Electrophysiology Flashcards
What is the difference between conductive and contractile myocytes?
Conductive myocytes create action potentials (AP) and cannot contract. Is spontaneous and initiated by Na current
while contractile myocytes contract when depolarized by an AP. Initated by SA and AV node.
What happens in phase 0-1 of contractile myocyte APs?
0- Na channel opens. 1- Na channel inactivated to allow greater Ca influx.
What happens after phase 1 in contractile myocyte APs?
2- Ca channels open and fast K closes. 3- Ca channel closes and slow K opens. 4- resting potential.
Why does it plateau at phase 2 of contractile myocyte AP?
There is equal inward Ca and outward K, prolonged to allow for maximum Ca influx for contraction.
What does VG Na self-limiting mean?
The inactivation gate closes at peak depolarization, preventing reentry and excessive depolarization.
What are the key channels in Phase 0 of conductive and contractile myocytes?
Conductive: L VG Ca. Contractile: Fast VG Na.
What are the key channels in Phase 2 of conductive and contractile myocytes?
Conductive: n/a. Contractile: L VG Ca and VG K.
What are the key channels in Phase 3 of conductive and contractile myocytes?
Conductive: VG K. Contractile: VG K.
What are the key channels in Phase 4 of conductive and contractile myocytes?
Conductive: slow Na (unstable). Contractile: K1 + membrane pumps (stable).
What is automaticity?
The ability for pacemaker cells to spontaneously depolarize and generate action potentials.
What are the inherent rates of automaticity for the SA node?
60-100 AP/min.
What are the inherent rates of automaticity for the AV node? Why?
40-60 AP/min.
Not as sensitive to the Vagus nerve which also delays AV conduction and AV nodal delay
What are the inherent rates of automaticity for Purkinje fibers?
15-40 AP/min.
What is the absolute refractory period?
All Na channels are inactivated and the cell cannot produce another action potential.
What is the relative refractory period?
The myocyte starts to repolarize and Na channels recover.
When is the absolute refractory time?
End of phase 0 to early phase 3.
When is the relative refractory time?
Mid phase 3 to start of phase 4.
What happens during activation of ion channels?
The activation gate opens at threshold (-60mv)
What happens during inactivation of ion channels?
The inactivation gate closes while the activation gate is open at peak depolarization.
What happens during deactivation of ion channels?
The inactivation gate opens and the activation gate closes when the myocyte repolarizes below threshold.
How is unidirectional AP propagation ensured?
Through the absolute refractory period.
How do contractile myocytes conduct electricity?
Through syncytium created by gap junctions and intercalated discs.
Outline the steps of the conduction system.
- SA node pacemaker cells depolarize. 2. Atrial contraction (L to R). 3. Depolarization wave (AV node to BoH to B branches to PF). 4. Ventricular contraction.
In what direction does ventricular contraction occur and why?
From endocardium to epicardium and apex to base, to pump blood towards the great vessels at the base.
Where is there a delay in the conduction system and why?
At the AV node, to allow for ventricular filling.
What happens if AV node conductance increases?
Ventricles contract prematurely –> decreasing time for ventricular filling –> decreased stroke volume and cardiac output.
What is the role of intercalated discs and gap junctions?
Intercalated discs connect cardiomyocytes via gap junctions to allow for a wave of excitation.
What is the role of the cardiac skeleton in conduction?
It electrically isolates atrial and ventricular conduction systems.
What is endocardial polarization timing and why is it important?
Endocardial cells depolarize first and repolarize last to prevent reentry and allow for coordinated contractions.
What is chronotropy?
The effect of a substance or process on the rate or timing of a physiological process, such as heart rate.
When is chronotropy affected by autonomic innervation?
In phases 0 and 4.
What is dromotrophy and when is it affected by autonomic innervation?
Dromotrophy refers to conduction velocity and is affected in phases 0 and 4.
What is inotropy?
The force of ventricular contraction.
What is lusitropy?
The rate of relaxation.
How does the PNS depress the SNS?
Through mAChRs that activate ACh-sensitive K channels and inhibit AC to slow VG Na channels.
How does the SNS depress the PNS?
NE stimulates B1 receptors, activating AC and increasing cAMP, which slows VG Na and reduces mAChR stimulation.
What does the PNS innervate in the heart and what is the impact?
The SA and AV nodes; it activates mAChRs leading to hyperpolarization of SA nodes, decreasing pacemaker current and heart rate.
Where does the SNS innervate?
The SA and AV nodes plus ventricular muscle.
What effect does SNS stimulation have on chronotropy?
It increases the ability of VGCCs to open, allowing SA node cells to produce action potentials more rapidly, resulting in tachycardia.
What effect does SNS stimulation have on dromotropy?
It increases AV nodal conduction velocity.
What effect does SNS stimulation have on inotropic level?
It increases the size of the Ca current, leading to increased Ca entry per depolarization.
What effect does SNS stimulation have on lusitropic level?
It increases the rate of repolarization and resequestration due to SERCA and PMCA stimulation.
What would damage to the SA node cause?
AV nodal rhythm as the AV takes over. results in a lower HR due to lower inherent automaticity
Describe the wave of AP in R and L bundle branches.
Left= bigger due to the large muscle walls in LV = rate of contraction is faster in L
moves in two phases:
a) fast DOWN L and slower in R as goes from base –> apex
b) deploraised L side of septum –> resting cells on Right side
why is the LV bigger?
It’s size is due to large muscular wall that excites and triggers contraction
what is the role of the Bundle of His in the conduction system?
delivers wave of AP into the ventricles after clears AV node
What does the SNS innervate in the heart and how?
AV node, SA node and ventricular muscle
NE activates B1 andregenic receptors
what happens if you give a patient a drug that blocks 25% of slow funny Na channels?
HR lowers
conductive
slower depolarisation = gap between 0 and 3 increase
waves of AP through most of free walls of ventricles run
apex ==> base
endo —> epi
