Cardiac Pacemakers

Question 1

Describe the action potential of a cardiac autorhythmic (AR) cell.

Answer 1

Rising phase: due to activation of voltage-gated Ca2+ channels.

Falling phase is due to activation of voltage gated K+ channels.

Inactivation of these channels then small slow drift to the next action potential.

Due to 3 ionic events

• Cyclical decrease in normal passive K+ efflux
• Special voltage-gated channels that open when cell becomes more negative, continuing on from the previous action potential (funny If chanels)
• increased influx of Ca2+ chanels which accounts for second half of the drift to threshold. Ca2+ moves through transient Ca2+ channels (T-Type) which takes the membrane to threshold.

Question 2

Why does the action potential pause when it reaches the AV node?

Answer 2

It pauses to allow atria to contract to fill the ventricles.

Question 3

Describe the action potential of a cardiac myocardial cell.

Answer 3

Resting Memb Pot. (-90mV)

1. AP is brought about by an explosive increas in membrane permeability to Na+
2. Na+ permeability the falls to low resting level (Activation of slow Ca2+ chanels.Marked decrease in membrane K+ permeability)
3. Membrane potential is maintained at this depolarised level for several hundred miliseconds
4. Plateau phase.
5. Inactivation of Ca2+ channels and activtion of K+ channels. = K+ efflux

Question 4

What is the significance of the plateau period in the cardiac action potential?

Answer 4

Prevents tetany by elongating the action potential to being the same time as the muscle contraction.

Question 5

What is the role of the T-tubule system in contraction & relaxation in cardiac muscle?

Answer 5

T-Tbules of myocardial cells are larger than those in skeletal muscle (also branched).

T-Tubules permit rapid transmission of the action potential into the cell, and also play an important role in regulating cellular calcium concentration.

Cardiac muscle depends upon Ca2+ moving from ECF to cause contraction. Ca2+ diffuses directly from T-Tubules to cytosol.

Extra Ca2+ helps establish plateau phase.

Question 6

What physiological events modulate strength of contraction in the heart?

Answer 6

Strength of contraction is proportional to the number of crossbridges formed in the muscle.

Increase in [Ca2+] = more crossbridges being formed.

Regulatory protein phospholamban is activated by phosphorylated protein kinase (cAMP) to enhance the Ca2+ATPase activity in the SR.

Question 7

What is the effect of the parasympathetic nervous system on heart function?

Answer 7

Slows heart rate by increasing K+ efflux & decrease Ca2+ influx

Question 8

What is the effect of the sympathetic nervous system on heart function?

Answer 8

Speeds heart rate by Increasing calcium and Funny channel flow.