What is the neuroanatomical integration of the pupillary reflex arc?

From the Guidelines

The pupillary reflex arc is a complex neuroanatomical pathway that involves the integration of afferent and efferent signals to control pupil size in response to light, with the most recent evidence from 2023 supporting the importance of assessing pupillary reactivity and afferent pupillary defects in pediatric eye evaluations 1. The pupillary reflex arc begins when light enters the eye and stimulates photoreceptors in the retina, which then activate retinal ganglion cells. These ganglion cells send afferent signals through the optic nerve (CN II), with most fibers continuing in the optic tract to the pretectal nuclei in the midbrain.

• Key structures involved in the pupillary reflex arc include: + Optic nerve (CN II) + Pretectal nuclei in the midbrain + Edinger-Westphal nuclei + Oculomotor nerve (CN III) + Ciliary ganglion + Pupillary sphincter muscle From the pretectal nuclei, signals travel to the Edinger-Westphal nuclei in both sides of the brain, which contain preganglionic parasympathetic neurons.
• The sympathetic pathway, which dilates pupils, involves a three-neuron chain from the hypothalamus through the spinal cord, superior cervical ganglion, and finally to the pupillary dilator muscle, allowing for precise control of pupil size in response to changing light conditions and other stimuli, as noted in the assessment of pupillary reactivity in pediatric eye evaluations 1. The integration of parasympathetic and sympathetic inputs is crucial for controlling pupil size, and any defects in this pathway, such as a relative afferent pupillary defect, can indicate underlying problems with the optic nerve or other areas in the anterior visual pathway 1.

From the Research

Neuroanatomical Integration of the Pupillary Reflex Arc

The pupillary reflex arc is a complex neuroanatomical pathway that involves the integration of various brain regions and nerve fibers. The key components of this pathway include:

• Retinal ganglion cells, which detect light and send signals to the olivary pretectal nucleus (OPN) 2
• The OPN, which projects to the Edinger-Westphal (EW) nucleus and plays a central role in the pupillary constriction response 3, 2
• The EW nucleus, which exerts parasympathetic control over the iris musculature via the ciliary ganglion 3
• The accessory oculomotor nuclei (AON) and the periaqueductal gray (PAG), which receive input from the OPN and influence the pupillary light reflex (PLR) 3
• The sympathetic nervous system, which causes mydriasis in response to arousing factors and projects to the superior cervical ganglion (SCG) 4, 3

Pathways and Projections

The pupillary reflex arc involves several pathways and projections, including:

• The classical reflex arc, which comprises retinal ganglion cells, midbrain interneurons, oculomotor nerve, and short ciliary nerves 5
• The OPN-EW pathway, which is the primary pathway for the PLR 3, 2
• The AON-PAG pathway, which provides indirect control over the PLR 3
• The OPN-PAG-spinal cord pathway, which transfers light-sensitive information to the sympathetic SCG 3
• The cortical pathway, which involves the visual cortex and is thought to play a role in the integration of suprageniculate neurons into the pupillomotor pathway 6

Integration and Control

The integration and control of the pupillary reflex arc involve the coordination of multiple brain regions and nerve fibers. The key features of this integration include:

• The bilateral projection from the OPN to the EW nucleus, which contributes to the consensual pupillary light reflex 2
• The bilateral retinal projection to the OPN, which is the more determinant component of the pathway 2
• The influence of the AON and PAG on the PLR, which provides indirect control over the pupillary response 3
• The role of the visual cortex in the integration of suprageniculate neurons into the pupillomotor pathway, which supports the involvement of a cortical pathway in the PLR 6