Regulation of heart rate

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Regulation of Heart Rate (HR)

Heart rate is primarily controlled by the autonomic nervous system acting on the SA node. Understanding the balance between sympathetic and parasympathetic inputs is crucial for clinical physiology.

1. Autonomic Control

Division Neurotransmitter/Receptor Effect on HR
Sympathetic Norepinephrine / \beta_1 Positive Chronotropy (Increase HR).
Parasympathetic Acetylcholine / M_2 Negative Chronotropy (Decrease HR).

2. Physiological Mechanisms

  • Sympathetic: Increases cAMP, which increases the I_f (funny current) and I_{Ca,L} (calcium current) in the SA node, leading to a steeper phase 4 depolarization.
  • Parasympathetic: Increases K^+ conductance (I_{K,Ach}) and decreases I_{Ca,L}, hyperpolarizing the cell and slowing phase 4 depolarization.
  • Bainbridge Reflex: Increased venous return (atrial stretch) causes reflex tachycardia mediated by sympathetic nerves.

Physiology Board Hint: Remember that at resting HR (~70 bpm), vagal tone is dominant. This is why atropine (a muscarinic antagonist) causes such a significant increase in HR. Conversely, if you block both autonomic systems, the “intrinsic” firing rate of the SA node is approximately 100 bpm.