Electrophsiology of heart

Ion channels in heart:

Ion	Channel type
Na+	Voltage
K+	Voltage & ligand

phase 0	initial rapid depolarization and the overshoot	due to opening of voltage-gated Na+ channels
phase 1	initial rapid repolarization	due to closure of Na+ channels & also opening of K+ channels
phase 2	plateau	due to a slower but prolonged opening of voltage-gated Ca2+ channels.
phase 3 to phase 4	Final repolarization to the resting membrane potential	due to closure of the Ca2+ channels and K+ efflux through various types of K+ channels.




Variations in the duration and shape of the cardiac action potential exist depending on its location in the heart:

• Different layers of the ventricle exhibit marked changes in the action potential

Phase	Blunted at	Prominent at
Phase 1	Endocardium	Epicardium
phase 2	Epi

• M cells found in the midmyocardium have the longest action potential duration and may contribute to the U wave seen on the surface electrocardiogram
• The J (Osborne) wave seen on the ECG in cases of hypothermia may be due to the increased prominence of phase 1 in the epicardial cells

Normal heart

SA node : at the lateral border of the superior vena cava and right atrial junction in the sulcus

Terminalis, measuring up to 2 cm long × 0.5 cm wide

: sinus node artery, branching from either the right coronary artery (55 to 60%) or the left

coronary artery (40 to 45%), runs through the middle of the sinus node

: Effect

:of adrenergic: -increases rate

:of cholinergic : decrease rate

: stretch receptors: ( coupled to Cl-channels ) : increase the atrial rate with increasing atrial pressure

AV node : arterial supply from the right coronary artery in 85 to 90% of cases

: Conduction is relatively slow through the AV node compared with atrial and ventricular tissue, in part because of the decreased density of gap junction proteins & intercalated discs leads to slower depolarization of neighboring cells -- PR interval depends on this slow AV node conduction

Bundle of His : Conduction through the His bundle is rapid,owing to the presence of rapidly acting sodium channels

: arterial supply of the bundle of His originates from the left anterior descending artery in 90% of cases

: right bundle branch trifurcates distally into a network that supplies the anterolateral papillary muscle, the low right septum, and the parietal band

Ventricular activation

from left to right septum

from apex to base

from endocardium to epicardium

Purinergic receptors are a third family of G protein-coupled receptors that are activated by adenosine

slowing of the pacemaker activity for SA and AV nodes and conduction delay through the AV node