Autonomic nervous system (Intrinsic nervous system…)

Abstract

Overview

  • Function: controls unconscious, involuntary, and visceral body functions, i.e., the cardiovascular, thermoregulatory, gastrointestinal, genitourinary, and pupillary systems
  • Components
Overview of the sympathetic and parasympathetic nervous system
Sympathetic nervous system Parasympathetic nervous system
Effect
Location
  • Thoracolumbar outflow
  • Craniosacral outflow
Characteristics
  • Short preganglionic fibers, long postganglionic fibers
  • Ganglia close to the spinal cord
  • Long preganglionic fibers, short postganglionic fibers
  • Ganglia in visceral effector organs
Neurotransmitter
Most common cotransmitters
Receptors

Types of receptors

Types of nerve fibers

  • Small myelinated fibers: transmit preganglionic autonomic efferents and somatic afferents (fast!)
  • Unmyelinated fibers (e.g., which innervate sweat glands): transmit postganglionic autonomic efferents and somatic/autonomic afferents (slow!)

Neurotransmitters

Transmitters of the sympathetic nervous system: preganglionicpostganglionic neurons: acetylcholine; postganglionic neurons → effector organ: norepinephrine!

Sympathetic nervous system

For a rapid overview see table “Overview of the sympathetic and parasympathetic nervous system”.

Sympathetic nervous system

  1. Descends from central nervous system to T1–L2; (thoracolumbar outflow): neuron cell bodies are located in the lateral horn of the spinal cord.
  2. Preganglionic axons (short myelinated, cholinergic fibers) exit the spinal cord through ventral roots, spinal nerves, and white rami communicantes. They either:
  3. Postganglionic axons: (long unmyelinated, mostly adrenergic fibers): leave the paravertebral or prevertebral ganglia and innervate end organs

Sympathetic ganglia

Paravertebral ganglia

Prevertebral ganglia (or preaortic ganglia)

Injury to the cervical sympathetic trunk (esp. the superior cervical ganglion, which supplies the visceral structures of the head and neck) may result in Horner syndrome (partial ptosis, miosis, anhidrosis)!

References:[1]

Parasympathetic nervous system

For a rapid overview see table “Overview of the sympathetic and parasympathetic nervous system”.

General information

  1. Exits central nervous system (dorsal motor nucleus) with cranial nerves CN III, CN VII, CN IX, CN X, and the sacral spinal roots (craniosacral outflow)
  2. Preganglionic axons; (long, myelinated cholinergic fibers): synapse in ganglia close to end organs
  3. Postganglionic axons; (short, cholinergic fibers): leave ganglia and innervate end organs

Cranial outflow

The cranial outflow supplies visceral structures of the head, neck and face via CN III, CN VII, CN IX and of the thorax and upper abdomen via CN X.

Sacral outflow

Supplies pelvis via 2nd–4th sacral spinal nerves

The sympathetic vs. parasympathetic nervous system

The sympathetic and parasympathetic nervous systems mediate numerous, sometimes antagonistic effects in the organs they innervate. In general, the sympathetic nervous system stimulates the body’s fight-or-flight response, while the parasympathetic nervous system controls homeostasis and the body at rest.

Target organ Sympathetic nervous system

Parasympathetic nervous system

Brain
  • ↑ Blood flow and alertness
  • M1: CNS (increase memory, attention)
Eye
Salivary glands
  • α1: ↑ mucous secretion
  • M3: ↑ serous secretion
Blood vessels
  • α1: vasoconstriction in skin and intestine
  • β2: vasodilation in skeletal muscle (transmitter: only epinephrine!)
  • Most blood vessels in the body do not have parasympathetic innervation.
  • Vasodilation
    • Via M3: e.g., deep arteries of the penis (cavernosal arteries) during erection
    • Indirectly: via withdrawal of tonic adrenergic vasoconstriction
Heart
Lungs
  • β2: bronchodilation
  • M3: bronchoconstriction
Digestive system

Reproductive organs

Bladder
  • α1: contraction of vesical sphincter muscle
  • β2 and β3: relaxation of detrusor muscle
Adrenal medulla
  • None
Kidneys
  • None
Skin
  • None
Blood
  • α2: ↑ platelet aggregation
  • None
Adipose tissue
  • α2: ↓ lipolysis
  • β1, β2: ↑ lipolysis
  • β3: ↑ lipolysis, thermogenesis (in skeletal muscle)
  • None

Sympathetic nervous system: fight or flight! Parasympathetic nervous system: rest and digest!

Enteric nervous system

The enteric nervous system has complex networks of afferent and efferent nerve fibers. It can operate independently of the brain and the spinal cord but its activity is usually modulated by the sympathetic and parasympathetic nervous systems.

  • Consists of
    • Enteric ganglia
    • Plexuses of the GI tract
      • Submucosal (Meissner) plexus
      • Myenteric (Auerbach) plexus
    • Interstitial cells of Cajal (ICCs)
      • Located in the wall of the intestine
      • Act as pacemaker cells for peristaltic motor activity of the gut
      • Connected to smooth muscle cells via gap junctions
      • Generate spontaneous electrical slow waves and thus rhythmic contractions of the smooth musculature (i.e., peristalsis)
  • Function: control of GI secretion and motility

Plexuses

Whereas the sympathetic system has an inhibitory effect on the gastrointestinal tract, the parasympathetic system promotes secretion and motility! However, removal of vagal or sympathetic connections with the gastrointestinal tract only has a minor effect on GI function because of the autonomy of the enteric nervous system!

References:[2]

Clinical significance