Electrical conduction of the heart Flashcards
Explain the initiation and conduction of the electrical impulse
1) Action potential (AP) initiated in the SA node (controlled by brain and blood), propagates to the AV node via internodal pathways in the atria.
2) The cells of the AV node transmit AP more slowly & delay the impulse.
3) the impulse spreads down to the ventricles along the bundle of his
4) The AV bundle divides into left and right bundle branches
5) Impulses spread to the contractile cells of the ventricle through purkinje fibres.
What is a functional synctium?
Cardiac muscles are so tightly bound that when one of these cells become excited, the action potential spreads to all of them, spreading from cell to cell.
What allows the spread of AP throughout the myocardium?
The gap junctions in the intercalated discs
What is the myocardium?
Muscular wall of the heart
Describe depolarisation
In the surface membrane of a cell there are protein carriers.
These actively pump Na+ (Sodium) ions out of the cytoplasm to the outside of the cell. At the same time, K+ (Potassium) ions are pumped from the outside in.
3 Na+ out & 2 K+ in
This means there is a net movement of positive ions out of the cell making the inside of the cell negatively charged, relative to the outside.
This charge is the resting potential of the cell and is about -70mV.
When a receptor is stimulated, it will create a positive environment inside the cell.
This is caused by a change in the concentrations of Na+ and K+ ions in the cell and happens in a number of steps:
There is a change in permeability (the ability of the cell membrane to let ions through it) to Na+ and K+ in the cell surface membrane at the area of stimulation, which causes Na+ channels in that area to open.
Na+ therefore floods into the cytoplasm down the concentration gradient.
As this happens the membrane depolarises (this means that the resting potential of the cells starts to decrease). If this depolarisation reaches a certain level, called the threshold level (about -55 to -50 mV), then an action potential has been generated and an impulse will be fired. If it does not reach this level, nothing will happen.
Once +40mV is reached the Na+ channels close and K+ channels open. K+ floods out of the cytoplasm so that the overall charge inside goes back down. This stage is called repolarisation.
Describe PQRST wave
P - atrial contraction
QRS - contraction of ventricles
T - relaxation of the ventricles
What does tetanic mean?
Sustained muscle contractions evolked when the motor nerve that innervates a skeletal muscle emits action potentials at a very high rate.
What is sinus arrythmia?
Normal variation (e.g increase) in heart rate during inspiration.
What is the variation during inspiration and expiration?
TIBE: Tachycardia - inspiration
Bradycardia - expiration
What are ventricular ectopic beats?
Most common: premature - causing ventricular contraction before the underlying rhythm would depolarise the ventricle
Which one of these is not compatible with life?
1) Atrial or 2) ventricular fibrillation
Ventricular fibrillation
