Cardiac electrical activity

Question 1

What is the resting membrane potential for a cardiac muscle cell and why

Answer 1

-90mV, near the Nernst or equilibrium potential for Potassium. At rest the predominant channels that are open are K+ channels meaning the RMP is closest to K+ EP.

Question 2

What is the duration of a cardiac muscle cell action potential compared with nerve AP

Answer 2

200-300 msec, nerve and skeletal muscle AP 1 msec.

Question 3

What is threshold potential for cardiac muscle cells?

Answer 3

-70mV. The point at which voltage gated Na+ channels open.

Question 4

Describe phase 0 of the fast action potential

Answer 4

Rapid influx of Na+ ions as voltage gated Na+ channels open. Na+ is moving inwards downs its concentration (Na+ much higher out of the cell than in the cell) and electrical (RMP is negative Na+ is positive) gradients.

Question 5

Why do the Na+ channels close almost as quickly as they open

Answer 5

Because they are voltage and time sensitive

Question 6

Describe phase 1 of the fast action potential

Answer 6

Early repolarisation. Whereby there is a transient outward K+ current. Membrane potential close to 0mV

Question 7

Describe phase 2 of the fast action potential

Answer 7

Plateau phase Na+ channels shut and are refractory, inward going current Ca2+ and outward K+ current. Na+/Ca2+ exchangers 3 Na+ in for 2 Ca2+ out. Also Ca2+ dependent Ca2+ release from SER. Net current 0.

Question 8

Describe phase 3 of the fast action potential

Answer 8

Voltage and time dependant K+ channels (iK) open and repolarisation occurs,

Question 9

What happens if there is early or delayed repolarisation?

Answer 9

Potential for cardiac arrhythmia

Question 10

Describe phase 4 of the fast action potential

Answer 10

Resting phase. K+ open and membrane potential at -90mV

Question 11

Where does the fast action potential occur

Answer 11

In the atria, ventricles and fast parts of the specialised conduction system

Question 12

Where does the slow action potential occur?

Answer 12

In the cells of sinoatrial node, and atrioventricular node. Maybe pacemaker cells.

Question 13

What is the difference in upstroke in the the slow AP compared with the fast?

Answer 13

Slower upstroke on depolarisation as the currents are not as intense.

Question 14

What causes the upstroke in the slow AP

Answer 14

Upstroke due to opening of Ca2+ channels and inward going calcium. Also ‘funny’ channels (If) where Na+ channels are opened in response to hyper polarised membrane

Question 15

Why is refractory period essential

Answer 15

To avoid tetanic contraction. Contract to eject blood, needs relaxation to fill.

Question 16

Describe relative refractory period (RRP)

Answer 16

Can generate an AP with higher than normal stimulus but weaker AP than normal.

Question 17

Describe super normal period (SNP)

Answer 17

When you can stimulate AP with smaller than normal stimulus but slower conduction of AP occurs.

Question 18

Describe absolute refractory period (ARP)

Answer 18

When there is no possibility of generating another AP

Question 19

Describe the properties of cardiac muscle for spreading AP

Answer 19

Cardiac muscle fibres are conductive. Activation spreads through the muscle cell to cell via gap junctions in the syncytium

Question 20

Pacemaker cells in the heart

Answer 20

Sinoatrial node, Atrioventricular node, purkinje fibres

Question 21

How does the PNS work to slow HR

Answer 21

ACh, the PNS NT, increases the permeability of the cell membrane to K+. Cell membrane becomes hyperpolarized. strong vagal stimulation can also stop the SAN or block the AVN.

Question 22

How does the SNS increase the HR

Answer 22

Noradrenaline at the SAN increases the slope of the pacemaker depolarisation through phosphorylating Ca2+ channels which generates longer opening of the channels, more inward Ca2+ faster depolarisation, faster upstroke.

Question 23

What is the role of the AV node

Answer 23

The AV nodes role is to slow the conduction of AP to the ventricles. This is so the atria contracts slightly before ventricles this assists in filling the ventricles before they contract.

Question 24

What is the speed of conduction of the bundle branches and purkinje fibres?

Answer 24

2-4/m/s fast to activate ventricular endocardium synchronously

Question 25

What is the speed of conduction of the SA node

Answer 25

Very slow

Question 26

What is the speed of conduction of the AV node

Answer 26

Slow 0.02- 0.05m/s to slow conduction allowing ventricular filling

Question 27

Conduction velocity depends on

Answer 27

• Radius of cells

- Rate of depolarisation and amplitude of action potential (driven by different currents, different channels)

Question 28

What would occur with blockage of the right coronary artery?

Answer 28

The right artery supplies the SA and AV nodes so rhythm disturbances may occur

Question 29

What may occur with blockage of the left coronary artery?

Answer 29

Supplies the circumflex and LAD arteries. Blockage results in pump failure.

Question 30

What is the name of the band shortening transmission in the right ventricle.

Answer 30

The septomarginal trabecular. Allows the right ventricle to contract uniformly with the left.