Cells Within The Heart (and How They Create Action Potentials)

Question 1

What are pacemaker cells?

Answer 1

Specialised myocytes that generate electrical events at regular intervals

Question 2

What are pace maker cells responsible for initiating?

Answer 2

The cardiac action potential

Question 3

In diastole what is the m.p of pacemaker cells?

Answer 3

There is no fixed value (most negative -60mV)

Question 4

How does a pacemaker cell creat an action potential?

Answer 4

1) resting around -60mV
2) HCN channels cause a funny current(slow sodium channel activation)
3) L-type Ca2+ channels open causing large depolarisation
4) Voltage gated K+ ion channels channels open- influx of K+ and m.p becomes more negative again

Question 5

What role do HCN channels have in the production of a cardiac action potential?

Answer 5

They cause the funny currents which is slow sodium channel activation.

They are activated by hyperpolarisation therefore the more negative the M.p the more likely it is that the HCN channels will open and cause an influx of Na+

Question 6

What ion are cardiac myocytes permeable to at rest?

Answer 6

Potassium

Question 7

How is the resting membrane potential of a cardiac myocyte set up?

Answer 7

Na/K ATPase sets up concentration gradients

K+ moves out of cell down conc gradient

Small movement of ions makes the inside negative with respect to outside

As charge build up an electrical gradient is established

Question 8

Why in cardiac myocytes is the rmp not equal to Equilibrium potential of potassium (Ek)?

Answer 8

There is very small permeability to other ion species at rest which depolarise the cell

Question 9

What does an action potential doe to the cystolic [Ca2+] in myocytes? Why is this necessary?

Answer 9

It increases it

A rise in calcium is required to allow actin and myosin interaction (generates contraction)

Question 10

Describe a cardiac action potential

Answer 10

1) rmp = -85mV
2) voltage gates na+ channels open - influx of na+ depolarises cell
3) at peak there is a transient outward K+ current- repolarises cell slightly
4) L type voltage gated Ca2+ channels open - some k+ channels still open so gradual repolarisation (plateau phase)
5) ca2+ channels inactivated, voltage k+ channels- repolarise cells

Question 11

How long does a cardiac action potential take?

Answer 11

~400 ms

Question 12

In terms of ion influx and efflux, what is happening during a cardiac action potential?

Answer 12

Na+ influx

K+ efflux

Ca2+ influx (K+ effux)

K+ efflux