Garman- CV2: Electrical Flashcards
What are 2 types of cell types of the heart?
Contractile cells and conductile cells
What are the contractile cells?
Bulk of atrial and ventricular tissues
-Work horses of heart
What are the conductile cells
Specialized cardiomyocytes
- sole puprose is to generate and propagate electrical activity across contractile cells
Found in:
- SA node
- Atrial internodal tracts
- AV node
- Bundle of His
- Bundle branhces (L and R)
- Purkinje Fibers
What are some characteristics of cardiomyocytes?
-sTRIATED
-50-100 uM long; diameter ~20 uM (shorter and thinner than skeletal muscle cells)
- Branched at its ends
Very small
-Mono/bi-nucleated centrally located (skeletal muscles multi-nucleated and peripherally located nuclei)
- Reduced SR system but extensive T tubule system
-Large/numerous mitochondria
What are intercalated disks?
Consist of desmosomes- mechanical coupling, gap junctions- electrical coupling
What is purpose of desmosomes?
Make sure cardiomyocytes do not tear apart at opposing plasma membranes. Keeps cells tightly bound together
What innervates cardiac muscle?
Autonomic nervous system (Brain tells heart to increase or decrease rate, or increase contractility)
What is source of Ca for cell?
SR and ECF
What causes removal of Ca in cardiomyocte?
Ca ATPase pumps (membrane and SR) AND Na/Ca exchanger (3Na/1 Ca)
When do atria contract to “top off” filling of ventricles?
End diastole
When are AV valves open and semilunar valves closed?
Passive diastole
When are all 4 valves closed?
Isovolumetric contraction and relaxation
When are semilunar valves open?
Systole
How long does propagation of signal to AV node take?
50 mseconds
What is the delay at AV node?
100 msec, total time 150 mseconds
How long does impulse take down bundle branch and purkinje fibers via moderator band?
175 msec elapsed time
How long does the impulse take to go throughout ventricular myocardium and begin contraction?
225 msec
What is normal spontaneous firing for SA node?
100 /min
What is the atrial internodal pathway?
- Specialized conducting cells ~50 msec
- Stimulus passed to contractile cells which spread it across both atria
- Stops at atria- myocardium of atria is not connected with ventricle
What makes up AV node:
- Smaller cells/ slows signal
- 100 msec to move through AV node
What is normal firing of AV node?
~40/min
What is overdrive supression?
Faster firing of SA node supresses other cells from acting as a pacemaker
What is the only electrical connection between atria and ventricle?
AV bundle or bundle of His
Which bundle branch is bigger?
Left
What are the purkinje fibers?
- larger cells
- fast conduction system
- move upward from apex to base to push blood upward
- Normal firing frequency 15-20/min
What ions are responsible for depolarization of SA node cells?
“funny” current for Na (trickle of Na INTO the cells) causes small upslope at baseline
- Calcium is main ion that flows into cell causing quick depolarization
What ion is mainly responsible for depolarization in ventricular myocytes?
Na
What is RMP of SA node cell?
-65 mV
What is RMP of ventricular myocytes?
-75/-80 mV
What is relative speed of conduction through heart?
Fast from SA to atria, slow through AV node, fast down purkinje fibers.
“Fast, slow, fast”
Do ventricular myocytes fire multiple action potentials at one time like skeletal muscle?
No, one and done so that heart can relax and refill in diastole
What happens in phase 0 of contractile cell?
Depolarization
- Quick opening of VG Na channel
- Na influx
- T-type VGCC open- minor Ca influx (Transient-type channels)
- Closing of K channels (inward rectifiers only)
- Voltage gated K not open yet
What are the inward rectifier K channels?
These channels CLOSE whenever cell is depolarized. This stops K from flowing out of cell and allows action potential to occur.
What happens during phase 1 of AP of contractile cell?
Early repolarization
- Na channels close
- T-type VGCC close
- K efflux through transient outward channels
- L-type VGCC not fully open yet
- Na/Ca reversal
- Small repolarization caused by positive charge leaving cell via K efflux and Na/Ca reversal
What is Na/Ca reversal?
- These utilize the Na/Ca exchanger which normally brings 3 Na+ into cell and 1 (2+) Ca out of cell.
- These channels are always open
- During depolarization, based on [Na], [Ca], and membrane potential, these channels SWITCH the movement of ions
- During reversal, 3 Na pumped OUT of cell with 1 Ca (2+ charge) pumped in
What happens during phase 2 of contractile cell AP?
- L-type VGCC OPEN!!
- Calcium influx (this regulates height of plateau)
- K channels (Ks and Kr) partially open, some K efflux
- Vm near reversal potential of Na/Ca exchanger
What happens during phase 3 of contractile cell AP?
- L type VGCC cloase
- K (Ks, Kr, and Ki) all fully open
- efflux in K causing rapid repolarization (inward rectifier open causing K to rush out)
- Influc of Na and efflux of Ca through Na/Ca exchanger
How does absolute refractory period of heart compare to skeletal muscle?
- Much longer absolute refractory period in cardiac muscle compared to nerve/skeletal muscle
- Limits frequency of AP
- This allows a built-in safety mechanism and prevents tetanic contractions and ectopic pacemakers from stimulating contraction
Longer absolute refractory period allows ventricle to ____
Fill
How long is the absolute refractory period in heart muscle?
200 msec
How does Ca activate excitation-contraction coupling in cardiomyocyte?
- Ca enters cell through L-type Ca Channels
- This Ca binds to ryanodine receptors on SR and stimulates release of Ca from SR
causes Calcium induced calcium release
How are the L-type Ca channel and ryanodine receptors different in cardiac muscle?
They are not connected.
How is Ca removed in cardiomyocyte?
- Ca pumped back into SR via SERCA pump (Ca-atp pump into SR)
- Ca also extruded to ECF with Na/Ca exchanger
Are all L-type channels the same in the body?
No, L-type in cardiac myocytes do not connect to ryanodine receptors, therefore meds can just target cardiac muscle (i.e. CCB)
What happens during phase 4 of cardiomyocyte action potential (contractile cell)
Diastole
- K (Ki) channels remain open- near nernst potential
- All other channels are closed
What happens during phase 4 of SA node AP?
Pacemaker potential
- Na channels open- funny current (influx)
- Voltage gated K channels closed– upward drift of membrane potential from funny currents
- T-type VGCC opens mid-phase
- Slow influx of Ca, slow depolarization
What happens during phase 0 of SA node AP?
Depolarization
- T-type VGCC closes
- L types VGCC opens
- Large influx of Ca and rapid depolarization
What happens in phase 3 of SA node AP?
Repolarization
- L type VGCC closes
- Influx of Ca stops
- VG K channels open
- Efflux of K
Are there phase 1 or 2 in SA node?
No
What is ECF concentration Na?
140 mEq/L
What is ICF concentration Na?
14 mEq/L
What is ECF concentration K?
4.5 mEq/L
What is ICF concentration K?
120 mEq/L
What is ECF concentration Ca?
2.5 mEq/L
What is ICF concentration Ca?
0.0001 mEq/L
How does sympathetic regulation (Beta 1 receptors/Norepi) influence HR?
- Positive chronotropic effect from increased firing rate of SA node
- This stimulation causes opening of Na and Ca ion channels, causing influx of Na and Ca and increases the steepness of pacemaker potential
- Cell has reduced repolarizaiton, and is therefore able to meet threshold for AP quicker causing increased HR
How does parasympathetic regulation affect heart rate?
- Negative chronotropic effect form decreasing firing rate of SA node/HR
Muscarinic/Ach receptors cause opening of K channels (via increased conductance for K)
- This causes efflux of K
- This hyperpolarizes cell and decreases steepness of pacemaker potential, taking MORE time to reach threshold causing AP
What is ERP in ventricular myocytes?
Effective refractory period- the time frame where another AP cannot be elicited
What is the RRP in ventricular myocyte?
Relative refractory period- more difficult to elicit and AP than during phase 4
What causes a noticeable spike on EKG?
- More mass, “noiser”= bigger spike
- In uniform, more noticeable (uniform P wave in SR vs no uniform p wave in afib)
What does P wave represent? Normal duration?
Atrial depolarization
0.08-0.10 sec
What does QRS complex represent? Normal duration?
Ventricular depolarization
0.06-0.10 sec
What does T wave represent?
Ventricular repolarization
no normal
What does PR interval represent? Normal duration
Atrial depolarization plus AV nodal delay
0.12-0.20 sec
What does ST segment represent?
Isoelectric period of depolarized ventricles
What does QT interval represent? Normal duration?
Length of depolarization plus repolarization- corresponds to A.P. duration
0.20-0.40
What does lead I resemble?
RA–> LA
What does lead II represent?
RA–> LL
What does lead III represent?
LA–> LL
What does aVr represnt?
Looking up toward right hand form center
What does aVL represent?
Looking up at left hand from center
What does aVF represent?
Looking down to LL from center
When current flows toward red arrowheads (on limb lead drawing), ______ deflection occurs in EKG.
Upward
When current flows away from red arrowheads, ______ deflection is seen in EKG.
Downward
When current flows perpendicular ot red arrows _____ deflection or biphasic deflection occurs.
No
When impulse orginates at SA node, a wave of depolarization spreads over atria, resulting in electrical vector directed ________ and to left. This causes an ______ deflection in ECG in tracing I and aVF
Downward; upward (positive)
After delay in AV node, impulse traverses common bundle of His and R/L BB and inters interventricular septum, causing myocardial depolarization with electrical vector directed to right and downward. This results in small _______ deflection in lead I and ______ deflection in lead aVF.
Negative (downward)= Q wave lead I
Positive (upward)= R wave in AVF
During apical and early ventricular depolarization, impulse continues along conduction system, causing depolarization of apical ventricular myocardium with electrical vector directed downward and to left. This results in large _________ deflection in lead I and extends R wave in lead AVF
Positive (upward)
During late ventricular depolarization, depolarization spread over ventricles and vector shifts to become directed superiorly and to the left, thus ______ Rwave in lead I and causing ______ deflection in avf
Extending; negative (downward)= S wave in avf
Whean heart is fully depolarized, there is no electrical activity for a preif period (ST segment). Then repolarization begins from epicardium to endocardium, producing electrical vector directed downard and to left causing _______ deflection in lead I and AVF..
Upward (positive)= T wAVE
Baseline on EKG means ____
No electrical activity occuring
Overall direction of vector during atrial depolarization?
Down and to left
Overall direction of vector in septal depolarization
Down and to right
Overall direction of vector in apical and early left ventricular depolarization
Down and to left
Overall direction of late ventricular depolarization vector?
Up at to left
Overall direction of repolarization vector?
Slight down and to left
