Heart Conducting System

Conducting System

• Specialized cardiac muscle fibers transmit signals through the heart.
• These fibers are autorhythmic, meaning they can spontaneously depolarize.
• Gap junctions vary in density depending on the region.
• Sarcolemma have various ion channel diameters depending to change ion movements.
• Signals are altered at various points on the pathway.
• Nodes consist of a lump or mass of specialized cardiac cells.
• The sinoatrial (SA) node is the first action potential (AP) generator.
• The SA node is located near the opening of the superior vena cava in the right atrium.
• The SA node depolarizes the fastest, setting the AP firing pace.
• The interatrial septum carries signals to the left atrium.
• The atrioventricular (AV) node is the second AP generator.
• The SA node overrides the AV node but the AV node will set the pace if the SA node malfunctions.
• The AV node slows the signal down in the interatrial septum near the coronary septum and AV valve.
• The AV node has smaller fibers and fewer gap junctions compared to the SA node.
• The AV bundle (bundle of His) carries signals through a small hole in the fibrous skeleton to reach the ventricles.
• Bundle branches consist of left and right branches with more gap junctions and a larger diameter than the AV node.
• Bundle branches extend beneath the endocardium and through the interventricular septum to the apex of the heart.
• Purkinje fibers are bundle branches that fold back up and travel along the outside of the ventricles.
• Purkinje fibers have a larger diameter but fewer myofibrils for fast transmission without contraction and have many gap junctions.
• The heartbeat occurs because of autorhythmic (create and transmit APs) and contractile (most) fibers.

Cardiac MAPs

• Cardiac resting membrane potential (RMP) is approximately -90 mV.
• Extracellular fluid has high [Na+] and [Ca2+], while intracellular fluid has high [K+].
• Step 1 involves rapid depolarization to ~20 mV.
• Voltage-gated fast Na+ channels open, causing Na+ inflow during rapid depolarization.
• Step 2 involves maintained depolarization to allow contraction (cusp then plateau).
• Voltage-gated slow Ca2+ and some K+ channels open, while Na+ channels close during maintained depolarization.
• Troponin can receive Ca2+ from the sarcoplasmic reticulum (SR) or from extracellular fluid.
• Calcium-induced calcium release (CICR) involves specialized receptors on the SR releasing more Ca2+ after the initial release.
• Calcium voltage-gated channels are found all over the sarcolemma.
• Step 3 involves repolarization.
• Ca2+ channels close, and more K+ channels open during repolarization (arccot).
• Cardiac muscle action potential takes about 0.3 seconds in total.
• The refractory period starts at the same time as depolarization and ends slightly after contraction ends, allowing relaxation to prevent tetanus.
• Contraction starts slightly after depolarization.

Autorhythmic MAPs

• The SA node rate is ~100 bpm (70-80*).
• The AV node rate is 60-70 bpm (40-60*).
• Purkinje fibers rate is 25-30 bpm (20-40*).
• The pacemaker will be the AV node if something is wrong with the SA node.
• The pacemaker will be an artificial pacemaker if the AV node fails too, to stimulate depolarization.
• There is no stable RMP; the lower end is -60 mV
• "Funny" sodium channels are non-gated leak-like channels allowing constant Na+ movement into the cell.
• Funny sodium channels widen as the voltage inside the cell becomes more negative below -50 mV.
• Pacemaker potential is caused by Na+ leakage into the cell, causing RMP to move towards the threshold.
• Transient calcium channels start to open as the cell becomes more positive, helping to reach the threshold.
• Potassium channels close during this period.
• During the depolarization phase, long-lasting Ca2+ channels open, and K+ channels are closed.
• During the repolarization phase, Ca2+ channels close, and K+ channels open.

Electrocardiogram

• ECG/EKG measures electrical signals sent through the heart via a series of leads (electrodes) placed on the skin.
• The P curve represents atrial depolarization.
• The P curve is a small bell curve.
• The QRS complex represents ventricular depolarization (and atrial repolarization).
• The QRS complex is identified as a dramatic dip (Q), a rise (R), and a bigger dip (S).
• The T curve represents ventricular repolarization.
• The T curve is a medium, right-leaning bell curve.
• The PQ (PR) segment indicates the start of atrial excitation to the start of ventricular excitation, lasting ~0.16 s.
• The QT segment indicates the start of ventricular depolarization to the end of ventricular repolarization, lasting ~0.36 s.