2016B02 Compare and contrast the actional potentials from the sino-atrial node and a ventricular myocyte.



·         Intro

·         Graphs and conductances

·         Pacemaker action potential

·         Ventricular myocyte 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 conductances:


 (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


Ventricular myocyte action potential:


·   Plateau -> time for systolic ejection and prevention of tetany

·   Variable timing -> co-ordinated contraction

o e.g. endocardial cells start earlier, finish later

Refractory periods

·   Absolute (ARP):no Na+ channels resting; AP impossible

·   Effective (ERP): insufficient Na+ channels resting; AP impossible

·   Relative (RRP): enough Na+ channels resting, but fewer than normal; AP possible with higher than normal stimulus

·   Supranormal period (SNP): AP possible with lower than normal stimulus
(? closer to threshold potential ? other mechanism)

Autonomic changes

·   SNS: shortens duration of action potential (don’t know how)

·   PSNS: don’t know

Pathological changes

·   Hypoxia: failure of Na+K+ATPase -> depolarisation to -50mV -> inactivation of fast-Na+ channel and activation of L-Ca2+ channel -> behaves like pacemaker cell -> arrhythmia


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