what are the main components of a ventricular myocyte action potential
- resting membrane Potential is typically -90 mV (close to Ek)
- AP is initiated when wave of depolarization enters cells
- AP is divided into four phases (0-4)
- long AP duration (200msec)
what are the main channels involved in generating the ventricular action potential
- voltage gated sodium channels
- voltage gated calcium channels
- different types of voltage gated potassium channels
explain the ion channel kinetics of ventricular myocyte action potentials
- AP shape is due to time dependent opening and closing of different voltage gated ion channels
- all of the voltage gated ion channels involved open in response to the initial membrane depolarization
-> differences in kinetics underlie the AP shape (some faster or slower to Inactivate/activate) - Na+ channels are very similar in function to those seen in neurons
-> fast activating
-> Fast inactivating
-> membrane must re polarize in order for them to reset (refractory period) - Ca2+ and K+ channels also open upon membrane depolarization but their activation kinetics are slower
what are the phases of ventricular myocyte action potentials and what happens at each phase
phases
0 - rapid upstroke/depolarization
-> sodium channels open, sodium flows in
1 - notch
-> Na+ channels inactivate, fast K+ channels open (Na+ flow stops, K+ flows out briefly)
2 - plateau
-> fast K+ channels close and slower K+ channels open, Ca2+ channels open (K+ flows out, Ca2+ flows in)
3 - rapid repolarization
-> Ca2+ channels inactivate, slow K+ channels open (Ca2+ flow stops, K+ flows out)
4 - rest
-> slow K+ channels close (K+ out through leak channels, Na+-K+ ATPase)
explain the refractory periods in ventricular contractile cells
- Na channels closed
Absolute refractory period
-> Na channels open
-> most Na channels inactivated
relative refractory period
-> some Na channels inactivated
why are ventricular myocyte action potentials longer than any other action potentials
- they have 2 depolarizing channels while the others only have 1
- Na+ = heavy lifting
- Ca2+ = continues the depolarization
what is the purpose of the long cardiac AP
- prevents summation and tetanus
- in cardiac muscle fiber the refractory period lasts almost as long as the entire muscle twitch
- long refractory period prevents tetanus
- summation is not possible because we need the heart to contract and relax rhythmically to pump blood
- in skeletal muscle fast twitch the refractory period is very short compared with the amount of time required for the development of tension
explain how pacemaker APs are driven by funny channels
- pacemaker potential gradually becomes less negative until it reaches threshold triggering an action potential
ion movements - pacemaker potential -> net Na+ in (If channels open)
- threshold (AP) -> Ca2+ in
- no stable resting membrane potential
-> pacemaker potential between APs due to If current through funny channels - hyperpolarization activated cyclic nucleotide gated (HCN) channels
- > conduct both Na+ and K+
- net Na+ in leads to depolarization
explain electrical conduction in myocardial cells
- autorhythmic cells spontaneously fire action potentials
- depolarizations of the autorhythmic cells then spread rapidly to the adjacent contractile cells through gap junctions (using connexins)
- action potentials rhythmically initiated in SA node cells set the pace of the heart
explain the conducting system of the heart
- SA note lies at the junction of superior vena cava with the right atrium
- the internodal pathways spread across both Atria
- AV node lies in the interatrial septum (electrical barrier)
-> only pathway for electrical conduction across connective tissue between Atria and ventricles
what is the ventricular conduction system pathway
AV node -> the common AV bundle (bundle of his) -> divides into left and right bundles -> the two bundles give rise to many strands of Purkinje fibers
explain how electrical conduction is initiated and focused by nodal cells (SA and AV node)
SA node
-> Set the pace of heartbeat at 70 bpm
AV node
-> routes the direction of electrical signals
->delays the transmission of action potentials
-> AV node can act as the pacemaker under some pathological conditions (fires at 50 bpm)
-> purkinje fibers also have pacemaker properties (35bpm)
what is heart rate set by
- set by the fastest pacemaker cells in the circuit
what is the pathway of excitation through the heart
- SA node depolarizes
- electrical activity goes rapidly to AV node via internodal pathways
- depolarization spreads more slowly across atria, conduction slows through AV node (signal is focused and routed through AV node)
- depolarization moves rapidly through ventricular conducting system to the apex of the heart
- depolarization wave spreads upward from the apex
how does the heart go from action potentials to contraction
-> action potentials are continually and spontaneously initiated in the SA node
-> action potential spread through electrical conducting system of the heart
-> all action potentials pass across atria and ventricles, depolarization spreads across myocytes
-> action potentials depolarized myocyte membrane
-> myocyte contracts
what is Einthoven’s triangle and the electrocardiogram
- An ECG represents the summed electrical activity of all cells, recorded from the surface of the body
-> predominantly measures speed of depolarization through heart muscle
-> measured from one lead at a time - lead l = left arm to right arm
- lead ll = left leg to right arm
- lead lll = left leg to left arm
(pos -> neg)
how does the direction of deflection of the ECG indicate the relationship between the direction of the vector of the electrical current flow and the axis of the lead
- upward deflection on an ECG means the current flow vector is toward the positive electrode
- downward deflection on an ECG means the current flow vector is toward the negative electrode
- a vector that is perpendicular to the axis of the electrode causes no deflection
define and describe P-wave, QRS complex, T-wave
P wave = atrial depolarization
QRS complex = atrial repolarization and ventricular depolarization
T wave = ventricular repolarization
explain the electrical events of the cardiac cycle
- P wave = atrial depolarization
- PQ or PR segments = conduction through AV node and AV bundle, Atria contract
- Q wave
- R wave
- S wave
- ST Segment, ventricles contract
- T Wave, Ventricular repolarization
what are the 4 common arrhythmias (irregular heart rate)
tachycardia = HR is faster than normal
bradycardia = HR is slower than normal
- fibrillation = electrocardiogram is disorganized
atrial fibrillation = heart still functions as a pump
ventricular fibrillation = heart does not function as an effective pump
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