A ganglion (plural ganglia) was a biological tissue mass, most commonly a mass of nerve cell bodies. In 2268, Leonard McCoy fretted that he could not reconnect the millions of ganglia and nerves in Spock's brain when he began to lose the surgical knowledge he had gained from the Controller. (TOS: "Spock's Brain" ) In 2375, The Doctor found that a number of the ganglia in Chakotay's visual cortex were hyperactive after a boxing simulation on the holodeck. This was due to the link between his mind and alien beings that lived in chaotic space. (VOY: "The Fight")