The Anatomy of the Ciliary Ganglion

A collection of nerve cells controlling the pupil and lens of the eye

The ciliary ganglion is one of four parasympathetic ganglia in the head. It is located behind the eye. Nerve fibers leave the brain and enter the ganglia.

These fibers synapse (a junction between nerves where they communicate) with neurons going to the eye. Nerves from the ciliary ganglion innervate the muscles that constrict the pupil, a hole in the iris that lets light enter the eye.

Other fibers from the ciliary ganglia go to muscles that control the shape of the lens in the eye. When these muscles are stimulated, the lens “balls up” and bends light more so you can focus on close objects. This process is called accommodation.

Adie syndrome is a rare disorder affecting the ciliary ganglion. This disorder results in a pupil that constricts slowly and a lens that has a slow response when trying to focus on near objects. The prevalence of Adie syndrome is unknown, though Adie’s pupil, also called tonic pupil, is fairly common.

Innervation of the left eyeball and extraocular muscles

Photon Illustration / Shocktrek Images / Getty Images

Anatomy

An individual nerve has three major parts: The cell body is where the cellular organelles are located, dendrites carry nerve impulses to the cell body, and an axon carries the nerve impulses away from the cell body.

The nervous system is divided into the central nervous system, which comprises the brain and spinal cord, and the peripheral nervous system, which includes all the nerves and special sense organs outside of the brain and spinal cord. Just as the central nervous system can be divided into the brain and spinal cord, the peripheral nervous system is divided into the somatic and autonomic nervous systems.

The somatic nervous system is involved with conscious perception and voluntary motor responses. Nerve fibers going toward the brain are sensory fibers, which carry information from your senses to your brain. Fibers leaving the nervous system are motor fibers. They innervate your muscles and bones, among other functions.

The autonomic nervous system handles the involuntary control of the body, such as temperature regulation or changes in the small muscles of the eye to facilitate vision. It maintains homeostasis, or a stable state, inside the body. The autonomic nervous system has two divisions: the sympathetic and parasympathetic nervous systems.

The ciliary ganglion is a parasympathetic ganglion, as it has sympathetic and sensory fibers passing through it. A collection of cell bodies outside the nervous system is called a ganglion.

Outside of the brain, 12 pairs of ganglia are symmetrically arranged. Eight of the ganglia are sensory, while the other four are part of the parasympathetic nervous system. These four parasympathetic ganglia are:

  • Otic ganglia
  • Ciliary ganglia
  • Submandibular ganglia
  • Pterygopalatine ganglia

Structure

The ciliary ganglion has parasympathetic nerve fibers coming from the oculomotor (third) cranial nerve. The parasympathetic fibers originate from the upper part of the midbrain in an area called the superior colliculus. The collection of cell bodies in the central nervous system where they originate is called the Edinger-Westphal nucleus. They travel along the third cranial nerve.

The nerve fibers enter the orbit of the eye through the superior orbital fissure. These preganglionic (before reaching the ganglion) fibers synapse with postganglionic fibers in the ciliary ganglion. The postganglionic fibers then enter the eyeball.

Sensory nerve fibers originate from the nasociliary nerve, which comes from the ophthalmic branch of the trigeminal nerve, and enter the eye through the superior orbital fissure.

The sympathetic root fibers travel with the internal carotid artery and enter the orbit through the superior orbital fissure. These sympathetic fibers, like the sensory fibers, pass through the ciliary ganglion uninterrupted.

Location

The ciliary ganglia are located behind each eye in the posterior part of the eye orbit, and are embedded in the fat surrounding the eyeball. The ganglia are about 2 mm horizontally and 1 mm vertically, and contain approximately 2,500 nerve cell bodies. The ciliary ganglia receive their blood supply from the posterior lateral ciliary artery and the lateral muscular arterial trunk.

Relative to other structures around the eye, the ciliary ganglion is:

  • About 1 cm in front of the medial (towards the middle) end of the superior orbital fissure
  • Between the lateral rectus and the optic nerve
  • Temporal (toward the ear) to the ophthalmic artery
  • About 1.5 to 2 cm behind the eyeball

Anatomical Variations

The ciliary ganglion may vary in position with respect to the surrounding structures, and it may vary in size from 1.16 to 3.19 mm in length, 0.81 to 1.93 mm in height, and 0.47 to 0.77 mm in thickness. It may also vary in distance from reference landmarks such as the optic nerve.

Additionally, the size of the bunches of nerve fibers entering and leaving the ciliary body may vary. This variation is important for surgeons to recognize.

Function

The pupil of the eye participates in two reflexes that are controlled by the ciliary muscle:

  • The light reflexes: If light enters the eye, the pupil of that eye contracts (the direct light reflex) along with the pupil of the other eye (consensual light reflex).
  • The near reflex: When you look at a close object, three things happen simultaneously: Your eyeballs converge (look inward toward your nose), the lens becomes more convex (balls up to bend light more), and the pupil constricts (lets less light enter the eye).

The parasympathetic nerve fibers that synapse in the ciliary ganglion travel to the circular pupillary muscles and the ciliary body. The circular papillary muscles surround the pupil, the opening in the center of the iris of the eye. When nerve fibers stimulate these muscles to contract, the pupil becomes smaller.

A decrease in pupil size is a natural response to bright light. Contracting the pupil filters out light rays coming in at an angle. When there is plenty of light, these light rays bounce around inside the eye, making the visual image blurry.

Parasympathetic fibers from the ciliary ganglia also go to the ciliary muscles. The lens of the eye bends light to focus it, much like the lens of a camera bends light to focus the image on the film.

When your eye is looking at a close object, nerve impulses from nerves in the ciliary ganglia stimulate the ciliary muscles to contract. When they contract, the lens of the eye “balls up” and becomes more convex. This process bends light rays more and focuses the image on the retina of the eye.

Sensory nerve fibers pass through the ciliary ganglion without synapsing to carry sensations from the cornea, iris, and ciliary body to the brain.

Sympathetic nerve fibers also pass through the ciliary ganglion without synapsing. They originate from the superior cervical ganglion, near the angle of the jaw. They innervate the muscles that dilate the pupil (make the pupil bigger).

Associated Conditions

Adie’s Pupil

Tonic pupil, also called Adie’s pupil, is fairly common, affecting approximately one of every 500 people. If you have this condition, your pupil will not exhibit the expected response to bright light, rapid constriction to filter out excess light.

It is theorized that the ciliary body is suddenly denervated and then slowly reinnervated. The sudden denervation of the ciliary ganglion results in a dilated pupil that will not respond to light and a loss of accommodation due to the inability of the lens to become more convex. Regeneration of the nerves in the ciliary body happens over one to two years.

Adie Syndrome

Adie syndrome, or Holmes-Adie syndrome, is a rare disorder in which both tonic pupil and absent or poor tendon reflexes are found. The cause is unknown, but it may be due to trauma, surgery, lack of blood flow, or infection. Tendon reflexes are elicited when your healthcare provider taps on a tendon, such as the one below your knee, with a rubber hammer.

Adie syndrome is more common in females than in males. The disorder is most commonly diagnosed in adults between the ages of 25 and 45.

Ross Syndrome

Ross syndrome is a variant of Adie syndrome that affects the ability to sweat as well. If you have Ross syndrome in addition to a tonic pupil and decreased tendon reflexes, you may notice that some parts of your body sweat excessively while others are unable to sweat properly. The inability to sweat can lead to heat intolerance.

The ciliary ganglion can also be injured directly or as a result of surgery. Surgical procedures to repair orbital fractures or remove tumors inside the bony eye orbit can damage the ciliary ganglion.

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8 Sources
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