SA Node and Slow Cells

SA Node and Slow Cells

• SA node, AV node, and His-Purkinje system each fire at specific frequencies.
• Slow cells are present in the SA and AV nodes.
• Autorhythmic cardiomyocytes spontaneously depolarize and fire action potentials.
• Spontaneous depolarizations are known as pre-potentials or pacemaker potentials.
• Resting membrane potential of SA nodal cells is -60 mV, different from muscle cells.
• Threshold potential is -40 mV.

Funny Channels

• Funny channels are responsible for spontaneous depolarization of SA nodal cells.
• Funny channels allow sodium current (If) into the cell, causing it to drift from -60 mV to -40 mV.
• Funny channels open upon repolarization back to threshold, not depolarization.
• Cyclic AMP regulates funny channels internally, increasing the probability the channel will be open.
• Sympathetic nervous system increases cyclic AMP levels via norepinephrine binding to beta-1 adrenergic receptors, increasing heart rate.
• Parasympathetic nervous system reduces cyclic AMP levels via acetylcholine on M2s, lowering heart rate.

Action Potential Phases in Slow Cells

• Phase 0: Voltage-gated calcium channels open, calcium influx occurs.
• Phase 3: Repolarization occurs due to potassium efflux.
• After repolarization the funny channel opens and sodium influx happens and it drifts back to threshold

Fast Cell Action Potentials

• Atrial and ventricular cardiomyocytes are stimulated by autorhythmic cells to contract.
• Fast cells include contractile cells and those in the Bundle of His and Purkinje fibers
• Resting membrane potential: -90 mV (due to potassium channels).
• Threshold potential: -70 mV (voltage-gated sodium channels open).
• Phase 0: Sodium influx, rapid depolarization.
• Around -40 mV, L-type calcium channels open.

Action Potential Phases in Fast Cells

• Phase 1: Voltage-gated sodium channels close, and voltage-gated potassium channels open.
• Phase 2: Plateau phase caused by balance of calcium influx and potassium efflux.
• Phase 3: Repolarization due to voltage-gated potassium channels opening.
• No phase 4: No spontaneous depolarization.

Plateau Phase

• The plateau phase is an absolute refractory period of about 200 milliseconds.
• It prevents tetanus in the heart.

Excitation-Contraction Coupling in Cardiac Muscle

• Action potential comes down the T-tubule
• DHP acts as a calcium channel, allowing calcium influx, which binds to RYR.
• Calcium influx causes RYR to open, releasing more calcium from the sarcoplasmic reticulum, known as calcium-induced calcium release.
• Cross-bridge cycling is similar to skeletal muscle.

Cardiac Muscle Contraction

• Atrial muscles contract, pumping blood into ventricles through AV valves.
• Ventricles contract, pumping blood through semilunar valves into pulmonary and systemic circuits.
• Atria contract simultaneously and then there is a short delay before the two ventricles contract at the same time.

Cardiac Mechanical Considerations

• Contraction decreases chamber volume, increasing pressure.
• Relaxation increases chamber volume, decreasing pressure.
• Blood fills into the heart during ventricular diastole.

Papillary Muscles and Chordae Tendineae

• Papillary muscles contract slightly before ventricular myocardium.
• Chordae tendineae are tethered to valve cusps, preventing AV valves from opening back into the atria.
• Proprioceptive fibers in papillary muscles sense tension and adjust accordingly.

Electrocardiogram (ECG)

• Body surface recording of electrical events in the heart.
• P wave: Atrial depolarization.
• QRS complex: Ventricular depolarization (and atrial repolarization).
• T wave: Ventricular repolarization.
• PR segment: AV nodal delay.
• ST segment: Ventricular contraction and emptying.
• TP interval: Ventricular relaxation, passive filling.

ECG Waveforms and Cardiac Events

• SA node firing corresponds to the P wave
• PR Segment is when Atria contract and then relax
• Green segment is signal at AV node corresponding to PR segment which is AV nodal delay
• QRS is Ventricular depolarization and atrial repolarization
• ST segment is isoelectric line corresponding to ventricles in systole
• T wave is ventricles in repolarization
• TP interval is flat line because little electrical activity but mechanically blood is passively filling ventricles

Cardiac Arrhythmias

• Normal Sinus Rhythm (NSR): Normal heart rate and rhythm.
• Sinus Bradycardia: Slow heart rate originating from the SA node.
• Sinus Tachycardia: Fast heart rate originating from SA node.

Heart Block

• Complete Heart Block: No electrical communication between atria and ventricles; atria and ventricles beat independently, requires a pacemaker.

Atrial Fibrillation (AFib)

• Irregularly irregular rhythm due to ectopic cells firing erratically in the atria.
• Atria quiver instead of contracting.
• Risk of stroke due to blood pooling and clotting in the atria.
• February waves (F waves) instead of P waves in ECG.
• Managed with blood thinners and antiarrhythmic drugs, cardiac ablation.

Ventricular Tachycardia

• Rapid ventricular rate (e.g., 150 bpm) due to ectopic cells, if pulseless is a form of sudden cardiac arrest.

Supraventricular Tachycardia (SVT)

• Rapid atrial rate (e.g., 150 bpm) due to ectopic cells in the atria.

Premature Atrial Contractions and Ventricular Contractions

• Premature Atrial Contractions (PACs): Early P wave with short TP interval and compensatory pause.
• Premature Ventricular Contractions (PVCs): Aberrant R wave with long compensatory pause, feel like "skipped beats".
• Triggers: Coffee, caffeine, stress, alcohol.

Ventricular Fibrillation

• Ventricles quiver, no effective contraction.
• Causes sudden cardiac arrest, requires immediate defibrillation.
• Leads to brain damage within four minutes due to lack of oxygen, because ventricles are not properly contracting for perfusion.

ECG Abnormalities

• ST Segment Depression: Ischemia near the endocardium.
• ST Segment Elevation: Transmural ischemia, indicative of heart attack.
• Checked for using troponin tests

Cardiac Cycle

• Systole is the contraction phase.
• Diastole is the relaxation phase.
• Atrial Systole: Atria contract.
• Atrial Diastole: Atria relax.
• Ventricular Systole: Isovolumetric contraction and rapid ejection.
• Ventricular Diastole: Isovolumetric relaxation and passive ventricular filling.

Cardiac Cycle Timing

• Two-thirds of the heart cycle is spent relaxing to allow for filling.
• Ventricles fill up 75% with blood during passive ventricular filling.
• Atrial systole tops off the last 25% (atrial kick).
• Ventricles fill up 75% with blood during passive ventricular filling.

Heart Sounds in the Cardiac Cycle

• First heart sound S1 (lub) occurs when pressure starts building, slams the AV valve shut (isovolumetric ventricular contraction).
• Second heart sound S2 (dub) occurs when isovolumetric ventricular relaxation forces blood back against the semilunar valve and slams them shut.

Wiggers Diagram

• X-axis is the time (cardiac cycle)
• Y-Axis is pressure in millimeters of mercury.
• Left Ventricle, Aorta and Left Atrium are represented
• Atrial systole pressure in the atria goes up and as a result the ventricular pressure goes up
• Left AV valve closing during that time causes first heart sound (early ventricular systole)
• As it contracts the isovolumetric ventricular contraction keeps rising
• Eventually ventricle becomes greater than aorta and semilunar valve opens
• Which causes the rapid ejection of late ventricular systole
• And then the ventricles go into diastole which is the early phase of isovolumentric relaxation
• and after ventricle relaxes long enough below that of the left atrium the AV valve opens
• and then enters the passive ventricular filling

Ventricular Volumes

• End Diastolic Volume (EDV): Volume of blood in the ventricle at the end of diastole (about 120 mL), includes atrial kick.
• End Systolic Volume (ESV): Volume of blood in the ventricle at the end of systole
• Stroke Volume: The amount of blood moved out per beat. Normally ends up at 70mL

Heart Sounds

• S1 (lub): AV valves close.
• S2 (dub): Semilunar valves close.
• S3 (lub-dub-da): Systolic heart failure, blood fills into a floppy ventricle; floppy ventricle happens post-heart attack because unable to push and squeeze
• S4 (da-lub-dub): Diastolic heart failure, a stiff ventricle strikes when atria fill it during ventricular contraction usually due to chronic hypertension.

Cardiac Output (the volume of blood pumped by each ventricle per minute)

• Calculated as heart rate times stroke volume.
• Normal cardiac output is approximately 5 liters per minute during rest.
• Heart Rate: Normal range is 60-100 beats per minute; the vagal brake keeps them between 60 and 100; intrinsically SA nodal cells are 100-110 beats
• Bradycardia is below 60 beats.
• Tachycardia is above 100 beats.

Regulation

• Parasympathetic NS: decreases heart rate
• Sympathetic: increases speed, dromatropy
• Heart innervated by SNS and PNS, SNS goes to the contractile cells as well, whereas PNS pretty much goes to SA and AV node