Physiological Properties of Heart Flashcards
Describe the path of electrical activity in the heart.
– SAN pacemaker potential starts the process
– Depolarisation spreads due to “functional syncytium”
– Around atria first, then ventricles (through AV node)
What does ECG measure ?
Total electrical activity of the heart
Describe the action potential of sinus nodal fibers and ventricular muscle fibers. Draw them.
1) SINUS NODAL FIBER AP
- no stable resting membrane potential (-55 to -60 mV. Less than ventricular muscle fiber due to leakier cell membrane to Na and Ca ions, causing influx of positive charge)
- no sharpness of movement (change in movement when treshold reached goes up, then down at more leisurely pace)
2) VENTRICULAR MUSCLE FIBER
- stable resting membrane potential (-85 to -90 mV)
- Rapid upstroke (due to fast Na channels), then plateau (due to slower opening of slow calcium channels + opening of K channels, balance between the two causes plateau) then quick repolarisation (due to opening of K channels and K leaking out)
Describe the different steps to atrial/ventricular depolarisation. Draw this.
Phase 0
• Rapid depolarisation due to ↑ Na+ permeability (gNa+) as fast (voltage gated) Na+ channels open
Phase 1
• Start of repolarisation as fast Na+ channels close
Phase 2
• Effect of Ca2+ entry via L-type channels (causes plateau)
Phase 3
• Rapid repolarisation as ↑ [Ca2+]i stimulates K+ channels to open and gK+ ↑
• Ca2+ L-type (dihydropyridine) channels close
Phase 4
• Stable resting membrane potential where gK+ exceeds gNa+ by 50:1
Draw a graph representing Ionic Currents for Calcium, Sodium, and Potassium outward and inwards over the length of a ventricular AP.
Refer to “Physiological Properties of the Heart” slide 5.
Describe the steps to sino-atrial node depolarisation. Draw the graph for it.
Phase 1
• Gradual drift ↑ in resting membrane potential due to ↑ gNa+ as “funny” F-type Na+ channels open and ↓ gK+ as K+ channels slowly close (pacemaker potential)
• Transient (T) Ca2+ channels help with
the “final push
Phase 2
• Moderately rapid depolarisation due to Ca2+ entry via slow (L) channels
Phase 3
• Rapid repolarisation as elevated internal Ca2+ stimulates opening of K+ channels and an ↑ in gK+ (K+ leaks out)
Which of the sinoatrial node or ventricular muscle fibers rely more on Calcium to achieve depolarisation ? more on Sodium ?
SAN: Calcium (depolarisation) and Potassium (repolarisation)
Ventricular muscle fiber: Sodium (depolarisation) and Potassium (repolarisation)
Draw the graph for permeability of the membrane to Sodium, Potassium, and Calcium respectively during an AP in the SAN.
Refer to “Physiological Properties of the Heart” slide 6.
What is peculiar about F type Na+ channels ?
More likely to open as membrane potential becomes more negative
Why is there no stable resting membrane potential in the SAN ?
Because of the presence of F type Na channels: as membrane potential becomes more negative, increase likelihood of F type Na channels opening, which then starts pushing membrane potential back up again
Which parts of the heart does each of the PSNS and SNS innervate ? Which one is more diffuse.
PSNS: Nodal areas (AVN and SAN), some atria
SNS: Nodal areas, ventricular muscle mass
SNS much more diffuse
Identify a nerve which specifically “interfaces with parasympathetic control of the heart”.
Vagus Nerve
Describe sympathetic stimulation of pacemaker activity.
– Noradrenaline acts on β1 receptors to ↑ cAMP production
– Increases rate of SAN phase 1 depolarisation by changing permeability of the membrane
• ↑ gCa2+
• ↑ gNa+ via “funny” channels
Overall, increase in heart rate (positive chronotropic effect)
Describe parasympathetic stimulation of pacemaker activity.
– Acetylcholine acts on M2 receptors which ↓ cAMP production
– Reduces rate of phase 1 depolarisation
– Hyperpolarises membrane potential to lower starting level
• ↑ extent and duration of opening of K+ channels ∴ ↑ gK+
Overall, negative chronotropic effect
Graph the effect of sympathetic and parasympathetic stimulation of pacemaker activity, explaining what’s going on.
Refer to “Physiological Properties of the Heart” slide 9.
SNS causes faster AP + higher starting membrane potential.
PSNS causes slower AP + lower starting membrane potential.
Describe the electrical pathway within the heart.
SAN –> AVN –> Bundle of His –> Purkinje fibers –> Ventricles
Identify the main function of the AV node.
Slows down pathway: allows electrical events within atria to have resolved, gathers electrical activity that has spread around internodal tracts and around the atria (therefore traveling at variety of different speeds) coalesces it at one point and allows coordinated impulse to travel to the ventricles.
How does the electrical pathway travel from SAN to AVN in the atria ?
Through internodal tracts
What is the rate of the depolarisation (uninfluenced by any innervation) of the following: Sinoatrial node (SAN) Atrioventricular node (AV node) Bundle of His Purkinje fibers Ventricles
Sinoatrial node (SAN): ~90/min Atrioventricular node (AV node): ~60/min Bundle of His: ~50/min Purkinje fibers: ~40/min Ventricles: ~30/min
What happens if conduction is blocked at the SAN ?
Downstream tissues assume their intrinsic rate
How fast is conduction in atrial and ventrical fibers ? in internodal pathways ? in the AV node and bundle ? in the Purkinje fibers ?
0.3-0.5 m/s
1 m/s
0.01 m/s
1.5-4 m/s
What time does it take for the electrical signal to reach the AV node from the SA node ?
~0.03s
How long is the signal delayed in the AV node ?
~0.09s
What is the penetrating portion of the AV bundle ? How long is the signal delayed there (in addition to the delay in the AV node) ?
Fibrous portion which insulates skeleton of the heart
~0.04s
Which part of the heart is the biggest determinant of how long the signal takes to get through ?
AV node
How long does the signal take to go from SA node to bundle of His ?
0.16s (0.03 + 0.09 + 0.04s)
Briefly explain what (microscopically) exactly in the heart an ECG detects ?
During the spread of depolarisation through the heart when the chambers contract, the extracellular fluid around the myocardium becomes more negatively charged. The polarised and repolarised regions of the heart remain relatively more positive. This creates a potential difference between the different regions, which ECG electrodes detect. The cardiac dipole is a vector which has both a direction (from the most negative to most positive regions of the heart) as well as an amplitude (voltage). Several electrodes are placed on the body to “look” at the cardiac dipole from different points of view.
Why doesn’t an ECG detect anything during resting membrane potential or during hyperpolarisation ?
Because there is no dipole (no ion influx or outflux) so no current is generated.
What does an ECG measure ?
Electrical activity of the heart over time
How many electrodes are used in an ECG ? What is their role ?
Four on the limbs
– One is an “earth”, used to remove background noise
– Three used to create virtual “leads” between each pair of electrodes (limb leads measure the sum of the electrical activity of the heart and the direction that electrical activity is moving in)
Six across the chest
– To give more specific, localised information about areas of the heart
What does it mean when an ECG trace goes up ? down ?
One end of each lead is designated “positive”
• Depolarisation moving towards the “positive” causes the trace to go up.
• Depolarisation moving away from the “positive” causes the trace to go down.
What is Einthoven’s triangle ?
“Imaginary formation of three limb leads (on R arm, L arm, L leg) in a triangle used in electrocardiography, formed by the two shoulders and the pubis. The shape forms an inverted equilateral triangle with the heart at the center that produces zero potential when the voltages are summed”
Identify the positive and negative electrode for each lead in Einthoven’s triangle.
Lead 1: + (LA) - (RA)
Lead 2: + (LL) - (RA)
Lead 3: + (LL) - (LA)
What factors determine the size of electrical signals from the heart ?
– Current (proportional to tissue mass)
– Direction of signal
Name the formula to calculate observed signal.
Observed signal = E x CosƟ, where
E = Electrical event
Ɵ = angle between event and ECG lead
Will a smaller or a larger angle get biggest observed signal ?
Small angle gets biggest observed signal
Explain what the event in each ECG wave.
P wave
– Atrial depolarisation
QRS wave
– Ventricular depolarisation
T wave
– Ventricular repolarisation
Why isn’t atrial repolarisation visible on an ECG ?
Because it occurs during ventricular depolarisation (so hidden by QRS wave)
Identify the important timing intervals on an ECG.
– P-R interval (0.12-0.2s)
– QRS complex width (0.06-0.12s)
– Q-T interval (0.25-0.35s)
Draw a typical ECG, annotating it to include PR interval, PR segment, QRS complex, ST segment, and QT interval.
Refer to “Physiological Properties of the Heart” slide 17
True or False: Many parts of the heart exhibit spontaneous, rhythmical depolarisation
True
