What is the function of myocardial cells? (2)
- Initiation and conduction of electrical impulses
* Contraction (myocytes)
The conducting system of the heart are specifically designed myocardial cells. What are they lacking compared to myocytes?
• Myofibril components
What is the process by which electrical cells conduct a signal to myocytes causing contraction called?
• excitation-contraction coupling
What factors modify the rate of electrical impulse generation and the force of myocardial contraction? (2)
- Autonomic input
* Stretch
Describe the coronary arterial system.
- Consistes of the right and left coronary arteries.
- They branch from the aorta immediately above two cusps of the aortic valve.
- Right coronary artery gives off the posterior descending coronary artery and the marginal branch.
- Left coronary artery divides into left anterior descending artery and the circumflex artery.
Where does the SAN lie?
•In the lateral and epicardial aspects of the junction between the SVC and the right atrium.
Why is the SAN the natural pacemaker of the heart?
• It generates impulses automatically by spontaneous depolarisation of its membrane at a rate quicker than any other cardiac cell type.
What modulates the rate of depolarisation of the SAN? (6)
- Autonomic tone
- Stretch
- Temperature
- Hypoxia
- Blood pH
- Hormonal influences (e.g. tri-iodothyronine and serotonin)
Describe the action potential process of atrial and ventricular myocytes.
- Resting membrane potential is due to small inflow of potassium ions through open ‘inward rectifier’ channels. Sodium and calcium channels are closed.
- Arrival of adjacent action potentials opens voltage-gated, fast, self-inactivating sodium channels, resulting in a sharp depolarisation spike.
- Followed by partial repolarisation due to activation of ‘transient outward’ potassium channels.
- Plateau phase (unique to myocytes) due to small, but sustained, inward calcium current through L-type calcium channels lasting 200-400ms. This plateau phase prevents early reactivation of myocytes and directly determines the strength of contraction.
- The gradual inactivation of the calcium channels activates delayed rectifier potassium channels, depolarising the membrane.
Where does the AV node lie?
• Beneath the right atrial endocardium within the lower intertribal septum.
Describe the conduction beyond the AV node.
- The AV nodes continues to the His bundle which conducts the cardiac impulse towards the ventricle.
- The His bundle divides into the right bundle branch and the main left bundle branch at the crest of the inter ventricular septum.
- The right bundle branch continues down the right side of the inter ventricular septum to the apex; from here, it radiates and divides to form the Purkinje network which spreads through the right ventricle.
- The main left bundle branch fans out into the anterior hemi-bundle (supplies anterior and superior surfaces) and the posterior hemi-bundle (supplies inferior and posterior surfaces).
Describe Starling’s law of the heart.
• Stroke volume increases in response to an increase in end diastolic volume.
(As a larger volume of blood flows into the ventricle, the blood will stretch the walls of the heart, causing a greater expansion during diastole, which in turn increases the force of contraction and thus the quantity of blood that is pumped into the aorta during systole)
What is the nerve supply of the myocardium?
- Adrenergic nerves supply atrial and ventricular muscle fibres, as well as the conduction system. Beta1-receptors predominate in the heart with both adrenaline and noradrenaline having positive inotropic and chronotropic effects.
- Cholinergic nerves from the vagus supply mainly the SA and AV nodes via M2 muscarinic receptors.
- Under basal conditions, vagal inhibitory effects predominate over the sympathetic excitatory effects, resulting in a slow heart rate.
When do the right and left coronary arteries fill?
•They fill during diastole when not occluded by valve cusps.
Describe the course of the right coronary artery.
- Arises from the right coronary sinus and courses through the right side of the AV groove.
- Gives off the marginal branch
- Continues as the posterior descending coronary artery, which runs in the posterior inter ventricular groove and supplies the posterior part of the inter ventricular septum and the posterior left ventricular wall.
Describe the course of the right coronary artery.
- Arises from the lest coronary sinus
- QUICKLY (2.5cm) divides into the left anterior descending artery and the circumflex artery.
- LAD runs in the anterior inter ventricular groove and supplies the anterior septum and the anterior left ventricular wall.
- The left circumflex artery runs in the left AV groove and gives off marginal branches.
What supplies the SA node and the AV node?
•The right coronary artery in 60% and 90% of people, respectively.
Briefly describe the coronary venous system.
- The majority of blood returns by veins that accompany the arteries, to empty in the right atrium via the coronary sinus.
- Some blood from the capillary beds in the wall of the heart drain directly into the cavities of the heart via tiny veins
Briefly describe the coronary lymphatic system.
• An extensive lymphatic system drains into vessels that travel along the coronary vessels and then into the thoracic duct.
