The conducting system Flashcards
Describe the conducting system of the heart
- sinoatrial (SA) node
- atrioventricular (AV) node
- right and left bundles of His.
- The left bundle divides into anterior and posterior fascicles.
The conducting system runs within the endocardium
The atrial muscle fibres are separated from those of the ventricles by a fibrous tissue ring, and normally the only conducting tissue between the atria and ventricles is the bundle of His
Describe the SA node (location, connection to AV node, function)
Located at the junciton of the SVC and the right atrium
Three bundles of atrial fibres contain Purkinje type fibres and connect the SA node to the AV node
* Anterior internodal tract of Bachman
* Middle internodal tract of Wenckebach
* Posterior internodal tract of Thorel
Normally the only pacemaker in control of the heart rate. Has the fastest discharge rate (70-80) so dominates over other slower subsidiary pacemakers
What are the inherent discharge rates of the cardiac pacemakers (4)
SAN: 70-80
AVN: 60
Bundle of His: 50
Ventricles (purkinje cells): 30-40
Describe the AV node and bundle of His
The AV node is in the right posterior portion of the interatrial septum and is continuous with the bundle of His
Bundle of His
* gives off a left bundle branch at the top of the interventricular septum and continues as the right bundle branch
* left bundle branch divides into anterior fascicle and posterior fascicle
* branches and fascicles run subendocardially down either side of the septum and come into contact wiith the Purkinje system
* Purkinje fibres spread to all parts of the ventricular myocardium
Describe the microscopic anatomy of the conducting system (and how the SA / AV nodes differ)
Mainly composed of modified cardiac muscle that has fewer striations and indistinct boundries
SA and AV nodes also contain P cells:
* Small round cells with few organelles which are connected by gap junctions
* Function as pacemaker cells
Describe the nerve supply to the conducting system of the heart
- SA node develops from the right side of the embryo, and the right vagus nerve is distributed to the SA node
- The AV node develops from the left side and left vagus
- Vagal fibres are endocardial
- The sympathetic innervation is similar. Most fibres from from the stellate ganglion. Cholinergic and noradrinergic fibres are epicardial
*
What are the five phases of the cardiac action potential (in a Purkinje cell)
0: Rapid membrane depolarization due to rapid influx of sodium into the cell.
1: Early repolarization.
2: Plateau, prolonging action potential due to slow inward sodium and calcium movement.
3: Rapid repolarization due to outwardly directed potassium currents.
4: Gradual reduction in trans-membrane voltage until the threshold potential for depolarization (phase 0) is reached.
Describe the action potential in a cardiac pacemaker cell
Monophasic action potential 3 phases:
* 4: Pacemaker potential: The resting potential (-60 mV) is caused by continuous leakage of potassium. The permeability decreases with time causing slow depolarization until the threshold potential (-40 mV) is reached.
* 0: Upstroke: When the threshold is reached calcium is let into the cell by voltage-sensitive calcium channels which open and depolarization occurs.
* 3: Repolarization: The calcium channels are rapidly inactivated soon after they open and phase 3 occurs when potassium permeability is increased, potassium leaves the cell and repolarization occurs back to the resting state.
What do the waves on the ECG represent?
p wave: electrical activity of atrial
QRS: electrical activity of ventricles
T wave; return of heart to its resting state
What are the three main requirements for effective heart pumping?
- **Substantial atrial to ventricular delay **(simultaneous contraction would lead to inefficiency and backflow. Achieved by electrical isolation of atria from the ventricles)
- **Co-ordinated contraction of ventricular cells **(contraction begins at the apex of the heart - conduction reaches here via the Bundle of His, so blood is squeezed towards the exit)
- Absence of tetany (After contraction there is a period of relaxation, in order that the heart fills up with blood again. Temporary inactivation of certain ion channels allows relaxation.)
What conduction abnormalities may lead to arrythmias?
- Altered automaticity (Depressed activity of the SA node may permit escape rhythms to occur or rapid ectopic pacemaker sites may arise elsewhere and suppress the SA node.)
- Re-entry (occurs when the propagating impulse does not die out after complete activation of the heart but persists to re-excite it after the end of the refractory period.)
What is the resting membrane potential of a
* myocardial cell
* cardiac pacemaker cell
Myocardial cell: -60mV
Cardiac pacemaker: -60mV
