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

· Intro

· Graph and conductance

· Pacemaker 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

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

Features	· 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] ∝ I_f ∝ slope of phase 4 ∝ heart rate o Note depth of repolarisation ∝ I_K ∝ 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 -> ↑I_f = ↑I_Ca > I_K -> ↑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: -> ↓I_f, ↑I_K -> ↓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