CARDIAC ELECTRICAL ACTIVITY-CONTROL

SUMMARY

1. Control mediated by constant balance between sympathetic & parasympathetic activity.

MECHANISMS

2. Parasympathetic stmulation via vagus nerve ⟹ release Ach at SA & AV nodes ⟹ in SA node Ach binds to M₂ muscarinic receptors ⟹ increased intracellular G protein ⟹ opens up special K⁺ channels ⟹ hyperpolarisation of membrane ⟹ it takes longer for cell to reach threshold ⟹ therefore reduce HR & AV nodal conduction.

3. Sympathetic stimulation via the thoracic splanchnics ⟹ release NA at the SA & AV nodes ⟹ binds B₁ receptors ⟹ increased intracellular cAMP ⟹ increases permeability of sarcolemma to Na & Ca ions ⟹ cells reach threshold quicker ⟹ therefore increases HR & AV nodal conduction.

Reference(s)

Barrett, K.E., Barman, S.M., Brooks, H.L., X, J. and Ganong, W.F. (2019). Ganong’s review of medical physiology. 26th ed. New York: Mcgraw-Hill Education