Origin and Conduction of the Heart Flashcards
WHERE in the heart does the EXCITATION NORMALLY ORIGINATE?
SINO-ATRIAL NODE (ORIGINATES in the PACEMAKERS CELLS)
where? upper RIGHT ATRIUM near where the Superior Vena Cava entry
SINUS RHYTHM
A heart CONTROLLED by the Sino-Atrial node
How does the CARDIAC EXCITATION NORMALLY ORIGINATE?
-The CELLS in the SA NODE have NO STABLE RESTING MEMBRANE POTENTIAL
-SA node cells generate regular spontaneous pacemaker potentials (PP).
-PP bring the membrane potential to threshold.
-Upon reaching threshold, an action potential is generated.
-This leads to the production of regular spontaneous action potentials in SA nodal cells.
WHERE IS PACEMAKER POTENTIAL ON THIS?
What is the Ionic Basis for Spontaneous Pacemaker Potential?
The pacemaker potential (slow depolarization to threshold) is primarily caused by:
-Funny Current (If): Slow Na⁺ influx via HCN channels (depolorisation)
-Reduced K⁺ efflux at the end of hyperpolarization.
-Ca²⁺ influx.
Once the threshold is reached:
The rising phase of the action potential (depolarization) occurs due to the opening of long-lasting (L-type) voltage-gated Ca²⁺ channels,
Leading to an influx of Ca²⁺ ions.
Repolarization occurs during the falling phase of the action potential.
L-type Ca²⁺ channels inactivate, reducing Ca²⁺ influx.
K⁺ channels activate, leading to K⁺ efflux.
HOW does the CARDIAC EXCITATION NORMALLY SPREAD across the heart?
-Atria & SA to AV node: Conduction is mainly cell-to-cell via gap junctions, with some internodal pathways.
-AV node delay: Ensures atrial systole precedes ventricular systole.
-Bundle of His, branches, and Purkinje fibers: Enable rapid action potential spread to the ventricles.
-Ventricular muscle: Conduction occurs cell-to-cell via gap junctions.
What is the AV node?
what? location? only? size?
-The AV node is a small bundle of specialized cardiac cells.
Located at the base of the right atrium, above the atria-ventricle junction.
It is the only electrical connection between atria and ventricles.
AV node cells are small in diameter with slow conduction velocity.
Action potential in contractile cardiac muscle cells (myocytes)
5 phases
Phase 0 (Plateau phase):
Fast Na+ influx
Phase 1:
Closure of Na+ channels and Transient K+ efflux
Phase 2:
Mainly Ca++ influx (L-type Ca++ channels)
Phase 3 (Falling Phase):
Closure of Ca++ channels and K+ efflux (REPOLARISATION)
Phase 4:
Resting membrane potential
How to record the spread of electrical activity through the heart from the skin surface
ECG
Electrocardiogram
What is an (ECG)?
The ECG records the heart’s depolarization and repolarization cycles as electrical currents detected by surface electrodes.
Describe ECG waves recorded from skin surface
P-wave: Atrial depolarisation
QRS complex: Ventricular depolarisation (masks atrial repolarization)
T-wave: ventricular repolarisation
PR interval: largely AV node delay
ST segment: ventricular systole occurs here
TP interval: diastole occurs here
Standard ECG Limb Leads:
Lead I: RA - LA
Lead II: RA - LL
Lead III: LA - LL
The Influence of the Autonomic Nervous System on Heart Rate?
sympathetic, para
Heart rate is mainly controlled by the autonomic nervous system:
-Sympathetic stimulation increases heart rate.
-Parasympathetic stimulation decreases heart rate.
-Changes in heart rate involve a reciprocal action.
Parasympathetic division
The vagus nerve (parasympathetic supply) continuously influences the SA node at rest. Vagal tone dominates under normal conditions, slowing the intrinsic heart rate from ~100 bpm to a normal resting rate of ~70 bpm.
Vagus nerve supplies the SA and AV nodes.
Vagal stimulation slows SA node firing and increases AV nodal delay, reducing heart rate.
Acetylcholine acts through muscarinic M2 receptors.
Normal Heart Rate, Bradycardia, Tachycardia
Normal- 60 and 100 BPM
Bradycardia- less than 60 BPM
Tachycardia- more than 100 BPM
Effect of VAGAL STIMULATION on Pacemaker Potentials
Effect of Noradrenaline on pacemaker cells
Effect of Autonomic Nervous System on Heart Rate
slope of PP, influx, AV delay, HR
