normal heart, defined as the ratio of work output to total chemical energy used?
The intrinsic ability of the heart to adapt to increasing volumes of inflowing blood is known as what?
What is the physiological explanation for the Frank-Starling mechanism?
What is the Bainbridge reflex?
Strong sympathetic stimulation can increase the heart rate in young adult humans from a normal rate of 70 beats/min up to what range?
Strong vagal (parasympathetic) stimulation can decrease the strength of heart muscle contraction by what percentage?
How does an excess of potassium ions in the extracellular fluids affect the heart?
What is the effect of a deficiency of calcium ions on the heart?
How does increased body temperature, such as during a fever, affect heart function?
What is the term for the concept that cardiac muscle, being composed of many interconnected cells, acts as a single functional unit?
What is the approximate total duration of the cardiac cycle if the heart rate is 72 beats/min?
When the heart rate increases, how does the duration of systole and diastole change?
What is the normal ejection fraction, which is the fraction of the end-diastolic volume that is ejected?
What causes the first heart sound (S1)?
In the context of cardiac function, what is the 'afterload' of the ventricle?
In a patient with aortic stenosis, how is the aortic pressure pulse typically affected?
How much faster is the velocity of the pressure pulse transmission in the aorta compared to the velocity of blood flow?
The progressive diminution of pressure pulsations in the periphery is called damping. What two factors are primarily responsible for this effect?
What is the average resting membrane potential in ventricular muscle fibers?
How many functional syncytia is the heart composed of, and what separates them?
At a normal heart rate of 72 beats/min, what fraction of the entire cardiac cycle does systole comprise?
What is the approximate normal end-systolic volume in each ventricle?
During which phase of the cardiac cycle do the atria function as primer pumps for the ventricles?
Which part of the ECG represents the repolarization of the ventricles?
What is the typical resting membrane potential of a ventricular muscle fiber during Phase 4?
How much larger is the diameter of the T tubules in cardiac muscle compared to skeletal muscle tubules?
What is the primary work output of the heart, also known as external work?