Cardiac Electrophysiology

Question 1

What are the basic parts that make up the cardiac action potential?

Answer 1

Step 1. The voltage gated sodium channel - I(Na): sodium in

Step 2. The L-type voltage gated calcium channel - I(Ca): calcium in

Step 3. Calcium binds the ryanodine receptors on the SR: calcium release from SR within the cell

-CONTRACTION-

Step 4.

A:The potassium efflux channels - I(Kr) and I(Ks): potassium out

B: The sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) pumps the calcium back from the cytosol in to the SR: Calcium (effectively) out

C: The sodium/calcium exchanger (NCX) pumps calcium out of the cell: Calcium out

-HYPERPOLARISED: REFRACTORY PERIOD-

Step 5. Funny currents slowly depolarise the myocyte

Question 2

When the heart rate is increased in sympathetic nervous stimulation, how does this occur in the cardiac myocytes?

Answer 2

Lusitropy

1. Sympathetic stimulation
2. Beta one receptors cause an increase of cAMP in the cytosol
3. cAMP dependant protein kinase A (PKA) phosphorylates phospholamban on the SERCA (calcium ATPase)
4. This increases the rate of calcium transport from the cytosol to the SR
5. Allowing the myocyte to recover, ready to contract again sooner

Question 3

When the force of contraction is increased in sympathetic nervous stimulation, how does this occur in the cardiac myocytes?

Answer 3

Inotropy - increased force and speed of contraction

1. More L-type calcium channels are opened all over the heart: More myocytes are recruited
2. More calcium is released in the myocytes
3. This means a far more rapid depolarisation with more myocytes contracting
4. This increases the force (force is proportional to number of muscle cells contracting) and the speed of depolarisation; contraction.
   - works only in combination with lusitropy