The

ciliary ganglion The ciliary ganglion is a bundle of nerve 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 parasympath ...

is a parasympathetic ganglion located just behind the eye in the posterior

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...

. Three types of axons enter the ciliary ganglion but only the preganglionic parasympathetic axons synapse there. The entering axons are arranged into three roots of the ciliary ganglion, which join enter the posterior surface of the ganglion.

Sympathetic root

The sympathetic root of ciliary ganglion is one of three roots of the ciliary ganglion. It contains ''postganglionic'' sympathetic fibers whose cell bodies are located in the superior cervical ganglion. Their axons ascend with the internal carotid artery as a plexus of nerves, the

internal carotid plexus The internal carotid plexus is situated on the lateral side of the internal carotid artery, and in the plexus there occasionally exists a small gangliform swelling, the ''carotid ganglion'', on the under surface of the artery. Postganglionic symp ...

. Sympathetic fibers supplying the eye separate from the carotid plexus within the

cavernous sinus The cavernous sinus within the human head is one of the dural venous sinuses creating a cavity called the lateral sellar compartment bordered by the temporal bone of the skull and the sphenoid bone, lateral to the sella turcica. Structure The cave ...

. They run forward through the superior orbital fissure and merge with the

long ciliary nerves The long ciliary nerves are 2-3 nerves that arise from the nasociliary nerve (itself a branch of the ophthalmic branch (CN V1) of the trigeminal nerve (CN V)). They enter the eyeball to provide sensory innervation to parts of the eye, and sympat ...

(branches of the

nasociliary nerve The nasociliary nerve is a branch of the ophthalmic nerve, itself a branch of the trigeminal nerve (CN V). It is intermediate in size between the other two branches of the ophthalmic nerve, the frontal nerve and lacrimal nerve. Structure The na ...

) and the

short ciliary nerves The short ciliary nerves are nerves of the orbit around the eye. They are branches of the ciliary ganglion. They supply parasympathetic and sympathetic nerve fibers to the ciliary muscle, iris, and cornea. Damage to the short ciliary nerve ma ...

(from the ciliary ganglion). Sympathetic fibers in the short ciliary nerves pass through the ciliary ganglion without forming synapses. ''Preganglionic'' sympathetic fibers originate from neurons in the ''intermediolateral column'' of the thoracic spinal cord, at the level of thoracic spinal nerve 1 (T1) and

thoracic spinal nerve 2 The thoracic spinal nerve 2 (T2) is a spinal nerve of the thoracic segment.

(T2). They form synapses in the superior cervical ganglion. The ratio of incoming to outgoing fibers (the “convergence”) in this ganglion is approximately 100:1. Sympathetic motor neurons in the spinal cord are controlled by ''supranuclear'' pathways that descend through the brainstem and spinal cord. Interruption of the sympathetic chain at any level (from the brainstem to the ciliary ganglion) will produce pupillary constriction (

miosis Miosis, or myosis (), is excessive constriction of the pupil.Farlex medical dictionary
citing: ...

) and eyelid droop ( ptosis) – the classic signs of

Horner's syndrome Horner's syndrome, also known as oculosympathetic paresis, is a combination of symptoms that arises when a group of nerves known as the sympathetic trunk is damaged. The signs and symptoms occur on the same side (ipsilateral) as it is a lesion ...

. Sympathetic fibers from the superior cervical ganglion innervate blood vessels (vasoconstriction), sweat glands, and 4 eye muscles: the dilator pupillae, the

superior tarsal muscle The superior tarsal muscle is a smooth muscle adjoining the levator palpebrae superioris muscle that helps to raise the upper eyelid. Structure The superior tarsal muscle originates on the underside of levator palpebrae superioris and inser ...

, the inferior tarsal muscle and the orbitalis. The ''dilator pupillae'' dilates the pupil; its action is antagonistic to the ''sphincter pupillae''. Pupil size is therefore under the dual control of sympathetic and parasympathetic nerves. Postsynaptic sympathetic signals that originate in the superior cervical ganglion are carried by the nasociliary nerve or directly extend from the internal carotid plexus and pass through the ciliary ganglion. The ''superior tarsal muscle'' elevates the upper eyelid. The

levator palpebrae superioris The levator palpebrae superioris ( la, elevating muscle of upper eyelid) is the muscle in the orbit that elevates the upper eyelid. Structure The levator palpebrae superioris originates from inferior surface of the lesser wing of the sphenoid bo ...

, which is supplied by a branch of the oculomotor nerve, also elevates the upper eyelid. Eyelid elevation is therefore under both voluntary and involuntary control. Interruption of either pathway will result in eyelid droop ( ptosis). The other two eye muscles with sympathetic supply (the ''inferior tarsal'' muscle and the ''orbitalis'') are vestigial in humans. They are variable and often incompletely developed.

Sensory root

Sensory fibers from the

eyeball Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and con ...

(the

cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...

iris Iris most often refers to: *Iris (anatomy), part of the eye *Iris (mythology), a Greek goddess * ''Iris'' (plant), a genus of flowering plants * Iris (color), an ambiguous color term Iris or IRIS may also refer to: Arts and media Fictional ent ...

, and

ciliary body The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the ciliar ...

) run posteriorly through the short ciliary nerves and pass through the ciliary ganglion without forming synapses. They leave the ciliary ganglion in the sensory root of ciliary ganglion, which joins the

—a branch of the

ophthalmic nerve The ophthalmic nerve (V1) is a sensory nerve of the face. It is one of three divisions of the trigeminal nerve (CN V). It has three branches that provide sensory innervation to the eye, the skin of the upper face, and the skin of the anterior sc ...

. From there, the signal travels back through the ophthalmic nerve to the

trigeminal nerve In neuroanatomy, the trigeminal nerve ( lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and chew ...

and back into specific nuclei in the thalamus where they are relayed to areas in the cerebral cortex. The exact distribution of sensory fibers, like the distribution of sympathetic fibers, is anatomically variable. There are other pathways to the eye for both sympathetic and sensory fibers, and the precise anatomy varies from person to person. Since the result is the same regardless of how the fibers reach the eye, the presence of sympathetic and sensory fibers in the ciliary ganglion (the contributions of the “sensory” and “sympathetic” roots) is of no functional significance.

Parasympathetic root

The parasympathetic root of ciliary ganglion provides parasympathetic supply to the ciliary ganglion. The ciliary ganglion is a ''parasympathetic'' ganglion. Incoming parasympathetic nerve fibers form synapses with the dendrites of nerve cells within the ganglion. However, the ciliary ganglion is not simply a relay station connecting ''preganglionic'' to ''postganglionic'' nerve fibers. There are roughly twice as many incoming parasympathetic fibers as outgoing parasympathetic fibers. Neural processing occurs as incoming signals ''converge'' onto target neurons. ''Presynaptic'' parasympathetic fibers originate in the Edinger-Westphal nucleus, the parasympathetic motor nucleus associated with the

oculomotor nucleus The fibers of the oculomotor nerve arise from a nucleus in the midbrain, which lies in the gray substance of the floor of the cerebral aqueduct and extends in front of the aqueduct for a short distance into the floor of the third ventricle. From t ...

in the brainstem. Axons from the Edinger-Westphal nucleus and the oculomotor nucleus run together in the brainstem and exit together as the oculomotor nerve. The oculomotor nerve passes through the lateral wall of the

and enters the orbit through the superior orbital fissure. It divides into branches that innervate the

and four of the six

extraocular muscles The extraocular muscles (extrinsic ocular muscles), are the seven extrinsic muscles of the human eye. Six of the extraocular muscles, the four recti muscles, and the superior and inferior oblique muscles, control movement of the eye and the othe ...

. Parasympathetic fibers initially run in the ''inferior division'' of the oculomotor nerve. They exit as one or two short “motor roots” that synapse in the ciliary ganglion. ''Postsynaptic'' parasympathetic fibers leave the ciliary ganglion in multiple (six to ten) short ciliary nerves. These nerves enter the posterior aspect of the eyeball to supply the sphincter pupillae and ciliaris muscles. The ''sphincter pupillae'' constricts the iris. The ''ciliaris'' muscle changes the shape of the lens, allowing the eye to focus on nearby objects ( accommodation). Neither of these muscles is under voluntary control.

References

External links

* https://web.archive.org/web/20080404062031/http://anatomy.med.umich.edu/modules/head_autonomics_module/autonomics_05.html {{Authority control Nerves of the head and neck Sympathetic nervous system