Autonomic Neurotransmitters Overview

Autonomic Neurotransmitters

• Both preganglionic sympathetic and parasympathetic neurons are cholinergic, releasing acetylcholine.
• Muscarinic receptors are found on postsynaptic cells and are activated by acetylcholine, initiating G protein-linked receptor pathways. The specific effect depends on the effector organ.
• Adrenergic receptors are activated by norepinephrine and epinephrine. There are four main types:
  • α1 receptors: Primarily excitatory, increasing intracellular calcium.
  • α2 receptors: Primarily inhibitory, decreasing cAMP levels.
  • β1 receptors: Activate cAMP, often associated with increased heart rate and contractility.
  • β2 receptors: Activate cAMP, often associated with bronchodilation and vasodilation.

Properties of Adrenergic Receptors

• α1 and α2 receptors have a higher affinity for norepinephrine than epinephrine.
• β2 receptors bind almost exclusively with epinephrine.
• β1 receptors have an equal affinity for norepinephrine and epinephrine.
• All adrenergic receptors are linked to G proteins.
  • β1 and β2 receptor activation increases cAMP.
  • α1 receptor activation increases IP3/Ca2+.
  • α2 receptor activation inhibits cAMP in target cells.

Effect of ANS on Various Organs

• Heart:
  • Sympathetic: Increases heart rate and contractility.
  • Parasympathetic: Decreases heart rate.
• Blood Vessels:
  • Sympathetic: Generally constricts blood vessels (except for those in skeletal muscles, which dilate).
  • Parasympathetic: Dilates blood vessels in the penis and clitoris.
• Bronchioles:
  • Sympathetic: Dilates.
  • Parasympathetic: Constricts.
• Pupil:
  • Sympathetic: Dilates (mydriasis).
  • Parasympathetic: Constricts (miosis).
• Digestion:
  • Sympathetic: Inhibits digestive processes.
  • Parasympathetic: Enhances digestion.

Neurotransmitters: Norepinephrine vs. Epinephrine

• Norepinephrine is released from:
  • Adrenal medulla (80% of secretion)
  • Sympathetic postganglionic nerve endings
• Epinephrine is released from:
  • Adrenal medulla (20% of secretion)
• Norepinephrine acts primarily on α receptors, while epinephrine acts on both α and β receptors.
• Norepinephrine has a stronger effect on the heart than epinephrine.
• Norepinephrine causes vasoconstriction, except in skeletal muscles where it causes vasodilation.
• Epinephrine is a vasoconstrictor, but it also has some vasodilatory effects.

Drugs Affecting the ANS

• Atropine blocks the effect of acetylcholine on muscarinic receptors, suppressing parasympathetic activity. This can be used to decrease salivary and bronchial secretions before surgery.
• Metaprolol selectively blocks β1 receptors, used for treating high blood pressure by lowering heart rate and contractility.

Regions of the CNS Involved in Autonomic Control

• Hypothalamus: Integrates autonomic, somatic, and endocrine responses associated with emotions and behavior.
• Medulla Oblongata: Responsible for autonomic output, including cardiovascular and respiratory centers.
• Spinal Cord Integrates certain autonomic reflexes, such as urination, defecation, and erection.

Explore the roles of cholinergic and adrenergic receptors in the autonomic nervous system. This quiz covers the types of receptors, their functions, and the neurotransmitters involved in modulating physiological responses. Test your knowledge on how these receptors influence different organs and systems.