The

ciliary ganglion The ciliary ganglion is a 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. T ...

is a

parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The autonomic nervous system is responsible for regulat ...

ganglion A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there are ...

located just behind the eye in the posterior

orbit In celestial mechanics, an orbit (also known as orbital revolution) 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 ...

. 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 The superior cervical ganglion (SCG) is the upper-most and largest of the cervical sympathetic ganglia of the sympathetic trunk. It probably formed by the union of four sympathetic ganglia of the cervical spinal nerves C1–C4. It is the only ...

. Their axons ascend with the

internal carotid artery The internal carotid artery is an artery in the neck which supplies the anterior cerebral artery, anterior and middle cerebral artery, middle cerebral circulation. In human anatomy, the internal and external carotid artery, external carotid ari ...

as a plexus of nerves, the

internal carotid plexus The internal carotid plexus is a nerve plexus situated upon the lateral side of the internal carotid artery. It is composed of post-ganglionic sympathetic fibres which have synapsed at (i.e. have their nerve cell bodies at) the superior cervical g ...

. 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 ...

. They run forward through the

superior orbital fissure The superior orbital fissure is a foramen or cleft of the skull between the lesser and greater wings of the sphenoid bone. It gives passage to multiple structures, including the oculomotor nerve, trochlear nerve, ophthalmic nerve, abducens nerve ...

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 sympa ...

(branches of the

nasociliary nerve The nasociliary nerve is a branch of the ophthalmic nerve (CN V1) (which is in turn 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 ner ...

) 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 may ...

(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 The thoracic spinal nerve 1 (T1) is a spinal nerve A spinal nerve is a mixed nerve, which carries Motor neuron, motor, Sensory neuron, sensory, and Autonomic nervous system, autonomic signals between the spinal cord and the body. In the human bod ...

(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 iris dilator muscle (pupil dilator muscle, pupillary dilator, radial muscle of iris, radiating fibers), is a smooth muscle of the eye, running radially in the iris and therefore fit as a dilator. The pupillary dilator consists of a spokelike ...

, the

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

, 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 () 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 bone, just above the optic foramen. It ...

, 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 An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the eye ...

(the

cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...

iris Iris most often refers to: *Iris (anatomy), part of the eye * Iris (color), an ambiguous color term *Iris (mythology), a Greek goddess * ''Iris'' (plant), a genus of flowering plants * Iris (given name), a feminine given name, and a list of peopl ...

, 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 ciliary ...

) run posteriorly through the short ciliary nerves and pass through the ciliary ganglion without forming

synapses In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...

. They leave the ciliary ganglion in the sensory root of ciliary ganglion, which joins the

—a branch of the

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

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

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

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. F ...

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, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements o ...

. The oculomotor nerve passes through the lateral wall of the

and enters the orbit through the

. It divides into branches that innervate the

and four of the six

extraocular muscles The extraocular muscles, or extrinsic ocular muscles, are the seven extrinsic muscles of the eye in human eye, humans and other animals. Six of the extraocular muscles, the four recti muscles, and the superior oblique muscle, superior and inferior ...

. 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