cardiac electrophysiology

Question 1

what is meant by myogenic?

Answer 1

The heart generates electrical activity and contraction without the need for nerves

Question 2

How does SAN generate APs?

Answer 2

generates action potentials (APs) with rhythmical pacemaker activity in absence of any nerve stimulation

Question 3

Hr and SAN

importance?

Answer 3

Firing rate of APs (resting about 1/s) equals heart rate (resting about 60-70bpm)
Therefore SAN controls HR

Question 4

SAN cells not contractile?

Answer 4

generate electrical activity

Question 5

why does SAN node keep generating AP?

Answer 5

unstable/non-equilibrium resting membrane potential (RMP)

Question 6

what are If channels? (funny current)

what do they do?

Answer 6

Hyperpolarisation-activated Na+ channel (sometimes termed Ih)
Na+ influx produces depolarising slope (Phase 4)

Question 7

explain phase 4 for SAN

what sets it off and what do they do?

Answer 7

If activated by hyperpolarisation

open If Na+ channels and the influx causes depolarisation till threshold hit

Question 8

threshold hit leading to phase 0, explain (SAN)

Answer 8

Voltage-gated Ca2+ channels open leading to Ca2+ influx and rapid Depolarisation so AP generated

Question 9

why is Ca2+ and Na+ depolarisation different? (3 things)

Answer 9

Na+ nerves
Ca2+ slow upstroke compared to Na+ due to opening slower
Ca2+ is open longer so wider AP

Question 10

Phase 3 (SAN)

Answer 10

Voltage-gated K+ channels open leading to K+ efflux and rapid Repolarisation

Question 11

conduction pathway from SAN through heart

Answer 11

conduction across both atrium to AV nodes to Bundle of His to purkinje fibres

Question 12

why does Av node slow down conduction rate?

how does it slow it down?

Answer 12

has fatty insulated tissues to slow down conduction rate so it allows time for the ventricles to fill with blood from atrium

Question 13

importance of purkinje fibres

Answer 13

allow both ventricles to be stimulated at the same time hence can contract and eject blood at the same time

Question 14

what allows FAST electrical conduction in heart

Answer 14

Via low resistance pathways between myocytes called intercalated discs

Question 15

how are APs generated in atrium/ventricles?

Answer 15

from electrical stimulation arising in S-A node

atrial/ventricular cells have a stable resting membrane potential

Question 16

phase 0 in a/v ap

what opens, why and what does it do?

Answer 16

AP from SAN lead to Voltage-gated Na+ channels open and high Na+ influx leading to Depolarisation

Question 17

phase 1 (a/v) - what opens?

Answer 17

k+ channels breifly open for repolarisation

Question 18

phase 2 - plateau stage (a/v)

what opens and why plateau?

Answer 18

Voltage-gated Ca2+ channels (VGCCs) open and Sustained Ca2+ influx leading to Depolarisation
plateau cos k+ out and ca2+ in

Question 19

importance of plateau stage

what does it prevent and what is it essential for?

Answer 19

Myocytes are unexcitable or REFRACTIVE – CANNOT fire more APs, no twitching
One AP only produce ONE contraction
ESSENTIAL for proper ejection of blood from heart
This is all due to Na+ channels being inactivated so can’t lead to another AP firing

Question 20

phase 2 to 3 (a/v) - what close/open?

Answer 20

VGCCs switch off and K+ channels switch on

Question 21

Phase 3 (a/v) - what open and effect?

Answer 21

Voltage-gated K+ channels open leading to K+ efflux and rapid Repolarisation

Question 22

How does a/v APs produce a contraction?

Answer 22

Contraction occurs by an INCREASE in cytosolic Ca2+ levels
Higher increases in Ca2+ lead to increased force of contraction
Ca2+ rises from 0.1 µM to about 2-10 µM

Question 23

difference between smooth and cardiac muscle contraction (2 things)

Answer 23

CICR (Ca2+-induced Ca2+ release) via RyR and contraction via troponin system are DIFFERENT to smooth muscle (IP3R + MLCK)

Question 24

Link between Electrical Excitability and Contraction

Answer 24

1 heart beat = SAN AP to atrial contraction to electrical activity to AV node to ventricular contraction

Question 25

Ecg and pqrst wave

what do the different waves correspond to?

Answer 25

P wave - atrial depolarisation
QRS wave - ventricle depolarisation
T wave - ventricle repolarisation

Question 26

what effect does stimulation of the sympathetic nerves have on the heart?

2 effects?

Answer 26

Stimulation of these nerves as an effect on,
Heart rate : Positive chronotropic effect
Contractility : Positive inotropic effect
↑ If —-> ↑ Pacemaker frequency —–> ↑ Heart rate

↑ ICa + ↑ RyR —-> ↑ influx of Ca2+ levels + ↑ CICR ——> Contractility

Question 27

what sympathetic nerves have an effect on the heart?

what nerves and what do they stimulate?

Answer 27

Sympathetic nerves from T1-T5 of spinal cord innervates both S/A node and atria/ventricles

Question 28

How does the sympathetic nerves have an effect in the heart?

what is released and act on what receptors?

Answer 28

Releases noradrenaline which acts at B1 adrenoceptors on S-A node cells and cardiac myocytes

Question 29

what effect does stimulation of the parasympathetic nerves have on the heart? ( 1 effect)

Answer 29

Heart rate : Negative chronotropic effect only

↓ If —-> ↓ Pacemaker frequency —–> ↓ Heart rate

Question 30

what parasympathetic nerves have an effect on the heart?

Answer 30

Parasympathetic nerves from brainstem innervate S/A node

vagus nerve

Question 31

How does the parasympathetic nerves have an effect in the heart?

what is released and what receptor?

Answer 31

Releases Ach which acts at muscarinic (M2) receptors

on S-A node cells