The ciliary ganglion is located on the optic nerve at the back of the eyeball. Parasympathetic fibers innervate the ciliary muscle, the pupil constricting sphincter pupillae muscle, and the inner eye muscles. Lesions in the ciliary ganglion can lead to failure of the eyelid closure reflex; ganglion blockers act nonspecifically to counteract overexcitation in the ganglia, but are less commonly used as medications today.
What is the ciliary ganglion?
The ciliary ganglion is an anatomic structure located on the optic nerve and thus behind the eye. With its 2500 cells, the ciliary ganglion innervates various muscles of the visual organ and represents the link to other ganglia. Neurons that immediately follow a ganglion are called postganglionic neurons. In the peripheral nervous system, ganglia form punctate nodes that are characterized by a particularly high density of nerve cell bodies. They are considered the evolutionary precursors of the central nervous system in general and, in particular, the precursors of the basal ganglia (nuclei basales), which are the core structures in the brain. The ciliary ganglion owes its name to the Latin word for “eyelash” (cilium), which refers to both its spatial and functional relationship to the eye.
Anatomy and structure
The ciliary ganglion has several fibers, each with its own tasks; however, not all of them are interconnected, and they belong to different cranial nerves. Significant for the eyes are the parasympathetic fibers of the ganglion ciliare, which belong to the third cranial nerve (nervus oculomotorius). Medicine includes the ciliary ganglion among the parasympathetic ganglia, since these parts are the main contributors to the anatomical structure and, unlike other fibers, are switched here. In addition, the ganglion includes sympathetic and sensory fibers; however, they have no functional effect on the ciliary ganglion, but merely traverse the nuclear area. It is only in the cervical superciliary ganglion that synapses transmit signals from the sympathetic fibers to the downstream neurons. Sensory fibers, which also pass through the ciliary ganglion, connect the brain to the conjunctiva and cornea. These tracts belong to the nasociliary nerve. Overall, the diameter of the ciliary ganglion is 1-2 mm.
Function and tasks
For the parasympathetic and sensory fibers, the ciliary ganglion is merely a passageway, and their nerve signals remain unchanged in the ciliary ganglion; its actual functions depend on the parasympathetic fibers. Part of it is important for the ciliary muscle (Musculus ciliaris), which attaches on one side to the Bruch’s membrane (Lamina basalis choroideae). The Bruch’s membrane lies between the pigment layer and the choroid and not only demarcates the two layers from each other, but also supports the optimal distribution of water and nutrients. On the other hand, the ciliary muscle is attached to the sclera of the eye as well as Descemet’s membrane. Descemet’s membrane or lamina limitans posterior is a layer in the cornea that has three levels. Zonular fibers connect the ciliary muscle to the lens and can make it more or less curved. The eye uses this mechanism, also known as accommodation, to see objects in focus at different distances. Accommodative disorders can therefore lead to nearsightedness or farsightedness. The nerve pathways that supply the sphincter pupillae muscle also pass through the ciliary ganglion. They belong to the oculomotor nerve. The muscle is responsible for pupil constriction (miosis) and in this way regulates how much light enters the eye. In this process, the nucleus oculomotorius accessorius (also called the Edinger-Westphal nucleus) in the midbrain triggers the signal for the muscle contraction.
Diseases
Lesions on the ciliary ganglion can cause the eyelid closure reflex to fail. Certain chemicals can affect the ganglia in general, and thus the ganglion ciliare. Medicine refers to them as ganglioplegics or ganglion blockers, but rarely uses them as drugs anymore because of their nonspecific effect and the resulting side effects. The mechanism of action in all ganglion blockers is based on molecules inhibiting or completely preventing the activity of neurons. They can therefore no longer trigger electrical signals or transmit information from other nerve cells.One of the ganglion blockers is the active ingredient hydroxyzine, which can be used in cases of extreme allergic reactions; neurodermatitis and severe hives (urticaria) in particular represent indications for hydroxyzine. In addition, the substance is potentially effective against overexcitation, sleep disorders, anxiety and tension states. Hydroxyzine is not approved for use in obsessive-compulsive disorder, psychosis, and thought disorders, but it may possibly alleviate these as well. A particularly potent ganglion blocker is tetraethylammonium ion, which is a neurotoxin because of its powerful effects. Tetraethylammonium ions prevent potassium ions from flowing through cell membrane channels, thereby repolarizing the nerve cell. Amobarbital is also a ganglion blocker and belongs to the barbiturates. The active ingredient is rarely used today and has hardly been on the market since the benzodiazepines replaced it as an important sedative and sleep aid. The situation is similar for carbromal, which has the same effect on the human body. The situation is different with phenobarbital, which can still be used today in the treatment of epilepsy and was once widely used as a sleep aid. The drug can cause side effects including fatigue, drowsiness, headache, dizziness, coordination problems and ataxia, as well as psychological and functional sexual side effects. Because of these side effects and because phenobarbital limits reaction time, patients should not drive machinery, drive a car, or perform other sensitive tasks after taking it. Phenobarbital further plays a role in anesthetic preparation, where such effects are desired.