The ciliary ganglion is a bundle of nerves, parasympathetic ganglion located just behind the eye in the posterior orbit. It is 1–2 mm in diameter and in humans contains approximately 2,500 neurons. The ganglion contains postganglionic parasympathetic neurons. These neurons supply the pupillary sphincter muscle, which constricts the pupil, and the ciliary muscle which contracts to make the lens more convex. Both of these muscles are involuntary since they are controlled by the parasympathetic division of the autonomic nervous system.
The ciliary ganglion is one of four parasympathetic ganglia of the head. The others are the submandibular ganglion, pterygopalatine ganglion, and otic ganglion.
Structure
The ciliary ganglion contains postganglionic parasympathetic neurons that supply the ciliary muscle and the pupillary sphincter muscle. Because of the much larger size of the ciliary muscle, 95% of the neurons in the ciliary ganglion innervate it compared to the pupillary sphincter.
Roots
Three types of axons enter the ciliary ganglion but only the preganglionic parasympathetic axons synapse there. The entering axons are arranged into three roots which join enter the posterior surface of the ganglion: The sensory root branches from the nasociliary nerve and travels through the ganglion forming part of the short ciliary nerves. These sensory axons supply the cornea, ciliary body and iris. The sympathetic root originates from the internal carotid plexus with cell bodies in the superior cervical ganglion. The axons pass through the ganglion and enter the eye without synapsing into the short ciliary nerves. The sympathetic root contains the postganglionic sympathetic axons that provide sympathetic supply to the blood vessels of the eye. Sometimes, they also supply the pupillary dilator muscle, however these axons usually travel from the nasociliary nerve to the long ciliary nerves to enter the eye. The parasympathetic root branches from the inferior division of the oculomotor nerve and carries the preganglionic parasympathetic axons from the Edinger-Westphal nucleus to the ciliary ganglion. Within the ganglion the axons synapse onto the postganglionic parasympathetic neurons. These neurons project axons through the short ciliary nerves to innervate the ciliary muscle and pupillary sphincter muscle.
Short ciliary nerves
Exiting from the anterior surface of the ciliary ganglion are the short ciliary nerves which contain the sensory, postganglionic sympathetic and postganglionic parasympathetic axons to the eye.
Clinical significance
Adie tonic pupil
Diseases of the ciliary ganglion produce a "tonic pupil", which is a pupil that does not react to light (it is “fixed”) and has an abnormally slow and prolonged response to attempted near vision (accommodation).
When a person with an Adie pupil attempts to focus on a nearby object, the pupil (which would normally constrict rapidly) constricts slowly. On close inspection, the constricted pupil is not perfectly round. When the person focuses on a more distant object (say the far side of the room), the pupil (which would normally dilate immediately) remains constricted for several minutes, and then slowly dilates back to the expected size.
Tonic pupils are fairly common – they are seen in roughly 1 out of every 500 people. A person with anisocoria (one pupil bigger than the other) whose pupil does not react to light (does not constrict when exposed to bright light) most likely has Adie syndrome – idiopathic degeneration of the ciliary ganglion.
Physiology
The strange behavior of tonic pupils was first explained by Irene Loewenfeld in 1979. The ciliary ganglion contain many more nerve fibers directed to the ciliary muscle than nerve fibers directed to the constrictor pupillae – roughly twenty times more. The ciliary muscle is also more massive than the constrictor pupillae, again by a factor of twenty. Based on these observations, Loewenfeld proposed an explanation of the tonic pupil. She noted that pathological destruction of nerve cells in the ciliary ganglion that is found in all cases of Adie pupil. In her own words: