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
Due to its intrinsic slow conduction, the __________ is vulnerable to blocks of the electrical wave.
AV node
Vagal stimulation depresses _________ responsible for phase 0 (depolarization).
ICa,L (L-type calcium channel)
Vagal stimulation causes _______ which may cause cardiac arrest, while adrenergic stimulation causes _______ which may cause arrythmias.
Bradycardia; tachycardias
SDD=
Slow diastolic depolarization
What is the site of Ca2+ regulation in cardiac muscles?
Troponin in thin filaments
REMEMBER: Site of calcium regulation in smooth muscle is myosin in thick filaments
What is the means by which gradation is accomplished in cardiac muscles?
Treppe
*Varying length of fibers (depending on extent of filling of the heart chambers)
What is the normal absolute refractory period of cardiac muscles?
250 ms
In cardiac muscle, _____% of Ca2+ released into the cytoplasm is pumped back into the SR and the remaining percentage goes back out of the cell.
85
*In skeletal muscle, 100% goes back into the SR
Pacemaker cells do not have a stable resting potential. Why?
There’s a slow depolarization process during diastole at the base of the pacemaker potential
The _______ (higher/ lower) the MDP, the longer it takes to reach threshold.
Lower
Does parasympathetic stimulation go to ventricular myocytes?
No. Only to SA and AV nodes
*This is why parasympathetic stimulaioncan reduce pacemaker activity but CAN NOT resuce the force of contraction
Pacemaker activity is regulated by both division of the ANS but ventricular myocyte activity is regulated by the __________ only.
Sympathetic
How many ATPs are used during excitation-contraction coupling in the heart?
2
What is the normal ejection fraction?
60-70%
Two steps to active contraction
Isovolumic contraction
Isotonic contraction
How does the heart respond to decrease in the ejection fraction?
Walls thickens and an attempt to respond to the body’s needs
In isovolumic contraction, force-velocity is equal to ________
0 because there is no actually movement of blood so there is no velocity
The greater the force, the _____ (bigger/ smaller) the velocity.
Smaller
Which structural factors of an action potential contribute to an increase in velocity?
- Increase in diameter of the fiber
- Increase in number and size of gap junctions
Which physiological factors of an action potential contribute to an increase in conduction velocity?
- Higher amplitude of the AP
- Greater velocity of phase 0 depolarization
In order to be available for activation, a channel has to be in the ________ state.
Resting
The greater the difference in electricity between two areas, the _________ (faster/slower) the transmission between them.
Faster
What are the effects of hyperkalemia on resting membrane potential?
Increasing resting membrane potential, this prevents sodium channels from recovering completely. This results in less Na+ channels being available for an AP
What are the 3 most important determinants of resting membrane potential?
- IK1 channels (voltage gated)
- Leak channels
- Na+/K+ pump
If you administer a drug that impacts Ca2+ activity, which conduction structure is most likely affected?
AV NODE
REMEMBER: Pukinje fibers and bundle of his are fast fiber, so they are activated by Na+, and the SA node has to high of a conduction to be truly affected.
The higher the slope, the higher the ________.
Automaticity
SA node> AV node> Purkinje fibers
The strong deactivation/ depolarization of IK channels in Phase 3 enhances the capability of _______ channels to depolarize the membrane quicker.
IF
