Electrophysiology

Question 1

What is the difference between conductive and contractile myocytes?

Answer 1

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.

Question 2

What happens in phase 0-1 of contractile myocyte APs?

Answer 2

0- Na channel opens. 1- Na channel inactivated to allow greater Ca influx.

Question 3

What happens after phase 1 in contractile myocyte APs?

Answer 3

2- Ca channels open and fast K closes. 3- Ca channel closes and slow K opens. 4- resting potential.

Question 4

Why does it plateau at phase 2 of contractile myocyte AP?

Answer 4

There is equal inward Ca and outward K, prolonged to allow for maximum Ca influx for contraction.

Question 5

What does VG Na self-limiting mean?

Answer 5

The inactivation gate closes at peak depolarization, preventing reentry and excessive depolarization.

Question 6

What are the key channels in Phase 0 of conductive and contractile myocytes?

Answer 6

Conductive: L VG Ca. Contractile: Fast VG Na.

Question 7

What are the key channels in Phase 2 of conductive and contractile myocytes?

Answer 7

Conductive: n/a. Contractile: L VG Ca and VG K.

Question 8

What are the key channels in Phase 3 of conductive and contractile myocytes?

Answer 8

Conductive: VG K. Contractile: VG K.

Question 9

What are the key channels in Phase 4 of conductive and contractile myocytes?

Answer 9

Conductive: slow Na (unstable). Contractile: K1 + membrane pumps (stable).

Question 10

What is automaticity?

Answer 10

The ability for pacemaker cells to spontaneously depolarize and generate action potentials.

Question 11

What are the inherent rates of automaticity for the SA node?

Answer 11

60-100 AP/min.

Question 12

What are the inherent rates of automaticity for the AV node? Why?

Answer 12

40-60 AP/min.

Not as sensitive to the Vagus nerve which also delays AV conduction and AV nodal delay

Question 13

What are the inherent rates of automaticity for Purkinje fibers?

Answer 13

15-40 AP/min.

Question 14

What is the absolute refractory period?

Answer 14

All Na channels are inactivated and the cell cannot produce another action potential.

Question 15

What is the relative refractory period?

Answer 15

The myocyte starts to repolarize and Na channels recover.

Question 16

When is the absolute refractory time?

Answer 16

End of phase 0 to early phase 3.

Question 17

When is the relative refractory time?

Answer 17

Mid phase 3 to start of phase 4.

Question 18

What happens during activation of ion channels?

Answer 18

The activation gate opens at threshold (-60mv)

Question 19

What happens during inactivation of ion channels?

Answer 19

The inactivation gate closes while the activation gate is open at peak depolarization.

Question 20

What happens during deactivation of ion channels?

Answer 20

The inactivation gate opens and the activation gate closes when the myocyte repolarizes below threshold.

Question 21

How is unidirectional AP propagation ensured?

Answer 21

Through the absolute refractory period.

Question 22

How do contractile myocytes conduct electricity?

Answer 22

Through syncytium created by gap junctions and intercalated discs.

Question 23

Outline the steps of the conduction system.

Answer 23

1. 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.

Question 24

In what direction does ventricular contraction occur and why?

Answer 24

From endocardium to epicardium and apex to base, to pump blood towards the great vessels at the base.

Question 25

Where is there a delay in the conduction system and why?

Answer 25

At the AV node, to allow for ventricular filling.

Question 26

What happens if AV node conductance increases?

Answer 26

Ventricles contract prematurely --> decreasing time for ventricular filling --> decreased stroke volume and cardiac output.

Question 27

What is the role of intercalated discs and gap junctions?

Answer 27

Intercalated discs connect cardiomyocytes via gap junctions to allow for a wave of excitation.

Question 28

What is the role of the cardiac skeleton in conduction?

Answer 28

It electrically isolates atrial and ventricular conduction systems.

Question 29

What is endocardial polarization timing and why is it important?

Answer 29

Endocardial cells depolarize first and repolarize last to prevent reentry and allow for coordinated contractions.

Question 30

What is chronotropy?

Answer 30

The effect of a substance or process on the rate or timing of a physiological process, such as heart rate.

Question 31

When is chronotropy affected by autonomic innervation?

Answer 31

In phases 0 and 4.

Question 32

What is dromotrophy and when is it affected by autonomic innervation?

Answer 32

Dromotrophy refers to conduction velocity and is affected in phases 0 and 4.

Question 33

What is inotropy?

Answer 33

The force of ventricular contraction.

Question 34

What is lusitropy?

Answer 34

The rate of relaxation.

Question 35

How does the PNS depress the SNS?

Answer 35

Through mAChRs that activate ACh-sensitive K channels and inhibit AC to slow VG Na channels.

Question 36

How does the SNS depress the PNS?

Answer 36

NE stimulates B1 receptors, activating AC and increasing cAMP, which slows VG Na and reduces mAChR stimulation.

Question 37

What does the PNS innervate in the heart and what is the impact?

Answer 37

The SA and AV nodes; it activates mAChRs leading to hyperpolarization of SA nodes, decreasing pacemaker current and heart rate.

Question 38

Where does the SNS innervate?

Answer 38

The SA and AV nodes plus ventricular muscle.

Question 39

What effect does SNS stimulation have on chronotropy?

Answer 39

It increases the ability of VGCCs to open, allowing SA node cells to produce action potentials more rapidly, resulting in tachycardia.

Question 40

What effect does SNS stimulation have on dromotropy?

Answer 40

It increases AV nodal conduction velocity.

Question 41

What effect does SNS stimulation have on inotropic level?

Answer 41

It increases the size of the Ca current, leading to increased Ca entry per depolarization.

Question 42

What effect does SNS stimulation have on lusitropic level?

Answer 42

It increases the rate of repolarization and resequestration due to SERCA and PMCA stimulation.

Question 43

What would damage to the SA node cause?

Answer 43

AV nodal rhythm as the AV takes over. results in a lower HR due to lower inherent automaticity

Question 44

Describe the wave of AP in R and L bundle branches.

Answer 44

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

Question 45

why is the LV bigger?

Answer 45

It's size is due to large muscular wall that excites and triggers contraction

Question 46

what is the role of the Bundle of His in the conduction system?

Answer 46

delivers wave of AP into the ventricles after clears AV node

Question 47

What does the SNS innervate in the heart and how?

Answer 47

AV node, SA node and ventricular muscle NE activates B1 andregenic receptors

Question 48

what happens if you give a patient a drug that blocks 25% of slow funny Na channels?

Answer 48

HR lowers conductive slower depolarisation = gap between 0 and 3 increase

Question 49

waves of AP through most of free walls of ventricles run

Answer 49

apex ==> base endo —> epi