Electrical System Flashcards
‘Intracellular charge is more negative than outside the cell’ - describes what potential state?
Resting membrane potential
‘Intracellular charge is less negative than outside the cell’ - describes what potential state?
Depolarisation
‘Ion exchange causing intracellular charge to reduce’ -describes what potential state?
Repolarisation
Describe phase 0 of AP (Depolarisation)
Rapid Sodium influx
Slow Calcium influx
Intracellular charge changes from - to + and triggers myocardial contraction
Describe phase 1 of AP
Sodium influx blocked
Slow Calcium influx continues
Intracellular charge becomes more negative
Describe phase 2 of AP (plateau phase)
Sodium influx still blocked
Slow Calcium influx continues
Potassium efflux starts
Calcium influx is equal to Potassium efflux
Cell supersaturated with IONS - No Stim possible regardless of strength
Describe phase 3 of AP (Rapid repolarisation)
Sodium influx blocked
Calcium influx blocked
Rapid Potassium efflux
These conditions lower intracellular voltage. Stim possible but only with high voltage
Described phase 4 of AP (Resting phase)
Sodium efflux via pump
Potassium influx via pump
Concentrations returned to pre-depolarisation levels. Ready for Stim.
What is the normal range for the SA node at rest
60-100 bpm
Define automaticity
Ability to initiate an impulse
Define excitability
Ability to respond to an impulse
Define conductivity
Ability to transmit an impulse
Where is the SA node located?
In the Right Atrium where the SVC enters
What are the 3 internodal tracts in the RA?
Anterior
Middle
Posterior
How does the impulse travel from the RA to the LA
Bachmann’s bundle - near simultaneous contraction
Where is the AV node located?
Inferior RA near the Cs Os
How long does the AV node delay the impulse?
0.04ms
Why does the AV node delay the impulse?
To allow ventricular filling from atrial contraction
Describe the bundle of HIS
Transports the impulse from the AV node. Bifurcates into the left and right bundle branches
What is the firing rate of the bundle of HIS?
40-60bpm
Which bundle branch is thicker and why?
The left due to thicker LV wall
Which bundle branch is faster and why?
The left is faster as more muscle mass must be stimulated in the same amount of time, otherwise contraction between LV/RV will not be synchronous.
Describe Purkinje fibres
Final part of the conduction system. They deliver the impulse from the endocardium to the ventricular myocardium and epicardium.
What is the pacemaker speed of Purkinje Fibres?
15-40bpm
