2020A09 Compare and contrast the actional potentials from the sino-atrial node and a ventricular myocyte.



·         Intro

·         Graph and conductance

·         Pacemaker action potential



Action potential

·   Allows rapid, co-ordinated signalling

·   Rapid depolarisation followed by repolarization

·   Ion channels cycle through resting -> activated -> inactivated states

Determinants of ion flux

·   Channel permeability: voltage dependent

·   Electrochemical gradients

o Na+ and Ca2+ into cell, K+ out of cell

o Mainly due to Na+K+ATPase


Graphs and conductance:


(note: I have found it difficult to find consensus regarding voltages, channel opening times etc.; please advise me if you find a solid source)


Pacemaker action potential:


·   Automaticity

o i.e. intrinsic, rhythmic, frequent depolaristion and contraction

o HCN channel produces ‘funny current’ – i.e. mixed Na+ and K+ influx

o [cAMP] If slope of phase 4 heart rate

o Note depth of repolarisation IK 1/(heart rate)

·   Post-repolarisation refractoriness

o RRP extends into phase 4

Refractory periods

·   ERP: insufficient HCN and Ca2+ channels resting; action potential (AP) impossible

·   RRP: enough HCN and Ca2+ channels resting but fewer than normal; AP possible with higher than normal stimulus

Factors increasing rate

·   ↑SNS -> ↑If = ↑ICa > IK -> ↑steepness all slopes, especially phase 4

·   ↑Temp -> ↑cell metabolic rate (enzymes, pumps)

·   ↓K+ -> less hyperpolarisation

·   Neonate: ↑resting HR (fixed stroke volume)

Factors decreasing rate

·   ↑PSNS: -> ↓If, ↑IK -> ↓steepness phase 4, increased hyperpolarisation

·   ↓Temp -> ↓cell metabolic rate (enzymes, pumps)

·   Hypoxia -> failure of Na+K+ATPase, unable to repolarize cell, HCN not reactivated

·   Elderly: fibrous/fatty change in conduction system, ↓β1 adrenoceptors



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