8 - Action Potential of the Heart

Question 1

What does an action potential trigger in the heart?

Answer 1

An increase in the calcium concentration

Question 2

What are the shapes of the action potential graphs for the SAN and the ventricle?

Answer 2

Question 3

Explain the shape of the ventricle cardiac action potential.

Answer 3

0 - Opening of fast v-gated Na channels

1 - Transient K+ outward i_to

2 - Voltage gated Ca channels and efflux of K+

3 - Ca channels inactivate and voltage gated K+ open so K efflux (i_KV)

4 - Resting

Question 4

What type of calcium channels are involved in the ventricular action potential?

Answer 4

L-type slow opening

Question 5

Explain the shape of the SA node action potential graph

Answer 5

1 - Pacemaker potential (Funny Current) that is a slow depolarisation activated by hyperpolarisation. Na influx

2 - Opening of voltage gated T and L Ca channels NOT Na

3 - Opening of voltage gated K+ channels, inacitivation of Ca channels

Question 6

What is the pacemaker potential?

Answer 6

• Occurs in the SAN as they can spontaneously depolarise
• Activated by hyperpolarisation below -50mv
• HCN gated channels that is permeable to K+ and Na+ but only Na comes in
• K+ current turned off
• T and L type channels activated at the end

Question 7

What is the resting potential of the SAN node?

Answer 7

Unstable resting membrane potential. Between -60 and -40

Question 8

Why is the SA the pacemaker?

Answer 8

It is the fastest node to depolarise so it sets the rhythm. AV can be pacemaker but would be slower

Question 9

What is the effect of hyperkalaemia?

Answer 9

• Higher resting m.p
• Shorter action potential
• Slower upstroke as some Na inactivated

Intially increased excitability or aystole due to Na channel inactivation

Question 10

What are the different extents of hyperkaelemia and how can it be treated?

Answer 10

Mild: 5.5-5.9 mmol/L

Moderate: 6 - 6.4 mmol/L

Severe > 6.5 mmol/L

• Calcium glutaronate
• Insulin (moves K into cells) and glucose

Question 11

What are the effects of hypokaelemia? <3.5mmol/L

Answer 11

Lengthens action potential and time for repolarisation so can lead to EAD’s, which cause oscillations in m.p.

This can lead to ventricular fibrillation

Question 12

How does an action potential cause the heart to contract?

Answer 12

• Action potential travels down T-tubule and causes L-type Ca channels to be opened by depolarisation.
• CICR (75% Ca)
• Ca binds to troponin C etc
• Pumped back out by SERCA, PMCA, NCX

Question 13

How do blood vessels contract?

Answer 13

Tunica media contains smooth muscle, especially in the arteriole.

Question 14

Where is the regulatory protein in smooth muscle?

Answer 14

Light chain on myosin head, instead of troponin on actin

Question 15

What can cause relaxation of smooth muscle?

Answer 15

• As Ca declines, MLCP dephosphorylates light chain
• PKA can phosphorylate MLCK so myosin can’t be phosphorylated as MLCK is inactive

Question 16

What are the main differences between cardiac muscle and smooth muscle contraction?

Answer 16

-

Question 17

Why is there a difference in length between the nerve and heart muscle action potential?

Answer 17

Heart is around 280ms because of the sustained calcium channel activity, during the plateau the calcium channels are open so influx of Ca, activating the efflux mechanisms

Question 18

Draw graphs for the pacemaker potential and the ventricular action potential, including voltage and time values.

Answer 18