Neural and Hormonal Control of Cardiac Activity

Extrinsic vs. Intrinsic Control

• Extrinsic Control: Signals from outside the organ regulate its activity. Examples include neural and hormonal signals.
• Intrinsic Control: Stimuli originate from within the organ itself.
• Many organs have both types of control.

Neural Control of Cardiac Activity

Sympathetic Nervous System

• Cardiac Accelerator Nerves: Branches of sympathetic nerves that innervate the heart.
• Innervates both conductive and contractile cells.
  • Conductive Cells: SA node, AV node, influencing heart rate.
  • Contractile Cells: Ventricular myocardium, influencing force of contraction.

Signal Transduction Mechanism

• Neurotransmitter: Norepinephrine (also epinephrine).
• Receptor: Beta one adrenergic.
  • Adrenergic receptors bind norepinephrine/epinephrine.
• Mechanism: G protein-coupled, activating adenylate cyclase.
  • Adenylate cyclase produces cyclic AMP (cAMP).
  • cAMP activates protein kinases.
  • Protein kinases phosphorylate (activate) extra sodium and calcium channels.
• Effects:
  • Conductive Cells: Increased slope of pacemaker potential → increased heart rate.
  • Contractile Cells: Increased calcium influx → stronger contraction (increased tension).
• Overall: Increases both heart rate and force of ventricular contraction → increased cardiac output.
• Contractility: Extrinsically influencing force of ventricular contraction.

Parasympathetic Nervous System

• Vagus Nerve: Parasympathetic nerve branches.
• Innervates SA and AV nodal cells (conductive) but not contractile myocardial cells.
• Neurotransmitter: Acetylcholine.
• Receptor: Muscarinic cholinergic.
  • Cholinergic receptors bind acetylcholine.
• Mechanism: G protein-coupled, directly influencing ion channels.
  • Closes calcium channels.
  • Opens extra potassium channels.
• Effects:
  • Decreased slope of pacemaker potential → slower heart rate.
  • No direct influence on force of contraction.
• Continuous Tug of War: Neither parasympathetic nor sympathetic systems are ever completely off; it's a balance.
  • Fight or Flight: Increased sympathetic activity, decreased parasympathetic activity.
  • Rest and Digest: Increased parasympathetic activity, decreased sympathetic activity.
• This ensures that we are increasing stimulation of the heart while decreasing inhibition to get the increase in heart rate and contractility we're looking for in fight or flight

Hormonal Control of Cardiac Activity

• Epinephrine can bind to beta one adrenergic receptors, similar to norepinephrine.
• There are lots of hormones that influence activity of the heart, epinephrine is just one example.
• Other hormones (glucagon, cortisol, estrogen, testosterone) can also influence heart activity.