The medical term "ganglion" derives from the Greek γάγγλιο (gánglio) and means "a knot". It was initially used by Hippocrates of Cos (460 BC - 370 BC) to denote a small mass (tumor) under the skin. The term is still used in medicine to name small synovial cysts found near joint or around small tendons. The term was later used by Galen of Pergamon (129 AD - 200 AD) to name small nervous masses. The common use for the term in tendons and nerves is that the Greek made no distinction between these structures calling them [νευρών] (nevrón) meaning both tendon or sinew and nerve. The plural form for ganglion is "ganglia".
Today, the term "ganglion" is mostly used to denote a an encapsulated aggregation of neuronal cell bodies in the peripheral nervous system that serves as a relay, integration, or sensory transduction site. A ganglion may be part of simple and complex reflex circuits. A ganglion has an external capsule formed by connective tissue, neuronal bodies, and supportive neuroglial cells.
This term should not be used to refer to structures found in the central nervous system (CNS). An aggregation of neuronal bodies in the CNS is called a "nucleus", the plural form is "nuclei".
When intercommunicated, ganglia can form complex networks (plexi) inside the muscular layers of organs that have rhythmicity, such as the enteric and cardiac ganglionated plexi. These neuronal networks function as autonomic relays and integration sites, reducing the workload of the CNS.
Interestingly, ganglia can be found in small animals that lack a vertebrate-type CNS, illustrating their function in complex reflexes. Some examples are:
- Echinodermata (sea stars, brittle stars/ophiuroids)
- Cnidaria (jellyfish, sea anemones)
- Flatworms (although it does have a cephalic ganglion that some contend is a "brain"
Sources:
1. Skinner, "The Origin of Medical Terms" 1970. William and Wilkins
2. Standring S, ed. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Elsevier; 2021.
3. Anderson, Douglas M "Dorland's Illustrated Medical Dictionary"; ISBN: 0721655777 USA: W.B. Saunders, 1994.
4. Ross, Michael H, et al. "Histology: A Text and Atlas"; ISBN: 0683073699 Baltimore: Williams & Wilkins, 1995
5. Haines, Duane E. "Neuroanatomy: An Atlas of Structures, Sections and Systems"; ISBN: 9780781746779 Philadelphia: Lippincott, Williams & Wilkins, 2004.
6. Drake RL, Vogl W, Mitchell AWM. Gray’s Anatomy for Students. 4th ed. Elsevier; 2020.



