Basal ganglia are not ganglia in the modern sense
A ganglion is a cluster of neuron cell bodies in the peripheral nervous system—dorsal root, cranial-nerve, or autonomic stations that relay and process signals outside the brain and cord. A pseudoganglion only looks the part: fibers without the cell-body crowd.
Strictly, a ganglion is a knot of nerve-cell bodies sitting outside the brain and spinal cord. Its bulk is made of somata and dendrites, bundled together, and neighbouring ganglia often link up into a web called a plexus. These clusters serve as relay stations, passing signals between the peripheral and central nervous systems. Vertebrates have three main kinds: dorsal root or spinal ganglia, which house sensory neurons; cranial nerve ganglia, such as the trigeminal; and autonomic ganglia. In the autonomic system, sympathetic and parasympathetic ganglia hold the cell bodies of the second neuron in each chain, so fibres running in from the central nervous system are called preganglionic and those heading out to an organ postganglionic.
The vocabulary is messier than the anatomy. Brain structures were named ganglia before the word was narrowed to peripheral clusters, and the basal ganglia kept their label even though they lie inside the brain. They are really nuclei wired to the cortex, thalamus and brainstem, involved in movement, thinking, emotion and learning. Terminologia Anatomica recommends calling them basal nuclei, but few have switched. Invertebrate biology ignores the restriction altogether: the main part of an insect brain is called the cerebral ganglion.
Not every bulge qualifies. A pseudoganglion is a local thickening of a nerve trunk that looks like a ganglion yet holds fibres rather than cell bodies; examples occur in the radial nerve and in the teres minor muscle.
Ganglion cells also matter in the eye. A South African study of 17 eyes in patients with tumours above the pituitary compared scans of the retinal ganglion cell complex with visual-field tests. Field defects were common, while the scans flagged only about 59 percent of eyes, suggesting the imaging helps but can miss milder loss. A historical review of vagal sensory neurons, meanwhile, credits microscopists from Santiago Ramón y Cajal and Albert von Kölliker to the overlooked Margaret Brown.
Source: Ganglion
