| Ciliary ganglion | |
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 Pathways in the ciliary ganglion. Green = parasympathetic; Red = sympathetic; Blue = sensory | |
| Anatomical terms of neuroanatomy |
Theciliary ganglion is aparasympatheticganglion located just behind the eye in the posteriororbit. Three types of axons enter the ciliary ganglion but only the preganglionic parasympathetic axons synapse there. The entering axons are arranged into threeroots of the ciliary ganglion, which join enter the posterior surface of the ganglion.
Thesympathetic root of ciliary ganglion is one of three roots of the ciliary ganglion. It containspostganglionic sympathetic fibers whose cell bodies are located in thesuperior cervical ganglion. Their axons ascend with theinternal carotid artery as a plexus of nerves, theinternal carotid plexus. Sympathetic fibers supplying the eye separate from the carotid plexus within thecavernous sinus. They run forward through thesuperior orbital fissure and merge with thelong ciliary nerves (branches of thenasociliary nerve) and theshort ciliary nerves (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 theintermediolateral column of the thoracic spinal cord, at the level ofthoracic spinal nerve 1 (T1) andthoracic spinal nerve 2 (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 bysupranuclear 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) and eyelid droop (ptosis) – the classic signs ofHorner's syndrome.
Sympathetic fibers from the superior cervical ganglion innervate blood vessels (vasoconstriction), sweat glands, and 4 eye muscles: thedilator pupillae, thesuperior tarsal muscle, theinferior tarsal muscle and theorbitalis.
Thedilator pupillae dilates the pupil; its action is antagonistic to thesphincter 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.
Thesuperior tarsal muscle elevates the upper eyelid. Thelevator palpebrae superioris, 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 (theinferior tarsal muscle and theorbitalis) are vestigial in humans. They are variable and often incompletely developed.
Sensory fibers from theeyeball (thecornea,iris, andciliary body) run posteriorly through the short ciliary nerves and pass through the ciliary ganglion without formingsynapses. They leave the ciliary ganglion in thesensory root of ciliary ganglion, which joins thenasociliary nerve—a branch of theophthalmic nerve. From there, the signal travels back through the ophthalmic nerve to thetrigeminal nerve 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.
Theparasympathetic root of ciliary ganglion provides parasympathetic supply to the ciliary ganglion.
The ciliary ganglion is aparasympathetic 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 connectingpreganglionic topostganglionic nerve fibers. There are roughly twice as many incoming parasympathetic fibers as outgoing parasympathetic fibers. Neural processing occurs as incoming signalsconverge onto target neurons.
Presynaptic parasympathetic fibers originate in theEdinger-Westphal nucleus, the parasympathetic motor nucleus associated with theoculomotor nucleus in the brainstem. Axons from the Edinger-Westphal nucleus and the oculomotor nucleus run together in the brainstem and exit together as theoculomotor nerve. The oculomotor nerve passes through the lateral wall of thecavernous sinus and enters the orbit through thesuperior orbital fissure. It divides into branches that innervate thelevator palpebrae superioris and four of the sixextraocular muscles. Parasympathetic fibers initially run in theinferior 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 thesphincter pupillae andciliaris muscles. Thesphincter pupillae constricts the iris. Theciliaris muscle changes the shape of the lens, allowing the eye to focus on nearby objects (accommodation). Neither of these muscles is under voluntary control.
Anatomy of theautonomic nervous system | |||||
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