Autonomic Neurotransmission

Autonomic Neurotransmission

1. Definition, scope & general principles

• Neurohumoral (chemical) transmission is the process by which a nerve impulse is carried across a synapse or neuroeffector junction by the release of a specific chemical messenger (neurotransmitter), rather than electrically (KDT 8e Ch.7, pp.104–5).
• The autonomic nervous system (ANS) — also called the visceral, vegetative, or involuntary nervous system — regulates visceral functions (heart, blood vessels, glands, smooth muscle) below the level of consciousness to maintain homeostasis (G&G 14e Ch.10, p.173; KDT 8e Ch.7, p.103).
• Functionally, autonomic pharmacology is built on the principle that drugs which mimic or block chemical transmitters can selectively modify autonomic function; every step of transmission is a potential drug target (Katzung Ch.6, p.94; G&G 14e Ch.10, p.200).
• Two efferent divisions plus an integrative network:
  • Sympathetic (thoracolumbar) — preganglionic cell bodies in the intermediolateral columns, T1–L2(L3) (G&G 14e Ch.10, p.174; KDT 8e Ch.7, p.106).
  • Parasympathetic (craniosacral) — preganglionic fibres in cranial nerves III, VII, IX, X and sacral segments S2–S4 (G&G 14e Ch.10, p.176).
  • Enteric nervous system (ENS) — sometimes called the "third division"; >150 million neurons in the gut wall organised into the myenteric (Auerbach) plexus and submucosal (Meissner) plexus, functioning semi-autonomously (Katzung Ch.6, pp.95–6; G&G 14e Ch.10, p.176).
• ⚠ Craniosacral vs "cranial autonomic": Katzung Ch.6 p.97 (Espinoza-Medina evidence) argues the sacral cholinergic preganglionic fibres are embryologically sympathetic precursors, suggesting the traditional "craniosacral" parasympathetic label be revised to "cranial autonomic." G&G 14e and KDT retain the classical craniosacral designation. (See Multi-source disagreements.)

Somatic vs autonomic nerves (key contrasts)

• Autonomic efferents supply all innervated structures except skeletal muscle (somatic) (G&G 14e Ch.10, p.173; KDT 8e Ch.7, p.103).
• The most distal autonomic synapse lies in a peripheral ganglion outside the cerebrospinal axis; somatic synapses are entirely within the CNS (G&G 14e Ch.10, p.173).
• Autonomic nerves form extensive peripheral plexuses (absent in the somatic system) (KDT 8e Ch.7, p.103).
• Postganglionic autonomic fibres are non-myelinated; somatic motor fibres are myelinated (G&G 14e Ch.10, p.173).
• After nerve section, denervated skeletal muscle is paralysed and atrophies, whereas smooth muscle/glands retain spontaneous activity and do not atrophy (KDT 8e Ch.7, p.103).

Sympathetic vs parasympathetic (organising contrasts) (KDT 8e Ch.7, Table II.2, p.105)

• Origin: dorsolumbar (T1–L2/L3) vs craniosacral (III, VII, IX, X; S2–S4).
• Distribution: wide (sympathetic) vs limited to head, neck, trunk (parasympathetic).
• Ganglia: away from organ (sympathetic) vs on/near organ (parasympathetic).
• Postganglionic fibre length: long (sympathetic) vs short (parasympathetic).
• Pre:post ganglionic ratio: 1:20 to 1:100 (sympathetic, allowing diffuse discharge) vs 1:1 to 1:2 (parasympathetic; up to 1:8000 in the myenteric plexus) (KDT 8e Ch.7, p.105; G&G 14e Ch.10, p.177).
• Functional role: "fight or flight" / stress & emergency (sympathetic) vs "rest and digest" / energy conservation and assimilation (parasympathetic) (Katzung Ch.6, p.104; KDT 8e Ch.7, p.105).
• The activity of a dually-innervated organ at any moment is the algebraic sum of sympathetic and parasympathetic tone — they are functional (physiological) antagonists, not merely check-and-balance opposites (KDT 8e Ch.7, p.104; G&G 14e Ch.10, p.178).
• Some organs receive single innervation only: most blood vessels, spleen, sweat glands, pilomotor (hair follicle) muscles → sympathetic only; ciliary muscle, bronchial smooth muscle, gastric & pancreatic glands → parasympathetic only (KDT 8e Ch.7, p.104).

Neurotransmitter map of the autonomic outflow

• ACh is the transmitter at: ALL preganglionic fibres (sympathetic and parasympathetic), most postganglionic parasympathetic fibres, a few postganglionic sympathetic fibres (sweat glands, some vasodilator fibres), the somatic NMJ, and the adrenal medulla (G&G 14e Ch.10, p.174; KDT 8e Ch.7, Table 7.1).
• NE (noradrenaline) is the transmitter at: most postganglionic sympathetic fibres (G&G 14e Ch.10, p.174).
• Adrenal medulla = a modified sympathetic ganglion; preganglionic (cholinergic, N_n) input triggers release of EPI (~80%) and NE (~20%) into the blood (G&G 14e Ch.10, pp.176, 190; Katzung Ch.6, p.94).
• Some postganglionic parasympathetic nerves use NO as transmitter → nitrergic nerves (G&G 14e Ch.10, p.174; KDT 8e Ch.7, p.108).
• Dopamine may be released by some peripheral sympathetic fibres (e.g. renal vasculature during stress) (Katzung Ch.6, p.94).