What are the two primary types of cells (types of conduction) in the heart? Where does each occur?
} Two primary types of cells in the heart } Calcium based conduction (Sinoatrial node, AV node) } Sodium based conduction (His-Purkinje network, myocardial tissue)
What are the 5 phases of cardiac conduction (sodium based)? Describe what occurs in each and how it occurs in detail.
Phase 0: Depolarization } Rapid upswing of action potential } Voltage activated sodium channels allow influx (positive ions) causing potential to rapidly change from negative to positive (-92 mV to +20 mV). Phase 1: Initial repolarization } Rapid inactivation of sodium channels } Voltage dependent potassium channels (ITO) open allowing potassium (positive charge) to leave cell, making it more negative Phase 2: Plateau } Complex phase with multiple channels involved } L-type calcium channel (voltage activated) allows calcium (positive ion) to influx making cell more positive }Calcium also needed for muscle contraction electromechanical coupling) } Sodium-Calcium pump increases positive charge in cell (three sodium ions in for every calcium ion out) } This is somewhat balanced by potassium efflux from cell Phase 3: Final repolarization } Potassium efflux from cell making cell more negative } Leads to a return to the resting potential Phase 4: Resting potential } High component of leak type potassium channels } Membrane resting potential very close to potassium Nernst potential (-92 mV)
Where does the calcium based AP occur? What are 3 major differences compared to sodium based AP?
} Sinoatrial node and AV node } Major differences compared to sodium based AP } Slower conduction properties } Resting potential gradient leads to Auto-activation } High input from Autonomic nervous system
What are the phases of calcium based AP? Describe what occurs in each and how it occurs in detail.
Phase 0 } Depolarization } Voltage activated calcium channels allow calcium influx into cell leading to a positive membrane potential Phase 3 } There is no phase 1 or 2 in calcium based cardiac cells } Repolarization } Potassium efflux from cell, similar to myocardial cells Phase 4 } Resting potential } “Funny current” (If) is a mixed sodium and potassium inward current and causes a slow rise (depolarization) in the resting potential
Explain why there has to be two different cardiac cells with different conduction mechanisms.
} Pacemaker cells need to auto-activate or else there would be no way for the heart to continue to contract } AV nodal cells are important to slow conduction to allow for atrial contraction prior to ventricular activation to allow A-V synchrony in contraction } “Hierarchy of Cardiac Conduction” } Myocardial cells and the His-Purkinje system need to have rapid conduction properties to allow muscle contraction to rapidly flow through the heart chambers to create a strong, efficient contraction to pump blood to the body
What are refractory periods? How are they caused on a cellular level? On a cellular level, describe the two parts of a refractory period. Describe the differences in refractory period between the two types of cardiac cells.
} Defined as the time that a cell or tissue can not be re-excited } On a cellular level, related to channel properties (open/closed) that block ion transport into the cell as the cell reestablishes its resting potential (repolarization) } Cardiac tissue has a long refractory period due to plateau phase (phase 2) that prolongs depolarization period } In fast response (sodium based) cells, refractory period is from phase 0 to late phase 3 } Based on recovery of fast sodium channels (occurs at about -50 mV) } In slow response (calcium based) cells, refractory period extends beyond period that full repolarization has been restored (post repolarization refractoriness) } On a cellular level, broken into two parts: } Absolute refractory period } No stimulus is capable of evoking a depolarization } Relative refractory period } Only stimuli that exceed the normal threshold can initiate a depolarization } Depolarization generally slower and of lower amplitude compared to normal cellular depolarization
What kinds of things will alter the refractory period? In what ways? How are refractory period and HR related in sodium based cells, especially purkinje cells? In AV nodal cells?
} Changes in cycle length will alter action potential duration and change the refractory period } Autonomic inputs also alter action potential duration and refractory periods } Sodium based cells, especially Purkinje cells, have an inverse relationship between refractory period and heart rate } AV nodal cells have an increase in refractory period with increase in heart rate
What are two ways in which cell to cell conduction occurs? Which is faster?
} Longitudinal and Transverse } Longitudinal conduction much faster } Intercalated discs
What do intercalated discs? What allows them to contribute to cell to cell connectivity? What do they contain? What do they create?
} Intercalated discs } Separate cardiac myocytes } Contain the gap junction which is characterized by nonselective channels that have pores } Pores create low electrical resistance channels (V=IR) that allow rapid longitudinal conduction
What else do gap junctions accomplish?
} In addition to allowing for rapid conduction, gap junctions are protective for cardiac myocytes } Low pH, high calcium closes connexon channels } Protects against calcium influx from adjacent cell death } Work like the bulkhead doors in boats to prevent flow of water from one compartment to the next
How do intercalated discs differ from cell to cell both generally and specifically? What allows them to do so?
} Density and type of intercalated discs vary in cardiac tissue } Higher density, faster conduction in His-Purkinje system } Low density, slower conduction in AV node } Gap junctions contain connexon proteins (multiple isoforms). Isoforms determine conduction properties of connexons
What are the components of the cardiac conduction system?
} Sinoatrial node (SA node) } Atrial myocardium } Bachmann’s bundle } Atrioventricular node (AV node) } Bundle of His } Bundle branches/ Purkinje network } Ventricular myocardium
What is the SA node? Location? Function? Activation?
} Cardiac Pacemaker } Located on epicardial surface of right atrium near the SVC junction } Surrounds the sinoatrial artery } Sinus rhythm originates here } Calcium type activation } (If) (sodium based ‘funny’ current) also plays a role } High level of sympathetic and parasympathetic inputs
What is Bachmanns bundle?
} Main pathway of intra-atrial conduction } Parallel aligned myocardial strands that cross superiorly through the intra-atrial septum
What is the AV node? Location? Function? Activation?
} Specialized compact area of cells that transmit conduction from atrium to ventricle } In a normal heart, this is the only way electrical conduction can pass from atrium to ventricle } Calcium type activation properties } “Electrical speedbump” slows conduction to allow atrial contraction to complete prior to ventricular activation } Important protective properties for atrial tachyarrhythmias } Large input from sympatheticand parasympathetic nervous system allow for modification of conduction properties
Waht is the HIS bundle? How does it work? Where is it located? What does it do?
} Located in the central
fibrous body of the
} Sodium type activation
} Rapidly transmits
conduction from the
AV node to the bundle
Waht are the bundle branches/purkinje network? How does it work? Where is it located? What does it do?
} Carries electrical
impulses rapidly from
the AV junction through
the septum and to the
} Divided into left and right
} Left bundle further divided
into anterior and posterior
} Sodium type activation
} Purkinje fibers are large
and have the greatest conduction velocity
In what direction does activation/contraction flow?
} Flow of electrical activity
from right to left and
superior to inferior
through the atrium
} Activation in ventricle
is endocardial to
epicardial, apex to base
} Repolarization tends to
What are the symp. effects on conductivity?
} Sympathetic Effects
} Release of
} Acceleration of diastolic
} Shortening of action
} Increase inward current
(Phase 0 activation)
What are the parasypm effects on conduction?
} Parasympathetic Effects
} Acetylcholine release
} Muscarinic receptors
channels and diminish
ionic currents of If
} Increases maximum
negativity of cells
} Reduces the slope of
What is meant by automaticity? How does it work?
} All cardiac cells have a slight positive deflection to
the resting potential
} This leads to the property of automaticity
} Automatic activation of cardiac cells at the resting
potential becomes more positive over time
} Eventually this leads to phase 0 activation of the cells
with the activation of voltage gated sodium or calcium
} Electrical activation in the heart is determined by the
quickest cells to fire; normally this is the sinoatrial
} So what happens if the sinoatrial node does not
What is meant by the hierarchy of cardiac conduction? How does it work? What escape rhythms are there?
} If the dominant pacemaker is lost (sinoatrial node),
automaticity will occur in the next most dominant
cluster of cells (AV node)
} If the AV node is not capable of activating, then the
bundle branches/Purkinje network will become the
pacemaker for the cell
} Because each of these cell types have different
resting potential gradients, there is a slowing of the
heart rate with each subsequent cell type becoming
the pacemaker for the heart
} Junctional (from AV node): 40-60 bpm
} Ventricular (from His-Purkinje network): 20-40 bpm