Autorhythmic Cells

Sinoatrial (SA) node

(1) Right atrial wall, near superior vena cava opening. Nodal cells here spontaneously depolarize generating an action potential which initiates heart beat. Pacemaker.

Atrioventricular (AV) node

(2) base of right atrium, near interatrial septum

AV Bundle & branches

(3) from AV node to interventricular septum; (4) forms right and left branches

Purkinje fibers

(5) Branch from AV bundles to ventricle muscles. Larger in diameter than other fibers for rapid AP propagation for simultaneous ventricle contraction.

Wolff-Parkinson-White Syndrome

0.2% of population has accessory pathway between atria and ventricles Causes abnormal conduction like AV reentrant tachycaria

Autorhythmic cells

1% of myocardial cells Gap junctions present Not typically contractile Exhibit Autorhythmicity No RMP Initiate and conduct electrical signals

Efficient pumping of the heart requires:

1) delay between atrial and ventricular depolarization 2) coordinated contraction of all contractile cells in a chamber 3) simultaneous contraction of both atria and separatly both ventricles

Contractile cells

99% of myocardial cells Gap junctions present Not autorhythmic Stimulated by conduction system Action potentials have plateau

Conduction sequence 4.

AP from Purkinje fibers spreads between ventricular cardiac muscles via gap junctions. Simultaneous stimulation and contraction of cardiac muscle cells in ventricular wall.

Conduction sequence 1.

AP initiated at SA node is spread between atria through gap junctions, excites all muscles of atria wall causing contraction.

Conduction sequence 3.

AP travels from AV node through the AV bundle, through right and left branches to Purkinje fibers.

Autorhythmicity

Capable of depolarizing and firing an action potential spontaneously without external influences

L-type Ca2+ channels

Fast Voltage-gated Ca2+ channel Opens during rising phase Ca2+ into the cell, action potential produced Closes during falling phase of action potential

Conduction sequence 2.

Insulting characteristics of fibrous ring only allow action potential to move through the AV node. Delay caused by small diameter fibers & few gap junction in AV node allow brief break before atria contraction and ventricle contraction. In this delay, ventricles fill with blood.

AV reentrant tachycaria

Occasionally electrical activity can spread from the ventricle back through the accessory pathway. In this case the atria can be prematurely stimulated to contract again; occurs suddenly (HR 60 --> 200 bpm instantly)

Voltage-gated K+ Channel

Opens at peak of action potential K+ fluxes out of cell

Defibrillator

Provides electrical shock to reset all autorhythmic cells; SA node first to recover, it drives regular sequence

Fibrillation

Random, uncoordinated excitation and contraction of cardiac cells. Ventricular fibrillation quite dangerous, no blood get's pumped.

Interatrial pathway

Rapid pathway that conducts depolarization from the SA node to left atrium

Funny channels (IF)

Slow Voltage-gated Na+ channel !!Only opens at negative membrane voltages (-60mV) Unlike normal VG Na+ channel!! Na+ into the cell, begins pacemaker potential Close when T-type Ca2+ channels open

Ventricular bradycardia

Slowness of ventricular rate, usually implying the presence of atrioventricular block.

AV nodal delay

Small diameter fibers & few gap junction within the AV node conducts the spread of depolarization very slowly, this ensures that atria have completely finished contracting before ventricular contraction begins

Ring of fibrous tissue

Surrounds heart valves also electrically insulates atria from ventricles.

AV block

Type of heart block in which the conduction between the atria and ventricles of the heart is impaired. The ventricles of the heart have their own pacing mechanisms, which can maintain a lowered heart rate in the absence of SA stimulation.

T-type Ca2+ channels

Voltage-gated Ca2+ channel Opens breifly during 2nd half of pacemaker potential, before action potential Ca2+ into the cell, threshold is reached

Pacemaker potential

slow drift towards threshold between action potentials caused by funny channels