Electrical Activity of the Heart

Question 1

Cardiac muscle is known as a functional syncytium, wtf does that mean?

Answer 1

Essentially it functions like one massive cell despite being made of many different cells.

Question 2

Whats the function of gap junctions in cardiac muscle?

Answer 2

They allow electrical connection between cells so when one depolarises so does the next one, allowing simultaneous contraction.

Question 3

What do desmosomes do?

Answer 3

They form a physical connection between cells.

Importantly this stops them all contracting away from eachother, instead they contract like one massive cell.

Question 4

What makes up an intercalated disc?

Answer 4

A gap junction/desmosome pair

Question 5

Why is the AP of cardiac muscle much longer than that of skeletal?

Answer 5

Its ~250ms rather than 2ms

Because much more calcium is released (in this case from outside the cell)

Question 6

How is the strength of contraction varied in cardiac muscle?

Answer 6

By the amount of calcium taken in from outside the cell. This is because the base level doesnt fully saturate the troponin.
(Ca++ from outside the cell triggers calcium dependant calcium release from the sarcoplasmic reticulum)

Question 7

Whats the point of cardiac muscle’s longer refractory period?

Answer 7

It ensures that the heart has relaxed fully before its capable of a 2nd contraction.
This prevents a tetanic contraction which would fucking kill you.

Question 8

Whats different about pacemaker cells?

Answer 8

They have a far more unstable resting membrane potential, its closer to -60mv which is very close to the threshold at -55mv. Hence the lack of stability.

Question 9

What is responsible for the plateau in non-pacemaker membrane potential after the initial depolarisation?

Answer 9

An increase in PCa++ (L Type Channels) and a decrease in PK+

Question 10

What causes repolarisation in non-pacemaker cells?

Answer 10

A decrease in PCa++ and an increase in PK+.

Question 11

Whats different about the depolarisation to threshold in a pacemaker cell vs a normal cell?

Answer 11

PAcemaker cells depolarise to threshold gradually and regularly each time.
Non-pacemaker cells depolarise suddenly to threshold

Question 12

What auses the gradual depolarisation of pacemaker cells?

Answer 12

• Decreasing PK+
• Slight Increase in PNa+ (PF)
• Increasing PCa++ (T Type channels)

Question 13

What channels open when threshold is reached in a pacemaker cell?

Answer 13

Ca++ L Type channels trigger the massive depolarisation of the AP.

Question 14

Whats the function of pacemaker cells?

Answer 14

They allow autorythmicity and modulation of the heart rate.

Question 15

What kind of drugs modulate electrical activity in the heart? And what is the end result.

Answer 15

Ca2+ channel blockers decrease the contraction force

Cardiac Glycosides increase the force of contraction

Question 16

How does temperature affect heart rate?

Answer 16

Every degree by which your body temp increases will increase your heart rate by ~10 bpm

Question 17

How does hyperkalemia affect heart function?

Answer 17

• > High plasma K+
• > Decreased K+ conc. gradient
• > K+ doesn’t leave the cell at rest
• > Cell gradually depolarises
• > Random contraction - Fibrillation.

Can also cause a blockage of APs travelling from the atrium to the ventricle - Heart Block

Question 18

How does Hypokalemia affect heart function?

Answer 18

-> Low Plasma K+
-> Cell starts to hyperpolarise
-> Suddenly loads of channels open and you get the same effects as hyperkalemia.
Even Prof Davies doesn’t know why, it is quote anomalous unquote.

Question 19

How does hypercalcaemia affect heart function?

Answer 19

Hypercalcemia increases HR & Force of Contraction

Question 20

How does hypocalcemia affect heart function?

Answer 20

Hypocalcemia decreases HR & Force of Contraction.

Question 21

Which pacemaker cells are responsible for the heart rhythm?

Answer 21

The faster pacemaker cells dictate the Heart rhythm. These are found in the sinoatrial node in the right atrium,

Question 22

What is the annulous Fibrosis?

Answer 22

The non-conducting insulating layer of fibrous connective tissue between each atrium & ventricle.
Prevents depolarisation travelling through so we dont get simultaneous contraction of the atrium and ventricle.

Question 23

What does the atrioventricular node do?

Answer 23

Slows AP conduction to about 0.05m/s so the atria can push blood into the ventricles before the ventricles contract.

Question 24

What carries depolarisation through the annulous fibrosis to the ventricles?

Answer 24

A bundle of His.

Purkinje fibres then conduct the signal rapidly through the whole ventricle so it all contracts at once. (5m/s)

Question 25

What is picked up by a ECG?

Answer 25

Lots of cells depolarising & repolarising at once evokes lots of small extracellular electrical potentials that summate into large waves that can be recorded at the periphery by an ECG.

Question 26

What are the 3 stages of an ECG and what electrical activity of the heart is responsible for each?

Answer 26

The P wave - Atrial Depolarisation
The QRS complex - Ventricular depolarisation
The T Wave - Ventricular repolarisation

We don’t detect atrial repolarisation because it happens simultaneously to the ventricular depolarisation and is swamped by it.