Electrical & Molecular Mechanisms In The Heart & Vasculature 4.2

Question 1

What is the resting membrane potential (or similar) and extent of depolarisation of an axon?

Answer 1

-70mV

+30mV

Question 2

What is the resting membrane potential (or similar) and extent of depolarisation of skeletal muscle?

Answer 2

-90mV

+40mV

Question 3

What is the resting membrane potential (or similar) and extent of depolarisation of the sino-atrial node?

Answer 3

-60mV

+30mV

Question 4

What is the resting membrane potential (or similar) and extent of depolarisation of cardiac ventricles?

Answer 4

-90mV

+30mV

Question 5

How do sinoatrial node myocytes differ from other myocytes?

Answer 5

They are specialised so they are worse at contraction but can spontaneously depolarise

Question 6

Compare the SA node and cardiac ventricular myocyte action potentials to a typical action potential

Answer 6

SA - doesn’t have a resting membrane potential as it is constantly depolarising (pacemaker potential). It is also activated by hyperpolarisation.
Ventricular myocytes - no hyperpolarisation

Question 7

What is the difference in the calcium requirement of cardiac vs other myocytes?

Answer 7

Cardiac need calcium ions outside the cell too as 25% comes from across the sarcolemma, whereas other myocytes just use the store in the sarcoplasmic reticulum

Question 8

Why is the SAN the ‘pacemaker’?

Answer 8

It is the fastest to depolarise; other parts of the conducting system in the heart also have natural automaticity but are slower

Question 9

What happens if action potentials fail in the heart?

Answer 9

Asystole

Question 10

What must the potassium plasma concentration be kept at to not cause problems?

Answer 10

3.5 - 5.5 mmol/L

Question 11

What is hyperkalaemia?

Answer 11

Plasma potassium concentration above 5.5mmol/L

Question 12

What is hypokalaemia?

Answer 12

Plasma potassium concentration below 3.5mmol/L

Question 13

What can hypokalaemia cause?

Answer 13

Ventricular fibrillation

Question 14

What can hyperkalaemia cause?

Answer 14

Asystole

Question 15

What are the differences in contraction between smooth and cardiac myocytes?

Answer 15

Cardiac - require calcium entry from outside cell, calcium binds to troponin C, initiated by spread of APs from nodes
Vascular smooth muscle -calcium binds to calmodulin, activation of MLCK phosphorylate myosin light chain, initiated by depolarisation or activation of alpha adrenoceptors