Initiation of heartbeat

Question 1

Compare the timings of a neuronal and a cardiac action potential.

Answer 1

Cardiac action potential much longer that neuronal action potential (200 > 2).
There is a larger proportion of time spent in the refractory period in the cardiac than in neuronal and a shorter proportion of time spent in the relative refractory period

Question 2

What is the cardiac QT interval and how does it change with HR?

Answer 2

It’s the action potential duration. It decreases to fit more in a minute when the rate increases

Question 3

Why is the refractory period of a cardiac action potential so long?

Answer 3

Twitch will have finished by the time refractory period is over so that there is no temporal summation of impulses leading to tetany

Question 4

Why do the SAN, AVN and Purkinje fibres have a clock/pacemaker function?

Answer 4

They have an unstable resting potential (funny current) which means depolarisation happens regularly

Question 5

Why do the atria and ventricles not have a clock/pacemaker function?

Answer 5

They have a stable resting action potential (no funny current) which means that depolarisation doesn’t happen spontaneously.

Question 6

What drug can block the funny current of the SAN?

Answer 6

Ivabradine

Question 7

How do the interdigitations of the cardiomyocytes help aid conduction of an impulse through cardiac muscular tissue?

Answer 7

They contain connexons (6x connexins) which allow the propagation of an action potential between cells.

Question 8

What is PQRST in an ECG

Answer 8

P = atrial depolarisation
Q = depolarisation of septum towards base
R = depolarisation of septum towards apex
S = depolarisation of ventricles towards base
T = repolarisation of ventricles

Question 9

How does relaxation of cardiac muscle occur in the cell?

Answer 9

There is a reduction in intracellular Ca concentration. Ca is pumped back out of the cell by the L-type channels which brought it in. And SERCA (sarcoendoplasmic reticulum Ca ATPase) pumps Ca back into the SR.

There is also a K+/Na+ ATPase which brings Na+ out of the cell so that a Na+/Ca2+ exchanger can work in the membrane removing more calcium from the cell.

Question 10

What is chronotropy?

Answer 10

Control of heart rate

Question 11

What is inotropy?

Answer 11

Control of heart contractility

Question 12

What is lusitropy?

Answer 12

Control of rate of relaxation of the heart