Pupillary light reflex
650978
221204896
2008-06-23T14:30:32Z
66.67.50.74
/* Test for nerve damage */
{{Unreferenced|date=March 2008}}
[[Image:Eye dilate.gif|thumb|250ppx|right|Pupillary light reflex]]
The '''pupillary light reflex''' is the reduction of [[pupil]] size in response to light.
The pattern of pupillary response to light can help determine which of the [[cranial nerve]]s is damaged.
==Types==
There are two types of response assessed for each eye:<ref name="url">{{cite web |url=http://www.med.yale.edu/caim/cnerves/cn3/cn3_12.html |title=yale.edu |format= |work= |accessdate=}}</ref>
*'''Direct pupillary reflex''': whether each pupil constricts with light shone into ''that eye''
*'''Consensual pupillary reflex''': whether each pupil constricts with light shone into ''the other eye''
==Mechanism==
The reflex pathway consists of [[ganglion cell|retinal ganglion cell]]s, which convey information from the [[photoreceptor]]s to the [[optic nerve]] which connects to the [[olivary pretectal nucleus|pretectal nucleus]]<!-- link is not working, should it be replaced with [[Pretectum]]? --> of the high [[midbrain]]. It bypasses the [[lateral geniculate nucleus]] and the [[primary visual cortex]]. From the pretectal nucleus neurons send axons to neurons of the [[Edinger-Westphal nucleus]] whose axons run along both the left and right [[oculomotor nerve]]s. Oculomotor nerve axons synapse on [[ciliary ganglion]] neurons whose axons innervate the constrictor muscle of the [[Iris (anatomy)|iris]].
==Test for nerve damage==
[[Emergency room]] [[physician]]s often assess the pupillary reflex because it is useful for gauging [[brain stem]] function. Normally, pupils react (constrict) equally. Lack of the pupillary reflex or an abnormal pupillary reflex can be caused by optic nerve damage, oculomotor nerve damage, [[brain death]] and depressant drugs, such as [[barbiturates]]. The optic nerve is responsible for the [[afferent]] limb of the pupillary reflex, or in other words, senses the incoming light. The oculomotor nerve is responsible for the [[efferent]] limb of the pupillary reflex and drives the muscles that constrict the pupil.
Normally, each pupil should constrict with light shone into either eye. On testing each reflex for each eye, several patterns are possible.
*[[Optic nerve]] damage on one side:
**The [[ipsilateral]] direct reflex is lost
**The [[contralateral]] direct reflex is intact
**The ipsilateral consensual reflex is intact (because light shone into the opposite eye can signal to the brain, causing constriction of both pupils via the normal oculomotor nerves)
**The contralateral consensual reflex is lost (because light shone into the eye on the damaged side cannot signal to the brain)
*[[Oculomotor nerve]] damage on one side:
**The ipsilateral direct reflex is lost
**The contralateral direct reflex is intact
**The ipsilateral consensual reflex is lost (because light shone into the opposite eye can signal the brain, causing attempted constriction of both pupils, but constriction fails on the damaged side)
**The contralateral consensual reflex is intact (because light shone into the damaged eye can still signal to the brain via the normal optic nerve, causing attempted constriction of both pupils, which works on the other side via the normal oculomotor nerve on that side)
==References==
<references/>
==See also==
*[[Pupillary response]]
*[[Slit lamp]]
==External links==
*[http://library.med.utah.edu/kw/hyperbrain/anim/reflex.html Animation of pupillary light reflex]
*{{MeshName|Reflex,+Pupillary}}
*[http://cim.ucdavis.edu/EyeRelease/Interface/TopFrame.htm A pupil examination simulator], demonstrating the changes in pupil reactions for various nerve lesions.
{{Reflex}}
{{Visual system}}
[[Category:Ophthalmology]]
[[Category:Reflexes]]
[[bg:Зеничен рефлекс]]
[[nl:Pupilreflex]]
[[pl:Odruch źreniczny]]