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