CVS S7 - Incomplete Cardiac Action Potentials + Drugs & the CVS

Question 1

Describe membrane permeability in a ventricular cardiac myocyte over the course of the cardiac cycle

Answer 1

In diastole, the resting membrane is close to EK
Initial depolarisation due to pacemaker cells
Once threshold reached, fast voltage gated Na channels open
Following this fast depolarisation towards Na’s equilibrium potential is a brief repolarisation caused by K outflow
Na channels deactivate and slower Ca channels activate
Influx of Ca causes intracellular Ca store release, triggering contraction
Ca channels close after ~250ms
Efflux of K returns membrane potential to resting

Question 2

Describe membrane permeability in a cardiac pacemaker cell over the course of the cardiac cycle

Answer 2

In diastole, membrane potential is ~-60mV
Slow Na channels open during repolarisation and are responsible for ‘funny current’ (If)
Once the cell reaches threshold, Ca channels open, giving a relatively slow depolarisation
Once Ca channels close, K efflux causes repolarisation.

Question 3

How would autonomic stimulation result in increased heart rate?

Answer 3

Sympathetic β1 adrenoceptor stimulation
Increased SAN pacemaker depolarisation speed
Therefore increased heart rate

Question 4

How would autonomic stimulation result in decreased heart rate?

Answer 4

Parasympathetic M2 cholinoceptor stimulation
Decreased SAN pacemaker depolarisation speed
Therefore decreased heart rate

Question 5

Describe baroreceptors

Answer 5

Located in the arch of the aorta and the carotid sinus
Stretch-sensitive
Detect high or low blood pressure
Feedback to the medulla, which causes compensatory autonomic stimulation

Question 6

How may arrhythmias arise?

Answer 6

Ectopic pacemaker activity
After-depolarisations
Re-entry loop

Question 7

Describe ectopic pacemaker activity

Answer 7

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Question 8

What is the cardiac membrane potential and what causes it?

Answer 8

~-90mV

Mostly due to high K+ permeability (EK is ~-80mV)