Ganglion versus nucleus
A ganglion is a collection of nerve cell bodies in the peripheral nervous system, the peripheral equivalent of grey matter.
Remember
Cell bodies cluster in a ganglion in the peripheral nervous system and in a nucleus in the central nervous system, and axons bundle into a peripheral nerve in the first and a tract in the second.
Grey matter and white matter are labels for the central nervous system (CNS), meaning the brain and spinal cord. They are not used for the peripheral nervous system (PNS), the part of the nervous system outside the brain and spinal cord. The PNS needs its own words.
When nerve cell bodies are collected together in the PNS, the structure is called a ganglion (plural ganglia), the equivalent of grey matter there. Axons in the PNS, gathered into a bundle, form a peripheral nerve, the equivalent of white matter.
Inside the CNS the same structures have different names. There, a collection of cell bodies is a nucleus, while a bundle of axons is a tract. So a nucleus and a ganglion are both clusters of cell bodies, one inside the CNS and one outside it.
The best-known example is the dorsal root ganglion. It forms an enlargement on the dorsal (sensory) root of a spinal nerve, beside the spinal cord. It holds the cell bodies of sensory neurones, the nerve cells that bring information in from the body towards the spinal cord.
A historical quirk can confuse. The name "basal ganglia" dates from before the rule (a cluster of cell bodies is a nucleus inside the CNS and a ganglion outside it) was settled: despite "ganglia" in the name, they are deep nuclei of the brain, so they are central structures.
Key facts
- Ganglion
- a collection of nerve cell bodies in the PNS, the peripheral equivalent of grey matter.
- Peripheral nerve
- a bundle of axons in the PNS, the equivalent of white matter.
- Nucleus
- a cluster of cell bodies inside the CNS.
- Tract
- a bundle of axons inside the CNS.
- Dorsal root ganglion
- holds the cell bodies of sensory neurones and lies beside the spinal cord.
- Basal ganglia
- despite the name, deep nuclei of the brain, so they are central structures.
Exam traps
- A nucleus is the term for a cluster of cell bodies inside the CNS, not for one in the PNS, where the matching term is ganglion.
- Grey matter is not the term for a collection of cell bodies in the PNS, because it is the term for cell bodies, dendrites and synapses in the CNS; the peripheral equivalent of grey matter is a ganglion.
- A fascicle within a peripheral nerve does not hold a cluster of cell bodies, because it is a bundle of axons, the peripheral equivalent of white matter.
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