The autonomic nervous system

Vivian Imbriotis | May 28, 2026

The ANS is a collection of peripheral nervous system efferents that are responsible for involunatary organ and tissue actions to maintain homeostasis.

It consists of sympathetic and parasympathetic parts.

In both, fibers exit the CNS, synapse at a ganglion, and then innervate target structures:

  • Preganglionic fibers are myelinated B fibers
  • Postganglionic fibers are unymelinated C fibers
  • The preganglionic neurotransmitter is ACh \(\to\) nicotinic receptors on postganglionic neuron

The sympathetic nervous system prepares the body for stress and intense muscle activity

Origin: Cell bodies in lateral horn of thoracic spinal cord from T1 to L2

Pathway: short preganglionic B fibers \(\to\) white rami \(\xrightarrow{synapse \ ACh \to N_2}\) sympathetic chain ganglia \(\to\) travel up or down levels in sympathetic chain \(\to\) long postganglionic C fibers \(\to\) grey rami \(\to\) visceral or spinal nerves \(\to\) release NorAd (or ACh for sweat glands) on target organ. Adrenal medulla = modified postganglionic neurons that release adrenaline directly into plasma

Anatomy:

  • Sympathetic chain ganglia is paired structure that runs parallel to spinal column from base of skull \(\to\) coccyx; subdivided into:
  • Cervical \(\to\) head, neck
  • Thoracic \(\to\) aortic, cardiac, pulmonary, renal plexi
  • Lumbar \(\to\) coeliac plexus
  • Pelvic \(\to\) pelvic plexus

Effects on...

  • Heart \(\to\) \(\beta_1 \ \to \) positive inotropy, chronotropy, lusitropy, dromotropy
  • Lung \(\to \ \beta_2 \ \to\) bronchodilation
  • Arterioles \(\to\) \(\alpha_1 + \beta_2 \ \to \) net vasoconstriction \(\to \uparrow\)TPR
  • Veins \(\to\) \(\alpha_1 + \beta_2 \ \to \) net vasoconstriction\(\to \uparrow\)MSFP
  • GIT \(\to \ \beta_2 \ \to \ \downarrow\)motility
  • Liver \(\to \ \beta_2 \ \to \ \uparrow\)glycogenolysis + gluconeogenesis + lipolysis \(\to\) lactate formation
  • Kidney \(\to \ \beta_1 \to \ \) renin release
  • Bladder \(\to\) detrusor relaxation (\(\beta_2\)), sphincter contraction (\(\alpha_1\))
  • Eye \(\to \ \alpha_1 \ \to\) mydriasis

The parasympathetic nervous system prepares the body for digestion and recovery.

Origins:

  • Edinger-Westphal nucleus \(\to\) CNIII \(\to\) eye
  • Superior salivary nucleus \(\to\) CNVII \(to\) parotid gland
  • Inferior salivary nucleus \(\to\) CN IX \(\to\) submandibular gland
  • Dorsal nucleus of X (most important, ~75% of outflow) \(\to\) CN X \(\to\) thoracic and abdominal organs
  • Sacral parasympathetic nucleus \(\to\) sacral nerves \(\to\) pelvic/sex organs

Pathway: long preganglionic B fibers \(\xrightarrow{synapse \ ACh \to N_2}\) plexus near or in target organ \(\to\) short postganglionic C fibers \(\to\) release ACh onto target organ

Anatomy:

  • L+R vagus nerve formed in medulla from 4 nuclei
  • n ambiguus \(\to\) motor
  • n of solitary tract \(\to\) visceral afferent
  • spinal n of V \(\to\) somatic afferent
  • Dorsal n of X \(\to\) parasympathetics
  • \(\to\) exits skull via jugular foramen \(\to\) travels in carotid sheath \(\to\) gives off pharyngeal, superior largyneal, recurrent largyneal branches \(\to\) enters thorax \(\to\) gives off cardiac and pulmonary branche\(\to\) oesophageal hiatus \(\to\) becomes anterior and posterior trunks \(\to\) hepatic, gastric, coeliac branches

Effects on...

  • Heart \(\to \ M_2 \ \to \) negative chronotropy, dromotropy
  • Lung \(\to \ M_2 \ \to\) bronchoconstriction,
  • Arterioles \(\to \ M_3 \ \to\) vasodilation
  • Veins \(\to \ M_3 \ \to\) vasodilation
  • GIT \(\to \ M_3 \ \to\) increase motility
  • Liver \(\to \ M_3 \ \to\) glycogenesis
  • Kidney \(\to \ M_3 \ \to\) no parasympathetic innervation!
  • Bladder \(\to \ M_3 \ \to\) detrusor contraction
  • Eye \(\to \ M_3 \ \to\) myosis, lactimation
  • Salivary glands \(\to \ M_3 \ \to\) salivation