Autonomic Nervous System


The autonomic nervous system (ANS) is a set of nerves and nerve cells that provide the innervation of blood vessels and the airways, heart, intestines and urogenital organs. These nerves regulate and coordinate bodily functions based on secretory activity of glands, on contraction and relaxation of smooth muscle and cardiac muscle, and on sensations arising from deep viscera.

Motor activity is a crucial function in hollow organs: muscle contraction propels fluids or solid bodies into tubes and ducts. Autonomic nerves control the motility of viscera, but there is also myogenic activity, generated spontaneously by the muscle itself. Myogenic and neurogenic controls are well balanced in the heart; myogenic control predominates in the muscles of intestine or uterus; neurogenic control predominates in the urinary bladder or the ductus deferens.

The sensory nerves provide ill‐localized but deep sensation about the state of the body, and can convey the sensation of pain (visceral pain).

Key Concepts:

  • A large array of nerves reaches the viscera and the vessels of the body from the central nervous system, and signals pass continuously in both directions.

  • Autonomic nerves control the motility (contraction, dilatation, tension) of cardiac muscle and of smooth muscle, both in viscera (visceral smooth muscle) and in blood vessels (vascular smooth muscle).

  • The musculature of viscera is under control of autonomic nerves, but, in some organs, it can also contract myogenically, that is, independently of external influences.

  • Autonomic pathways are chains of neurons and nerves linking brainstem and spinal cord to the peripheral organs.

  • Autonomic pathways contain nerve ganglia where nerve fibres synapse onto ganglion neurons and new nerve fibres are issues and travel peripherally.

  • Nerve fibres of the autonomic nerves are thin and often without a myelin sheath, and their conduction velocity is slower than in somatic nerves.

  • The autonomic nervous system (ANS) comprises sensory fibres, emerging from ganglion neurons in the dorsal root ganglia, which provide deep sensation about the state of the viscera and can convey the sensation of pain.

  • A special set of nerves forms the enteric nervous system, self‐contained and independent from other nerves.

  • In many parts of the ANS, at the surface of ganglion neurons, nerve endings and muscle cells there are receptors that are sensitive to neurotransmitters, to other neurochemicals, to hormones, to signalling molecules and to exogenously administered substances (pharmacological agents or drugs).

Keywords: autonomic ganglion; autonomic nervous system; enteric nervous system; neuro‐muscular junction; parasympathetic nerves; smooth muscle; sympathetic nerves; unmyelinated nerves

Figure 1.

The basic plan of the peripheral layout of the somatic (left side) and the autonomic (right side) nerves providing motor and sensory innervation to the muscles of the locomotor system (skeletal muscles) and to the muscles of blood vessels and viscera (smooth muscles). In the somatic component a motor neuron in the spinal cord provides a long axon extending to the surface of muscle fibres where, after branching, it forms large single neuro‐muscular junctions. In the ANS, the motor neuron in the spinal cord (called preganglionic neuron) is smaller, is located more laterally in the ventral horn and it emit a thinner axon that terminated inside an autonomic ganglion, forming a synapse on a ganglion neuron. Axons emerging from ganglion neurons extend to the peripheral organs, branch and form expansions in passing (or varicosities) which make innumerable contacts with the smooth muscle cells. As to the sensory innervation, the somatic fibres form spindles and other specialized terminations in the skeletal muscles; the nerve fibres project back onto the motor neurons of the spinal cord, directly or via an interneuron. In the autonomic system the afferent fibres are diffuse within the tissue of the viscera, and have less prominent terminations; however, they project back to the spinal cord allowing for the occurrence of reflex responses.

Figure 2.

General plan of the distribution of autonomic nerves and ganglia, as they are spread between the spinal cord (left) and the peripheral organs. The central component is represented by some nuclei in the brainstem (top left) and a long column of neurons in the spinal cord.

Figure 3.

Synapse of a preganglionic fibre and the dendrite of a ganglion neuron in a sympathetic ganglion. The incoming axon, originating in the spinal cord, ends with an expansion, or bouton, packed with clear agranular vesicles (approximately 50 nm in diameter), a few granular vesicles and some mitochondria. The dendrite is cut along its length and it contains mitochondria, microtubules and some endoplasmic reticulum. In the area of contact both the axon and the dendrite show a membrane thickening, the presynaptic density and the post‐synaptic density. The latter is particularly prominent and is the site where an assembly of receptors, channels, messengers, ligands and enzymes detect the arrival of neurotransmitter released from the synaptic bouton and transform it into an electric signal that is transmitted along the axon to the cell body of the neuron. Microscopic field approximately 6 μm wide.

Figure 4.

A large nerve ending, or bouton, abutting on the surface of a muscle cell of the urinary bladder and forming an efferent ending or a neuro‐muscular junction. The nerve ending is packed with lucent vesicles (approximately 50 nm in diameter) and a few mitochondria. The gap between nerve ending and muscle cell is of uniform width and covers more than 1 micrometre of the perimeter of the muscle cell. A cytoplasmic rim of a Schwann cell covers the top of the nerve ending. Other muscle cells are visible around the nerve ending, and sectioned collagen fibrils (small grey circular elements) occupy the spaces between cells. The image is characteristic of autonomic nerves that provide tight control of the activity of a visceral muscle, as is the case of the urinary bladder; in other muscles, in the gut and blood vessels, the terminal relation between nerves and muscle is looser and more at a distance. Microscopic field approximately 6 μm wide.

Figure 5.

In this preparation the small intestine has been stained as a whole, to highlight the myenteric plexus, which lies a short distance below the peritoneal surface of the gut. The pattern of the myenteric plexus is formed by elongated ganglia, spreading circumferentially around the intestine, and connecting strands. The image at bottom right shows a single ganglion, with many ganglion neurons (dark cell bodies with a light, circular nucleus). The faintly stained background in all the images is the musculature of the intestine. Field of view approximately 10 mm across (top), approximately 4 mm (middle), approximately 1 mm (bottom left and bottom centre) and approximately 200 μm (bottom right).


Further Reading

Brading A (1999) The Autonomic Nervous System and its Effectors. Oxford: Blackwell.

Matthias C and Bannister R (eds) (2001) Autonomic Failure: A Textbook of Clinical Disorders of the Autonomic Nervous System. Oxford: University Press.

Robertson D, Biaggioni I, Burnstock G, Low P and Paton J (eds) (2011) Primer of the Autonomic Nervous System. New York: Academic Press.

Wilson‐Pauwels L, Stewart PA and Akesson EJ (1997) Autonomic Nerves, Basic Science, Clinical Aspects, Case Studies. London: Dekker.

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Gabella, Giorgio(Aug 2012) Autonomic Nervous System. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000081.pub2]