Download PUPILLARY LIGHT REFLEX PATHWAY

Kufa college of medicine, Department of medical physiology
Dr.Zaid Yousif Shukur
M.B.Ch.B , M.Sc. , ICO , D.O
PUPILLARY LIGHT REFLEX PATHWAY
Kufa college of medicine
Department of medical physiology
By Dr.Zaid Yousif Shukur
M.B.Ch.B , M.Sc. , ICO , D.O
The pupil is round, regular shape and nearly equal in size. There may rarely be a
physiological difference in pupil size (physiological anisocoria). Each pupil is
located with its center a little below and slightly to the nasal side of the center of
the cornea. The pupils are dilated in excitement, in the dark, and with mydriatic
agents like atropine, conversely they are constricted in neonates, during sleep, with
ocular convergence as part of the near reflex, with use of the miotic agents like
acetylcholine and pilocarpine, and also they are constricted with light directed
toward the eye. The pupils reach their maximum diameter at about the age of 21
years of age and constricted thereafter to become small in old age (involutional
miosis). The pupil is regarded miotic if its diameter is less than 2 mm and
mydriatic if they are more than 6 mm.
Light reflex
The light reflex consists of a simultaneous and equal constriction of the pupils in
response to illumination of one or the other eye. The afferent pupillary pathway
originate in the retina , the axons of retinal ganglion cells pass into the optic nerve
and decussate in the chiasm and pass with the optic tract to the midbrain, the
pupillary fibers do not synapse with the visual fibers in the lateral geniculate
body(LGB)but pass to the pretectal nuclei (PTN) at the level of the superior
colliculus with intercalated fibers that pass as the efferent pupillary pathway to the
Edinger-Westphal nucleus (EWN)of the oculomotor nerve (CN III)on each side
(both sides) Preganglionic parasympathetic fibers run in the oculomotor nerve as it
leaves the brain stem. The fibers pass downward to lie inferiorly in the inferior
division of the third nerve as it enters the orbit. These fibers synapse in the ciliary
ganglion and give rise to postganglionic parasympathetic myelinated short ciliary
nerves, about 3%-5% of which are pupillomotor. The rest are designated for the
ciliary muscle and are concerned with the near reflex.
Baseline Pupil Size
The most important influence on pupillary size is the amount of ambient light.
The status of retinal adaptation also affects pupil size. Sleep results in
decreased sympathetic activity and therefore somewhat smaller pupils. Pupils
also, overall, become smaller with age. Arousal, excitement, or startle
increases pupil size. Elevated intraocular pressure may result in enlargement
of the pupil, possibly by inducing ischemia in the iris. Anoxic brain injury
often results in pupillary dilatation, probably from midbrain ischemia.
Anisocoria:
Efferent disturbances of the pupil size are usually unilateral, which produces
inequality in the diameters of the two pupils. This condition is called
anisocoria. Although it is important for the clinician to look for anisocoria
when assessing the pupil, the presence of anisocoria does not necessarily
represent an abnormality. In many individuals, anisocoria is physiologic.
Physiologic Anisocoria
Physiologic anisocoria (also known as simple or essential anisocoria) is the
most common cause of a difference in pupillary size. About 20% of
individuals have noticeably different diameters in their two pupils. A pupillary
inequality of 0.2 mm can be detected clinically, and anisocoria of 0.4 mm is
readily visible. Anisocoria is more common in older adults. The amount of
anisocoria in an individual can vary from day to day.
The Pupillary light reflex
Pupillary light reflex should be studied in illumination just bright enough to
permit observation of the pupils, the subject is directed to gaze in a distance,
attention should be directed to any difference in size(anisocoria),
shape(dyscoria), or position(corectopia). When the amount of light falling on
the eye is increased, the pupil will constrict, there is a latent period of(0.18)
second then constriction begins, and maximum contraction of the sphincter
pupillae occur about 1 second after the beginning of the stimulation. Light
falling on one eye will cause pupillary constriction of the other eye
simultaneously and to a similar degree (Indirect or consensual pupillary light
reflex)