Electrical Activity of the Heart Flashcards
What is automoticity?
¨It is the slow spontaneous depolarization from the maximal diastolic potential towards the threshold potential
Which tissues have the property of automoticity?
SA node
AV node
Purkinje fibers
Bundle of His
¨Velocity of conduction depends on:
–Structural: # gap junctions and fiber diameter
–Physiological: APA (amplitude) and dV/dt (cardiac action potential)
Define excitability.
Excitability is the ability of a cell (cardiac. nerve. or muscle cell) to respond to a stimulus by depolarizing and firing an action potential. Measure of responsiveness
Refroctoriness is directly related to the level of recovery from ____________.
Inactivation
What is the relationship between functional refractory period and action potential duration in fast fibers?
Equal
What is the relationship between the functional refractory period and action potential duration in slow fibers?
FRP> APD
Cardiac cells have two refractory periods. When are they?
The first from the beginning of phase 0 until part way through phase 3; this is known as the absolute refractory period. This is immediately followed, until the end of phase 3, by a relative refractory period, during which a stronger-than-usual stimulus is required to produce another action potential.
_______________ cells are specialized cells within the heart with intrinsic automaticity.
Pacemaker cells
- SA NODE: Leading
- AV node: Silent
- Purkinje fibers: Silent
What is the main role of the SA node?
To initiate action potentials of the heart, so that it can pass through the heart and cause contraction
What is the clinical significance of the SA node?
Sinus node dysfunction describes an irregular heartbeat caused by faulty electrical signals of the heart. When the heart’s sinoatrial node is defective, the heart’s rhythms become abnormal – typically too slow or exhibiting pauses in its function or a combination, and very rarely faster than normal.
The AV node receives two inputs from the right atrium. What are they?
Posteriorly, via the crista terminalis (location of SA node), and anteriorly, via the interatrial septum.
AV conduction during normal cardiac rhythm occurs through two different pathways. What are they?
The first “pathway” has a slow conduction velocity but shorter refractory period
The second “pathway” has a faster conduction velocity but longer refractory period
An important property that is unique to the AV node is decremental conduction. Explain.
The more frequently the node is stimulated the slower it conducts. This is the property of the AV node that prevents rapid conduction to the ventricle in cases of rapid atrial rhythms, such as atrial fibrillation or atrial flutter.
Bundle of His transmits the electrical impulses from the ____________ to the point of the ____________ via the bundle branches.
AV node; apex of the fascicular branches
____________ allow the heart’s conduction system to create synchronized contractions of its ventricles, and are, therefore, essential for maintaining a consistent heart rhythm.
Purkinje fibres
The Purkinje fibres carry the contraction impulse from both the left and right bundle branch to the __________of the ventricles.
myocardium
*This causes muscles of the ventricles to contract and eject blood out of the ventricle to either pulmonary or systemic systems.
Bachmann’s bundle is a branch of the anterior internodal tract that resides on the inner wall of the ________.
left atrium
*Bachmann’s bundle, during sinus rhythm, is one of the first connections to activate the left atrium
The interatrial or internodal conduction tracts are bands of specialised myocytes which are believed to lie between the ___________and the ____________.
sinuatrial node; atrioventricular node
There are three internodal tracts. What are they and where do they travel?
Anterior:
- leaves anterior surface of SAN
- passes posteromedially to interatrial septum and then inferiorly
- splits into two bundles:
- one passes to left atrium (Bachmann’s bundle)
- the other passes posterior to torus aorticus to reach AVN
Middle:
- leaves the posterior and superior surface of SAN
- passes inferiorly and medially around orifice of inferior vena cava
- reaches interatrial septum where it passes inferiorly to AVN
Posterior:
- leaves the posterior and inferior surface of the SAN
- passes inferiorly through the crista terminalis
- passes medially around valve of the inferior vena cava
- reaches posterior surface of AVN
Conduction time for AV node
0.2 m/s or 40-60 times/min
Conduction time for SA node
60-100 times/ min or 1.0 m/s
Conduction time for Purkinje fibers
1.5-4.0 m/s or 15-40 times/min
