The Excitable Heart

Question 1

What is the difference between the electrical and contractile cells of the heart?

Answer 1

Electrical cells make up 1% of the hearts cells, they are ‘pale’ and are not very striated (because they have low acid and myosin.
Compared to the contractile cells which make up 99% of the heart and have a striated appearance and high actin and myosin content.

Question 2

Describe how action potentials propagate along the surface membrane of electrical and contractile cells

Answer 2

• Depolarisation starts at the sinoatrial node (SAN)
• the signal spreads to neighbouring cells through gap junctions in the intercalated disks between cells.
• in a contractile cell: increased amounts of Ca2+ levels mean more cross bridge attachment and contraction

Question 3

What connects cardiac cells?

Answer 3

Intercalated disks connect most of the cells of the heart.
They contain gap junctions:
- pores with low resistance to ionic current
- allow current flow between adjacent cells

Question 4

Describe gap junctions and the spreading of impulse

Answer 4

• Begins at SAN
• conduction cells conduct the signal rapidly, but they also send the signal through gap junctions to the contractile cells too - which spread to contractile cells connected to them by the spreading of Ca2+.
• Increased speed of the impulse throughout the heart
• millions of cardiac cells behave as one - so the heart is therefore called a functional syncytium.

Question 5

Describe the path of the conduction pathway of the heart

Answer 5

1. SA node: it is the pacemaker of the heart and sends signals by itself. These signals go in three directions (right atrium, left atrium (via the interartrial bundle) and the ventricles (via the internodal bundles, AV node, AV bundle, bundle branches, and then purkinje fibres).
   - interatrial bundle is like a bridge that sends the signal from the SA node to the left atrium.
2. Internodal bundles: Signal from the SA node comes through these bundles and then collects at the AV node.
3. AV node: the atrioventricular node collects the signal from the internodal bundles and then pauses It for a while, allowing the atria to contract and push blood into the ventricles. once the atria start to relax, then the AV node will release the signal again.
4. AV bundle & bundle branches: transmit the signal down through the inter-ventricular septum and then into the purkinje fibres
5. Purkinje fibres: conduct the signal up into the ventricular walls on either side.
   The reason why the signal is transmitted to the base of the heart is so that the contraction starts from here to allow the ventricles to squeeze as much blood from the heart as possible like a toothpaste tube.

Question 6

Describe the excitation and conduction pathway of the heart

Answer 6

1. Quiescence ends when excitation spreads from the SA node
2. The atria are fully depolarised and contract
3. Atria repolarise and relax, while AV node sends excitation to ventricles
4. Vetricles fully depolarises and contract
5. Ventricles begin to repolarise and relax
6. Ventricles fully repolarised and relaxed, heart is back to quiescence

Question 7

What does quiescence mean?

Answer 7

When the heart is fully relaxed and there are no signals being sent (shown as fully grey on diagram)

Question 8

Describe an Electrocardiogram (ECG)

Answer 8

• ‘Lead’ - virtual line between two surface electrodes.
• A single lead detects a difference between electrodes
  
  • depolarisation and repolarisation
    There are three waves (the P wave, QRS complex, and T wave).
• ECG measures the change in electrical signals (NOT the voltage)

Question 9

Describe the phases seen on an ECG in relation to the hearts excitation and conduction pathway

Answer 9

1. Atrial depolarisation, initiated by the SA node - causes the P wave (because it picks up the electrical impulse from the SA node)
2. With atrial depolarisation complete, the impulse is delayed at the AV node - we see a flat line because no electrical signals (and so the line flattens out to baseline again)
3. Ventricular depolarisation begins at apex, causing the QRS complex (lots of electrical signalling occurring so big spike) Atrial repolarisation occurs.
4. Ventricular depolarisation is complete (line flattens out again as no electrical signals and it is at baseline again)
5. Ventricular repolarisation begins at apex, causing T wave - which is slightly larger than P wave
6. Ventricular repolarisation is complete, and impulse line flattens out again to baseline.

Question 10

Briefly describe the pressure changes of the heart on that graph thing

Answer 10

• Pressure increases when the heart is in systole (so when it is contracting the atria or the ventricles).
• There is a massive jump up in the graph during isovolumetric contraction phase because it’s the same volume of blood and the valves are closed.
• Biggest heart pressure in vascular ejection, and then massive drop in pressure in isovolumetic ventricular relaxation phase.