Electrophysiology Flashcards
Descirbe the conductive system of the heart
(how the wave of depolarisation flows through heart)
- Spontaneous depolarisation of sinoatrial node (SA node) located at the base of the superior vena cava
- Electrical impulse (AP) spreads through both atriavia interatrial braches and to atrioventricular (AV) node via internodal pathways
- AV node located in the interatrial septum
- There is a delay at the AV node. This allows the atria to fully contract before the ventricles contract
- The wave of depolarisation then travels from the AV node -> bundle of His -> left and right bundle branches (L faster than R)
- Reaches apex of heart -> up ventricular wall via Purkinje fibres (apex to base; endocardium to epicardium)
What is the function of conductile myocytes?
Conduct the AP through the heart
What is the special characteristic of conductile myocytes?
Automaticity
i.e. ability to spontaneously depolarise without input neural from the brain
What are the rates of depolarisation of the major groups of condactile myocytes of the conductive system?
- SA node: 60-100 AP/min
- AV node: 40-60 AP/min
- Purkinje fibres: 15-40 AP/min
What would happen if the pacemaker cells are damaged?
The next fastest group of conductile myocytes will takeover as the pacemaker
This will be slower and may result in an abnormal sequence of contraction
How is the resting membrane potential (RMP) of a conductile myocyte different to a contractile myocyte?
In a contractile myocyte, the RMP is constant. Depolarisation only occurs once it recieves an AP
In a conductile myocyte, the RMP is not constant. Slow voltage gated Na+ channels (funny channels) result in a slow depolarisation - this is known as the pacemaker potential.
Describe the depolarisation of conductile myocytes?
What are the K1 channels?
Special type of potassium channel that is active at very negative voltages which maintains the very negative membrane potential (-87mV) of contractile myocytes
Describe the depolarisation of contractile myocytes?
- Phase 0: Depolarisation - fast voltage gated sodium channels
- Phase 1: Notch - period of K+ channel dominance
- Phase 2: Plateua - voltage gated potassium channels and L type calcium channels opening. Increased Ca2+ → increased contraction
- Phase 3: Repolarisation - voltage gated K+ currents
- Phase 4: RMP - Maintained by K1 channels, Na+/K+ ATPase, Ca2+ ATPase, sarcoendoplasmic reticulum ATPase (SERCA)
When does the Absolute refractory period occur in contractile myocytes?
- This occurs during the activation period (phase 0)
- All the voltage gated sodium channels are open(both activation and inactivation gates open)
- The cell becomes positive enough to repel the activation gate, thus opening it
- This also occurs during the inactivation period (phase 1,2 and first part of 3)
- Positive charge in the membrane repels the positively charged innactivation gate → Inactivation gates closes→ prevents further Na+ current through the channel despite the membrane potential being depolarised enough to trigger the activation of the channel → meaning there is no way you can get another action potential.
When does the relative refractory period occur?
This occurs during the “recovery from inactivation” period
- Period when K+ channels have opened and membrane potential is negative enough to begin to open the positively charged inactivation gates
- At the same time the activation gate closes
- This also occurs during the deactivationperiod
- Period when the membrane potential is lower than -60mV causing the activation gates to close ( the inactivation gates stay open)
- The RRP starts mid to late stage 3 to the start of 4
What is chromotropy?
Heart rate
What is dromotropy?
Conduction velocity
What is ionotropy?
Strength of contraction
What is luisotropy?
Speed of relaxation
