Cardiovascular: The Excitation of Heart L7

Question 1

What are some structural differences between electrical and contractile cells.

Answer 1

Electrical cells make up about 1% of the heart. They are contractile cells that are specialised for generating and conducting electrical signals quickly. These cells have actin and myosin like contractile cells, but in smaller amounts. Electrical cells thus look paler and less striped than contractile cells if you look at them under a microscope. Contractile cells on the other hand, are striated and dense in actin and myosin for creating a lot of force in the form of heart beats.

Question 2

How do electrical cells influence other cells in heart contraction?

Answer 2

Electrical cells generate action potentials. These can be passed on to other electrical cells via gap junctions (Pores with low resistance to ionic current) much like electricity being passed along a wire. 
Action potentials can also be passed on from electrical cells to contractile cells (also via gap junctions), stimulating contraction. 
Gap junctions (within intercalated discs) also link contractile cells, so when an action potential is initiated in one cell, it can be passed on to other contractile cells in the heart. This is how left and right pumps of heart contract in unison.

Question 3

Where do you find the SAN? What is the function of the sinoatrial node? (SAN)

Answer 3

Right atrium near the superior vena cava. SAN has the intrinsic ability to cause conduction by themselves. This is why it is informally known as the pacemaker (it will still function without stimulation from the brain, just that the brain only controls increase/decrease of heart rate). Conduction cells will then propagate signals along specific pathways.

Question 4

From the sinoatrial node, what does it conduct and innervate first?

Answer 4

The SAN in the RA goes down a conduction pathway to the interatrial bundle where both the LA and the RA contract simultaneously.

Question 5

The SAN conducts a pathway that is delayed, what is this pathway, why does it delay, and what does the pathway innervate?

Answer 5

The SAN node conducts down to the atrioventricular node which has a delay. The delay is to allow the interatrial bundle conduction pathway to innervate and contract the atria first, so that all the blood goes into the ventricles firstly, then the ventricles can contract from the AV node’s innervation.

Question 6

What is the conduction pathway following from the AV node?

How does flow upwards in the ventricles to go to the aorta and pulmonary trunk?

Answer 6

After the AV node conducts, it passes to the Left and Right bundles of AV bundles, down the interventricular septum, and then up the wall of the heart via Purkinje fibres back to the top. This is to push blood upwards from ventricles where aorta and pulmonary trunk are.

Question 7

Explain the excitation and conduction pathway steps.

Answer 7

1. Quiescence (neither depolar or repolar)
2. SAN pulse excitation of intra and inter atrial conduction pathway. Start of atrial depolarisation.
3. Complete atrial depolarisation.
4. AVN pulse excitation of AV bundles and Purkinje Fibres
5. Atrial Repolarisation and Ventricular depolarisation
6. Start of ventricular repolarisation
7. Complete ventricular repolarisation.
8. Quiescence.

Question 8

In the ECG trace, what occurs at the P wave?

Answer 8

SAN wave + Atrial depolarisation + atrial contraction. This tops up the blood into the ventricles from passive filling, causing muscles to stretch. Priming of the ventricles for stronger contraction, push blood into ventricles.

Question 9

In the ECG trace, what occurs at the QRS complex?

Answer 9

Ventricular depolarisation + ventricular contraction, atrial repolarisation. Rise in ventricular pressure (Isovolumetric ventricular contraction), ejection of blood (when pressure in ventricles exceeds aorta). Fall in ventricular volume. Rise in aortic pressure.

Question 10

In the ECG trace, what occurs at the T wave?

Answer 10

Ventricular repolarisation + relaxation. Decrease in ventricular pressure (Isovolumetric ventricular relaxation). AV valves open, passive filling occurs.

Question 11

What causes the lubb dubb sounds?

Answer 11

Valves snapping shut = heart sound. Lubb= AV. Dubb = Semilunar. Each sound is just after QRS and T waves respectively.