Lecture 5: Cardiac Ion Channels And The Heart Beat

Question 1

Why does the Cardiac action potential have a plateau and what causes it?

Answer 1

• longer depol of the membrane allows for longer contraction time
• caused by slow Ca and Na/Ca channels which open just as fast Na channels are about to close
• they allow lots of Ca and Na in and stay open longer than the fast channels
• as soon as the AP starts the permeability to K decreases which prevents early depol, but when the slow channels close the Pk increases rapidly = repol

Question 2

Name a slow Ca channel:

Answer 2

L-type Ca channel

Question 3

Which 2 ions are the main contributors to the resting MP in Cardiac cells

Answer 3

K and Cl

Question 4

What is the voltage sensing region of an ion channel?

Answer 4

S4

Question 5

What is the difference between a fast response AP and a slow response AP?

Answer 5

Fast has a plateau phase

Slow does not

Question 6

During a fast response AP, _____ rapidly increases to cause depol, _____ increases to cause the plateau phase and _____ remains effectively 0 until repol phase.

Answer 6

G Na
G Ca
G K

Question 7

Outline the 4 phases of the of the Fast response AP:

Answer 7

Phase 1 - rapid increase in G Na then decrease due to closure of time and voltage dependent channels. Activation and subsequent inactivation of the transient outward K channels
Phase 2 - influx of Ca due to increase in G
Phase 3 - delayed rectifier current of K = repol
Phase 4 - inward rectifier current returns cell to resting MP

Question 8

Outline the key events of the slow response AP:

Answer 8

Phase 0 - slow increase in L-type Ca G, no Na
Phase 3 - slow decrease in G Ca and increase in Ik = repol
Phase 4 - lower resting G K and higher resting G Na (Maximum Diastolic Potential)
Late in phase 4, G Ca increase
Diastolic depol is caused by a decrease in Ik and/or steady inward current of Na

Question 9

How does sympathetic stimulation affect the heart beat?

Answer 9

• increases the automaticity in the pacemaker cells
• increase contractility of the atrial and ventricular cells
• alters the AP configuration

Question 10

How does parasympathetic stimulation affect the heart beat?

Answer 10

• decreases the rate of SA and AV node firing
• decreases conduction velocity
• antagonises sympathetic stimulation in all areas of the heart

Question 11

Why do cardiac cells need extracellular calcium in order to contract and how is this calcium sourced?

Answer 11

• intracellular stores are less developed and don’t have enough Ca for full contraction
• to compensate there are large t-tubules filled with mucopolysaccharides that bind Ca so they can quickly diffuse into the cell