Cardiac Conduction Anatomy & Physiology > Cardiovascular > Cardiovascular

Home , Bundle branches, Cardiac cycle, Cardiac output, Sinoatrial node

NOTES

CARDIAC MUSCLE CELLS Contractile (99%) cells contract and relax. Autorhythmic (1%) initiate and transmit action potential. CONDUCTION PATHWAY OF A SINGLE CARDIAC CONTRACTION 1. SINOATRIAL NODE

• The sinoatrial (SA) node is located in the upper wall of the right atrium, near the opening of the superior vena cava.

• Pacemaker: fastest rate of autorhythmicity, therefore, sets heart rate.

— Action potential originates here. 2. ATRIOVENTRICULAR NODE

• The AV node is located at the base of right atrium, adjacent to septum.

• It is the only electrical communication between the atria and the ventricles, and that it delays impulses to facilitate peak cardiac output.

— AV nodal delay: delays impulses, maximizes stroke volume, increases cardiac output. 3. BUNDLE OF HIS Originates at AV node, splits at interventricular septum into left and right bundle branches*. 4. PURKINJE FIBERS

• Spread upward through ventricular walls.

— Ventricles contract. CARDIAC MUSCLE CELL ACTION POTENTIAL Draw an action potential curve with a plateau phase.

PHASE 0: DEPOLARIZATION

The initial rise of the curve. Sodium moves rapidly into cell; calcium moves slowly into the cell. PHASE 1

Peak of curve. Voltage-gated sodium channels close. PHASE 2: PLATEAU PHASE

Curve plateaus. Potassium moves rapidly out of cell, while calcium moves slowly into the cell. — Calcium enters from both the extracellular space and sarcoplasmic reticulum, and is the cause of the plateau. PHASE 3: RAPID REPOLARIZATION

1 / 4 Curve declines. Calcium channels close and potassium moves rapidly out of cell. Potassium and sodium ion positions in regards to the sarcolemma are reversed. PHASE 4: RESTING POTENTIAL

Low curve. The resting potential is maintained by leaky potassium channels. The sarcolemma is impermeable to sodium during this period.

• Long absolute refractory period in cardiac muscle cells: phase 0 to phase 3

Second action potential cannot be initiated; thus, it is a protective mechanism against tetanus (state of maximal contraction). Review: ECG Clinical Correlations: AV node block; Supraventricular arrhythmias; Ventricular arrhythmias

FULL-LENGTH TEXT

• Here we will learn how the heart maintains its rhythmic beat.

• First, start a table.

• Denote that there are two specialized types of cardiac muscle cells known as contractile cells, which comprise about 99% of all cardiac muscle cells, and autorhythmic cells, which comprise the remaining 1 %.

• Indicate that autorhythmic cells initiate the action potentials responsible for contractile cell function and serve as a pacemaker for the cardiac cycle.

• Denote that autorhythmic cells lie in four specialized areas of the heart known as the sinoatrial node, atrioventricular node, bundle of His and Purkinje fibers.

Now, let's illustrate the conduction pathway of the heart.

• First, draw a cross section of the heart.

• Show that the sinoatrial (SA) node is located in the upper wall of the right atrium, near the opening of the superior vena cava.

• Draw the atrioventricular node (AV) node at the base of the right atrium adjacent to the septum.

• Draw the bundle of His, which originates at the AV node and splits at the interventricular septum into left and right bundle branches that continue down the septum.

2 / 4 • Show that the left and right bundle branches terminate in Purkinje fibers that spread downward to the apex of the heart and then upward through the ventricles.

• Indicate that the SA node is known as the pacemaker of the heart because it maintains the fastest rate of autorhythmicity.

• Show with arrows that the action potential originates at the SA node and spreads downward through the atria where it reaches the AV node.

- The atria contract almost simultaneously because the action potential travels through both atria very rapidly.

• Indicate that the AV node is the only electrical communication between the atria and the ventricles, and that it delays impulses to facilitate peak cardiac output.

- The AV nodal delay allows the ventricles to completely fill before they contract, which maximizes stroke volume.

• Show with arrows that the action potential spreads from the AV node to the bundle of His and upward through the ventricles via the Purkinje fibers.

• Write that this conduction pathway represents a single atrial and ventricular contraction.

Now, let's illustrate the action potential in a cardiac muscle contractile cell, which is very similar to that of a skeletal muscle cell.

• Draw an action potential curve with a plateau phase.

• Demarcate the rise of the curve as depolarization, which is phase 0; it results from sodium moving quickly into the cell.

• Show that calcium also begins to slowly move into the cell in this phase.

• Indicate that the peak is phase 1; it occurs when the voltage-gated sodium channels close.

• Show that the plateau is phase 2; it occurs when potassium moves quickly out of the cell, as calcium slowly moves into the cell.

- In cardiac muscle cells, calcium diffuses directly from the extracellular fluid into the cytosol and induces a greater release of calcium from the sarcoplasmic reticulum.

3 / 4 - This is unlike skeletal muscle in which voltage-gated calcium channels are limited to intracellular T tubules.

• Write that the prolonged depolarization that results distinguishes cardiac muscle cells from skeletal muscle cells.

• Demarcate the descending segment as rapid repolarization, which is phase 3; it results when calcium channels close and potassium moves rapidly out of the cell.

• Write that although the cell repolarizes in phase 3, the positions of potassium and sodium ions on either side of the contractile cell's plasma membrane, also known as the sarcolemma, are reversed.

• Indicate that sodium-potassium pumps restore the proper orientation of ions.

• Demarcate the final segment as the resting potential, which is phase 4; leaky potassium channels maintain it.

• Note that our illustration also includes phase 4 of the previous action potential.

• Indicate that in phase 4, the sarcolemma is impermeable to sodium.

• Show that an absolute refractory period begins during phase 0 of the cycle and continues into the beginning of phase 3.

- The long refractory period of cardiac muscle functions as a protective mechanism against tetanus, a state of maximal contraction in which repetitive stimulation prevents the muscle fiber from relaxing.

• Write that a second action potential cannot be initiated during a refractory period.

• Cardiac muscle cells are syncytial, they are connected in series by intercalated disks and their continuous interior allows for the spread of action potentials.

- This facilitates synchronized muscle contraction.