Wednesday, June 19, 2013

Eyes and Eyelids

Innervation of the Eyes 

The eyes and eyelids have both sympathetic and parasympathetic innervation. Two sets of muscles surround the pupils – radial and sphincter. Radial muscles, which have mostly sympathetic innervation, dilate the pupil; while sphincter muscles, which have mostly parasympathetic innervation, serve to constrict.

The pupils will respond to changes in lighting by attempting to allow a more-or-less constant amount of light onto the retina, constricting if the light is too bright, dilating when it gets darker. Pupils also respond to various emotions; dilating with passionate love, for example.

Pupil responses to emotion operate in a classic reciprocal fashion, as sympathetic tone goes up, parasympathetic goes down, and the pupil dilates. Curiously, pupil responses to light are almost entirely mediated by the PSNS. An increase in parasympathetic tone constricts the pupil, withdrawal dilates it.

Eyelids operate both voluntarily and autonomically. An increase in sympaththetic tone will serve to open the eyelid extra wide, for extra visibility and alertness. This is also a body language cue of sexual attraction.

Horner’s Syndrome

The eyes and eyelids receive post-ganglionic innervation from about C1-T1, just above the level normally targeted in ETS surgery. If T1 is accidentally damaged, sympathetic innervation to the eyes and eyelids may be interrupted, resulting in “Horner’s Syndrome”.

Horner’s Syndrome is characterized by an overly constricted pupil, and a drooping eyelid. The eyeball may sink lower in the eye socket, and suffer soreness, redness and a lack of moisture.