What cells in the heart are spontaneously depolarized?
The SA node has the highest rate of spontaneous depolarization and therefore suppresses the other pacemakers. In the denervated heart, the SA node discharges at a rate of approximately 100 times min−1. Vagal tone leads to a lower heart rate in healthy subjects at rest.
How do cardiac cells spontaneously depolarize?
There is a spontaneous depolarization of the membrane potential towards the threshold for producing an action potential by the opening of L-type Ca channels. Phase 4 is the primary determinant of automaticity and heart rate! During phase 4 there is a “leak” current that allows Na+ to enter the cell.
What triggers ventricular muscle cell depolarization?
Conductive cells contain a series of sodium ion channels that allow a normal and slow influx of sodium ions that causes the membrane potential to rise slowly from an initial value of −60 mV up to about –40 mV. The resulting movement of sodium ions creates spontaneous depolarization (or prepotential depolarization).
Does the ventricular contract during depolarization or repolarization?
Animation of a Normal ECG Wave: The red lines represent the movement of the electrical signal through the heart. The T Wave indicates ventricular repolarization, in which the ventricles relax following depolarization and contraction.
Which stage is indicative for depolarization?
Phase 0 is the phase of depolarization; Phase 1 through 3 is the phases during which repolarization occurs; Phase 4 is the resting phase with no spontaneous depolarization. During phase zero, the phase of rapid depolarization, voltage-gated Na+ channels open, resulting in a rapid influx of Na+ ions.
What happens during ventricular depolarization?
The P wave represents depolarization of the atria and is followed by atrial contraction (systole). Atrial systole extends until the QRS complex, at which point, the atria relax. The QRS complex represents depolarization of the ventricles and is followed by ventricular contraction.
What is the difference between depolarization and repolarization of the heart?
Depolarization of the heart is the orderly passage of electrical current sequentially through the heart muscle, changing it, Repolarization of the heart is the orderly return of each cell to its polarized state, cell by cell, until all are polarized again.
Why do pacemaker cells spontaneously depolarize?
Closure of ion channels causes ion conductance to decrease. As ions flow through open channels, they generate electrical currents that change the membrane potential. These depolarizing currents cause the membrane potential to begin to spontaneously depolarize, thereby initiating Phase 4.
What happens during ventricular diastole?
During ventricular contraction, the atria relax (atrial diastole) and receive venous return from both the body and the lungs. Then, in ventricular diastole, the lower chambers relax, allowing initial passive filling of the thick-walled ventricles and emptying of the atria.
What is the difference between repolarization and depolarization?
Depolarization is caused when positively charged sodium ions rush into a neuron with the opening of voltage-gated sodium channels. Repolarization is caused by the closing of sodium ion channels and the opening of potassium ion channels.
Is depolarization positive or negative?
Depolarization brings positive charge inside the cells in an activation step, thus changing the membrane potential from a negative value (approximately −60mV) to a positive value (+40mV).
What is ventricular depolarization?
Ventricular depolarization occurs in part via an accessory pathway (AP) directly connecting the atrium and ventricle and thus capable of conducting electrical impulses into the ventricle bypassing the AV-His Purkinje conduction system.
What causes depolarization of atrial / ventricular myocytes?
Depolarization of atrial/ventricular myocytes is a result of sodium ions entering the cell (“summit”), whereas depolarization of pacemaker cells is the result of calcium ions entering the cell (“climb”).
When does a cardiac myocyte generate an action potential?
Once the threshold cell membrane voltage is met, then an action potential is generated. The action potential will first “climb” as the voltage across the cell membrane becomes more positive. This is referred to as depolarization and it is phase 0 of the action potential (similar to non-pacemaker cardiac myocytes).
How are ventricular myocytes affected by PVC?
A guide to PVC nomenclature can be found in Table 1. In PVCs, ventricular myocytes spontaneously depolarize to create an extra systole that creates mechanical dyssynchrony with the cardiac cycle. 2,3 Affected cells are triggered by cyclic adenosine monophosphate-mediated and calcium-dependent delays in after-depolarizations. 3
When does depolarization occur in a San cell?
This phase consists of a rapid, positive change in voltage across the cell membrane (depolarization) lasting less than 2ms, in ventricular cells and 10/20ms in SAN cells. This occurs due to a net flow of positive charge into the cell.