Heart Electrophysiology

Question 1

Type of cells that spread the electrical activity

Answer 1

nodal
conducting cells
myocytes

Question 2

Position of Sino-atrial node and its conduction speed

Answer 2

15mm x 5mm x 2mm (L W T/D)
Posterior aspect
Junction superior vena cava and right atrium
Conduction – 0.05 m/s

Question 3

Pathway through the heart of electrical activity

Answer 3

1. Initiated at sino-atrial node (pacemaker)
2. Conduction to atria and atrioventricular node. Then atrioventricular ring.
3. Passage through bundle of His
4. Purkinje system distribution to ventricular muscle cells

Question 4

What is the Conduction rate via atrial myocardium or Bachmann’s bundle (interatrial tract to left atrium)

Answer 4

1.00m/s

Question 5

Structure and positon of the AV node and conductance rate

Answer 5

22mm x 10mm x 3mm (L W T/D)
Posterior aspect - Right side interatrial septum Three sub-zones: AN, N, NV
Slow conduction through AN – N = 0.05 m/s

Question 6

AV delay and refrationess meaning?

Answer 6

AV delay - allows atrial contraction to finish

AV refractoriness - Prevents excess ventricular contraction and Increases at high heart rate

Question 7

Sub nodal conduction rates

Answer 7

Fast conduction via His = 1 m/s
causing Septal activation
Purkinje fibres = 4 m/s
Ventricular muscle = 1 m/s

Question 8

role of spiral muscle

Answer 8

Spiral muscle contraction evokes a torsion causing the twisting of heart to ensure all blood ejected

Question 9

Other than SA node what can act as a pacemaker?

Answer 9

purkinje fibres and AV node

Question 10

Pacemaker Action Potential pathway ?

Answer 10

Slow depolarisation caused by a decrease in K+ permeability (pacemaker potential) = IK
If (funny current) occurs when cation influx Na+ slowly inflowing using HCN4 channel

Reahces threshold and fast Ca2+ inflow causing faster depolarisation through Cav1.2 and Cav1.3

If current – hyperpolarization-induced (HCN4), inactive when +ve potential
Fast depolarisation - ICa (L type Ca2+ channels, e.g. Cav1.2 & 1.3)

Repolarisation IK - K+ efflux

Regulated by innervation, temp, other pacemakers

Question 11

Sympathetic Action on nodal cells ?

Answer 11

noradrenaline at beta 1+2 impacts the pre potential phase by increasing the slope steepness and increasing the firing rate as reaches the threshold value quicker.

Question 12

Parasympathetic action on the nodal cells ?

Answer 12

vagal fibres via Ach cause hyperpolaristation and decrease the pre potential slope so it starts from a more negative potential. Takes longer to get back to the threshold so reduces ability of cells to send action potential so slow HR

Ach causes decrease in M2 so decrease cAMP so slows HR

Question 13

What regulates the funny current?

Answer 13

cAMP, enhances the probability so channels open more, increase beta 1+2

Question 14

Calcium clock oscillations definition

Answer 14

spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) is the mechanism for sinus rhythm generation. When the SR is full, the probability of spontaneous Ca2+ release increases. On the other hand, when the SR is empty, the chances for spontaneous Ca2+ release decreases. The rhythmic alterations of SR Ca2+ release is referred to as the Ca2+ clock.

This suggests that the SAN must exist normally in a state of calcium overload.

Question 15

Cardiac muscle action potential overview

Answer 15

No automaticity
Long plateau phase
Propagated and prolonged action potential
Fast depolarisation and overshoot

Question 16

cardiac muscle action potential mechanism

Answer 16

1. volt gate Na+ channels open (Nav1.5)
2. Na+ inflow depolarises membrane triggers opening of more Na+ channels so +ve feedback
3. Na+ channels close when cell depolarised and voltage peaks at +30 mV
4. Ca2+ entering through slow calcium channels (Cav1.2) prolongs the depolarisation of membrane creating a plateau
5. plateau falls as K+ leakage
6. Ca2+ channels close and K+ channels open (Kv11.1/ HERG and Kv7.1/kVLQTt1) so rapid influx to resting potential