Neuroscience Clerkship

 

 

NEUROANATOMY OF THE PUPILLARY LIGHT REFLEX


 

The pupil provides a window from the outside world to the retina, allowing for the strict regulation of light. This mechanism bestowed an evolutionary advantage to our ancestors, who relied on a keen visual sense to allow them survive from predators, and to hunt for prey in a multitude of illuminations.

This learning objective details the pupillary light reflex, which allows for the constriction of the pupil when exposed to bright light. This reflex serves to regulate the amount of light the retina receives under varying illuminations. The pupillary light reflex two main parts: an afferent limb and an efferent limb.

The diagram below shows the neuroanatomical pathways of the pupillary light reflex. The details of the pathway are detailed below the diagram.
 

 

Afferent Pathway of Pupillary Light Reflex (solid yellow above):

Light enters the pupil and stimulates the retina.

Retinal ganglion cells transmit the light signal to the optic nerve

The optic nerve enters the optic chiasm where the nasal retinal fibers cross to contralateral optic tract. while the temporal retinal fibers stay in the ipsilateral optic tract

Fibers from the optic tracts project and synapse in the pretectal nuclei in the dorsal midbrain in the collicular region

The pretectal nuclei project fibers to the ipsilateral Edinger-Westphal nuclei and also to the contralateral Edinger-Westphal nucleus via the posterior commissure

 

Efferent Pathway of Pupillary Light Reflex (black dotted above):

The Edinger-Westphal nucleus projects pre-ganglionic parasympathetic fibers, which exit the midbrain and travel along the oculomotor nerve (CN III) and then synapse on post-ganglionic parasympathetic fibers in the ciliary ganglion

Ciliary ganglion post-ganglionic parasympathetic fibers (short ciliary nerves) innervate the sphincter muscle of the pupils resulting in pupillary constriction.

The physiological result of the neuroanatomical pathways as described above is that light shined in one eye will result in pupillary constriction in both the ipsilateral pupil (direct pupillary light reflex) and the contralateral pupil (consensual pupillary light reflex).

 

Normal Pupillary Light Response (from top to bottom):

Row 1: pupils in a dark room without light stimulation.

Row 2: intact direct and consensual responses for right eye

Row 3: intact direct and consensual responses for left eye

From the above, we can conclude that both the afferent and efferent limbs of both eyes are intact in the above patient.

In the case of optic nerve dysfunction, such as in optic neuritis, as phenomenon called a relative afferent pupillary defect (RAPD) results. This is due to an impaired afferent limb of the pupillary reflex so that stimulation with light of the ipsilateral affected eye will not result in as much pupillary constriction as stimulation of the normal contralateral eye. A RAPD can be demonstrated by a test called the “swinging-flashlight test” as shown below.

 

Row 1: Unstimulated pupils in a dark room

Row 2: Stimulation of the right eye produces bilateral pupillary constriction, indicating intact afferent right limb, and intact bilateral efferent limbs.

Row 3: When moving the light source from the right to left eye, the left eye paradoxically dilates. This indicates a faulty left eye afferent limb, most likely from left optic nerve dysfunction. Note that the afferent limb of the left eye is not completely non-functional as the pupils are still more constricted then the pupils seen in the unstimulated row 1.

Row 4: Demonstrates again that the right afferent pathway is functioning normally and that the problem is with the left eye’s afferent pathway.