Pupillary light reflex 

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Pupillary light reflex

The pupillary light reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retina of the eye. Greater intensity light causes the pupil to become smaller (allowing less light in), whereas lower intensity light causes the pupil to become larger (allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye.1

Contents

Mechanism

The pupillary reflex pathway begins with retinal ganglion cells, which convey information from photoreceptors to the optic nerve (via the optic disc). The optic nerve connects to the pretectal nucleus of the upper midbrain, bypassing the lateral geniculate nucleus and the primary visual cortex. From the pretectal nucleus, axons connect to neurons in the Edinger-Westphal nucleus, whose axons run along both the left and right oculomotor nerves. Oculomotor nerve axons synapse on ciliary ganglion neurons, whose axons innervate the constrictor muscle of the iris.1

To summarize: The optic nerve is responsible for the afferent limb of the pupillary reflex - it senses the incoming light. The oculomotor nerve is responsible for the efferent limb of the pupillary reflex - it drives the muscles that constrict the pupil.1

Clinical significance

In addition to controlling the amount of light that enters the eye, the pupillary light reflex provides a useful diagnostic tool. It allows a physician or opthamologist to test the integrity of the sensory and motor functions of the eye.1

Under normal conditions, the pupils of both eyes respond identically to a light stimulus, regardless of which eye is being stimulated. Light entering one eye produces a constriction of the pupil of that eye (the direct response), as well as a constriction of the pupil of the unstimulated eye (the consensual response). Comparing these two responses in both eyes is helpful in locating a lesion.21

For example, a direct response in the right pupil without a consensual response in the left pupil suggests a problem with the motor connection to the left pupil (perhaps as a result of damage to the oculomotor nerve or Edinger-Westphal nucleus of the brainstem). Lack of response to light stimulation of the right eye if both eyes respond normally to stimulation of the left eye indicates damage to the sensory input from the right eye (perhaps to the right retina or optic nerve).1

Emergency room physicians routinely assess the pupillary reflex because it is useful for gauging brain stem function. Normally, pupils react (i.e. 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.

Normally, both pupils should constrict with light shone into either eye alone. On testing each reflex for each eye, several patterns are possible.3

References

  1. ^ a b c d e f Purves, Dale, George J. Augustine, David Fitzpatrick, William C. Hall, Anthony-Samuel LaMantia, James O. McNamara, and Leonard E. White (2008). Neuroscience. 4th ed.. Sinauer Associates, 290-1. ISBN 978-0-87893-697-7. 
  2. ^ "Cranial Nerve III—Oculomotor Nerve". yale.edu. Retrieved on 2008-07-27.
  3. ^ Colman, Andrew M. (2001). A Dictionary of Psychology. Oxford University Press, USA. 

See also

External links

v • d • e
Nervous system physiology: neurophysiology - reflex
Cranial nerve
Tendon reflexes
Primitive reflexes
Lists
Other
v • d • e
Sensory system – Visual system and eye movement
Vision
Tracking → PPRF
Horizontal gaze
Vertical gaze
Pupillary dilation
Pupillary light reflex (constriction)
Accommodation
Vestibulo-ocular reflex
Circadian rhythm