paraganglia
Meaning of paraganglia: Scattered nests of cells left over from the developing nervous system, some releasing hormones and some sensing the blood.
Definition of paraganglia
Paraganglia are small clusters of hormone producing cells that develop alongside the autonomic nervous system and sit near blood vessels and nerves. The largest and best known is the core of the adrenal gland. Some clusters release adrenaline and noradrenaline into the blood, while others, mostly in the head and neck, act as sensors monitoring oxygen and carbon dioxide.
Where are they found?
| Group | Location | Main function |
|---|---|---|
| Adrenal medulla | The core of each adrenal gland | Releases adrenaline into the blood |
| Sympathetic paraganglia | Along the spine, in the abdomen and pelvis | Release noradrenaline |
| Carotid body | Where the carotid artery divides in the neck | Senses oxygen, carbon dioxide and acidity |
| Jugulotympanic paraganglia | Near the skull base and middle ear | Sensory, not hormone releasing |
The division matters. Clusters attached to the sympathetic branch of the autonomic nervous system generally make hormones, while those in the head and neck, attached to the parasympathetic branch, generally do not.
What do they do?
The hormone producing clusters form part of the body's rapid stress response. When sympathetic nerves fire, these cells release adrenaline and norepinephrine straight into the bloodstream, lifting heart rate and blood pressure and freeing sugar for use. Most of that output comes from the adrenal gland. The sensing clusters do something quite different. The carotid body samples blood on its way to the brain and signals the brainstem to breathe harder when oxygen falls, the reflex that drives faster breathing at altitude.
Where do they come from?
Both types develop from the neural crest, a band of cells in the embryo that also produces nerve cells, the sheaths around nerves and the pigment cells of skin. Cells from this band migrate widely and settle in scattered positions, which is why paraganglia turn up in so many places and why tumors arising from them can appear almost anywhere along the route those cells traveled.
Why do they matter?
Tumors of these clusters are the reason the word appears in clinical writing at all. A tumor arising in the adrenal core is called a pheochromocytoma, and one arising elsewhere a paraganglioma. Those in the abdomen often produce hormones and cause episodes of high blood pressure, headache, sweating and palpitations. Those in the head and neck usually produce none, and instead show up as a painless neck lump, or as hearing changes and a pulsing sound in one ear.
Used in a sentence
The scan was performed to look for extra-adrenal paraganglia as a possible source of the catecholamine excess.
Good to know
Not every paraganglion releases hormones: clusters on the sympathetic branch generally do, while those in the head and neck usually work as oxygen sensors and cause no hormonal symptoms even when a tumor forms in them.
Word origin
Where “paraganglia” comes from
From the Greek para, meaning beside, and ganglion, meaning a knot or swelling, since the clusters lie next to nerve ganglia.