The trigeminal ganglion is a mass of nerve cell bodies that are part of the trigeminal nerve. It is also called the Gasserian ganglion, semilunar ganglion or Gasser's ganglion. It resides in a cavity, called Meckel’s cave, on the temporal bone in the area between the eye socket and the ear. The trigeminal ganglion is a sensory nerve ganglion, and it contains the cell bodies of incoming sensory fibers. This means that it is responsible for processing the sensory aspects of the trigeminal nerve, as in the sensations of touch or pressure.
This is also where the three branches of the trigeminal nerve, also called the fifth cranial nerve or simply CN5, converge. The trigeminal nerve, in terms of its sensory functions, is responsible for the sensations of touch on the side of the face. It also has motor nerve fibers, nerve fibers that are responsible for motion — in this case, chewing, biting and swallowing — but those fibers only pass through this ganglion. The motor cell bodies of the trigeminal nerve are located in the nucleus of another nerve, and the fibers converge there.
The function of the trigeminal ganglion is to act as a relay between external stimulation of the trigeminal nerve and the central nervous system. The sensory fibers of the trigeminal nerves pass to these ganglions, which in turn pass the nerve impulses through a single large sensory root that enters the brain stem. This is a very basic description, because the nerve pathway is a complex process.
The most notable function of the trigeminal ganglion, clinically speaking, is that it is used in a procedure known as a Gasserian ganglion nerve block, or simply Gasserian block. This procedure is done when trigeminal neuralgia, a condition where the trigeminal nerve is inflamed and causes pain, does not respond to oral medication or surgery. A Gasserian block consists of a small amount of local anesthetic, sometimes with a steroid, being injected into this ganglion. This blocks the sensory input and hopefully provides relief from the extreme pain that trigeminal neuralgia can cause.
The trigeminal ganglion can also be damaged by infection or surgery, a condition known as trigeminal trophic syndrome. This syndrome can cause numbness or a burning sensation and can lead to damage over time. Viruses can also linger in the trigeminal ganglion even after the initial infection is cleared, as in the case of a herpes infection, and can lay dormant over time until activated again.