Electrical activity of the heart

Question 1

In skeletal muscle which are the thin vs thick filaments? WHich are attached to the Z line?

Answer 1

thin = actin
thick = myosin

Actin are attched to the Z line

Question 2

What is the membrane surrounding the muscle called? What is a T Tubule?

Answer 2

Sarcomere, T-tubules are transverse tubules run deep into muscle, allow calcium/ions to reach the whole muscle.

Question 3

Where is the sarcoplasmic reticulum and what does it release? What Voltage do normal muscle cells remain at?

Answer 3

Membrane structure within the muscle. Calcium is released. Normal muscle cells remain at -90mV

Question 4

What binds to troponin?

Answer 4

Calcium

Question 5

Skeletal vs cardiac muscle

- connections and what do they allow through?

Answer 5

Skeletal muscle are muscle cells that have been merged together, whereas cardiac muscle have intercalated discs around the whole cell allowing signals to be distrubuted in every angle

Question 6

What is a gap junction?

Answer 6

A protein channel connecting one cells cytopolasm to another, allowing small signalling molecules through.

Question 7

What forms the intercalated disc? What is the advantage of hr intercalated disc?

Answer 7

Alternating gap junctions (allowing the signalling molecules through) and desmosomes (keeping cell-cell adhesion). Allows the whole heart to contact from one signal.

Question 8

How long is the repolarisation in skeletal vs cardiac muscle? Why is there a difference?

Answer 8

Skeletal muscle = 1-2ms
Cardiac muscle = 250ms

Because cardiac muscle has the extra calcium channels and they take longer to repolarise. This means that the heart is unable to have tectantic contraction and so will contract and then relax.

Question 9

How is the strength of contraction regulated?

Answer 9

By the amount of calcium channels that are open and so how much calcium is allowed to enter the cell from the outside which would depolarise the heart further leading to a faster and harder contraction.

Question 10

What is a tetanus? Why is there no tetanus from cardiac muscle?

Answer 10

In skeletal muscle when the there are consecutive action potentials that summate and can maintain/increase force exerted by the muscle, eg useful whilst holding a baby,

In cardiac muscle we don’t want tetanus as we want the heart muscle to relax to allow it to fill with blood. Because it also has calcium channels this means that they take longer to repolarise and there is a longer refractory period, meaning that the heart muscle is almost fully relaxed before the next action potential can have an effect.

Question 11

Do all cardiac muscle cells have an unstable resting potential? WHat do they act as?

Answer 11

No, not all, but those that do act as pacemaker cells.

Question 12

resting skeletal muscle, which ions are where? which channels open upon an AP, and what is the rmp?

Answer 12

Skeletal muscle - K+ inside the cell mainly, leaky K+ channels open, Na+ outside the cell, it’s channel is closed, Ca2+ outside the cell, closed channel.

To depolarise, the voltage gated Na+ channels open.

Resting membrane potential is -70mV

Question 13

in skeletal muscles, what is the main channel to depolarise a cell?

Answer 13

voltage gated sodium channels

Question 14

Discuss the action potential in a non-pacemaker cardiac muscle

Answer 14

resting potential (approx. -90mV), caused by leaky potassium channels (high relative membrane permeability).

Question 15

What keeps the plateau? And then what causes the repolarisation?

Answer 15

The calcium ion channels (L-type, L for LARGE, so lots calcium release and long lasting) and decrease in Potassium permeability (leaky potassium channels shut)

REpolarisation is caused by the Ca2+ channels shuttiing and the leaky potassium channels reopening.

The graphs from the powerpoint can be helpful if stuck (CVS 2.1)

Question 16

Discuss pacemaker action potential

Answer 16

The action potential is due to the opening of Ca2+ (L-type)

Question 17

Discuss pacemaker cells pacemaker potential (or pre-potential). What is the inerrant heart rate of the pace maker cells?

Answer 17

Gradual decrease in permeability to K+ (closing the leaky potassium channels)
The early increase in permeability of Na+ (aka PF - F for funny, because they open on repolarisation),
And lastly there is the late(r) opening of PCa2+ (T-type, T for transient, only let a little bit of Ca2+ into the cell).

These all gradually depolarise the cell to the threshold to create an action potential.

Heart rate is around 100bpm

Question 18

What type of channel is a PF channel? what is it opend by?

Answer 18

Funny Sodium. Opened early on repolarisation from the previous action potential.

Question 19

What are PCA2+ T-type gated channels?

Answer 19

Transient Ca2+ gated channels that are stimulated to open at mre hyperpolarised potentials, a late increase that finally pushes the potential over the threshold.

Question 20

What determines heart rate?

Answer 20

The rate of the fastest pace maker cell

Question 21

What is heart block? What can it be caused by?

Answer 21

Hyperkalemia. (High Potassium), this high potassium levels depolarises myocardial cells and triggers spontaneous and uncoordinated contraction (fibrillation).

Increase K+ outside cell. Reduced concentration gradient to pull K+ outside cell but bigger pull inside cell (from charges), so K+ conc increases inside cell, making inside cell more +ive and less negative and so closer to depolarise threshold.

Then also less +ive gradient for other +ive ions to enter cell, slowing down the condction of the action potential. Conduction through the AV node may fail and Atria depolarisation never reaches ventricles. This is what is known as HEART BLOCK.

Question 22

A 1 degree rise in body temp leads to what in the heart?

Answer 22

Increase in heart rate by 10bpm

Question 23

What effect does calcium channel blockers have on the rate and force of the heart?

Answer 23

So if you block the calcium channels (used for pacemaker pre potential and action potential or them), you will slow down the rate and strength of contraction of the heart.

Question 24

Where are the fastest pacemaker cells found in the heart?

Answer 24

In the sinoatrial node

Question 25

Are all pacemaker cells in the sinoatrial node?

Answer 25

Not all, but a lot and most importantly the fastest ones are.

Question 26

What is the annulus fibrosus?

Answer 26

A strip of insulating fibre between the atrium and ventricles to stop the whole heard contracting at the same time. Allows the signal to be delayed through the AV node before hitting the ventricles, giving time for the ventricles to fill with blood.

Question 27

How does the heart still spread its signal despite the Annulus fibrosus? How fast does this part conduct? Whys is this important?

Answer 27

Through the Atrioventricular node (AV node) -which can be seen as a delay box, so conducts very slowly (at 0.05m/s, 10x slower than across the atria (0.5m/s)). This is important to give the ventricles time to fill with blood from the atrium before contracting.

Question 28

Post AV node, what does the potential reach? What does this split into? How fast are they and why is this important?

Answer 28

It reaches the Bundle of His, which splits into the left and right branches which further split into purkinje fibres, which are a RAPID conduction system and conducts at 5/sec (!!!) Important so that the whole of the ventricle contracts at the same time.

Question 29

What enables ECGs to record?

Answer 29

The fact that all the cells of the atria and ventricles contract at the same time, causing mass extracellular depolarisation, which is conducted through the bodies fluid and can be detected through the skin.

Question 30

What is a P wave?

Answer 30

Representative of the atrial depolarisation

Question 31

What causes the QRS complex?

Answer 31

The depolarisation of the ventricles

Question 32

What corrosponds to the T wave?

Answer 32

The repolarisation of the ventricles

Question 33

What doesn’t the ECG tell you?

Answer 33

ECGs only tell you about the depolarisation and repolarisation of the heart, it doesn’t tell you anything with regards to how efficiently the heart is pumping or if the muscles are working fully. (the pumping ability of the heart, eg faulty valves etc)

Question 34

How long is 1 large square?

Answer 34

0.2s

Question 35

Large delay P - QRS complex indicates what?

Answer 35

1st degree heart block - something is going wrong with getting the signal down from the atria to the ventricles, most likely in the AV node delay box.

Question 36

How long should the gap be between the P wave an the QRS complex?

Answer 36

less than 0.18s (so under 1 big square)

Question 37

What is 2nd degree block?

Answer 37

When there is increasing length in time between the atrial contractions and the ventricle contractions, so after a few (often 4 atrial contractions), a ventricle contraction is skipped.

Question 38

What is 3rd degree block?

Answer 38

This is uncordinated atrial contractions and ventricle contractions, both just going to their own rhythm, ventricle usually much slower. no conduction down through the AV node.

Question 39

Blocks are disorders of what?

Answer 39

Condution

Question 40

What is Atrial flutter and how would it look on an ECG?

Answer 40

Fast atrial contractions so that the repolarisation of the ventricles is happening at the same time that the atrial is contracting. also known as supra ventricular tachycardia (SVT). (150bpm)

Question 41

What happens in Atrial fibrillation?

Answer 41

The atrial is just firing randomly (no coordinated P waves) and then the ventricle also will contract occasionally too when the depolarisation gets through the AV node, but not a steady ventricular contraction.

Question 42

Why is ventricular fibrillation lethal and how does the defib work?