Pupillary Light Reflex (PLR)

Your pupillary light reflex, or light reflex, is a reflex of the eye that automatically makes your pupils get bigger or smaller.

Your pupils are the black holes in the middle of your irises. They manage how much light gets into your eyes. For example, your pupils shrink if you leave a room without windows for one with lots of windows that let light in. They get bigger to help you see in dim light or darkness. Experiencing strong emotions or pain also triggers this reflex.

The reflex affects both of your eyes, even if the change in light affects one eye. Healthcare providers may refer to this as a consensual reflex. For example, if a healthcare provider shines a light into your right eye, the pupils in both eyes shrink.

How does the light reflex work?

Your pupillary light reflex is a team effort led by your parasympathetic and sympathetic nervous systems. The team includes your retina, iris, brainstem and optic nerve.

Parasympathetic PLR

Your parasympathetic nervous system handles many activities that you don’t control. Making your pupils shrink is one example. This process starts with your photoreceptors in your retina. Photoreceptors are cells that detect light and colors. When a bright light shines into your eyes:

• Photoreceptors turn the light into signals they send to your optic nerve.
• Your optic nerve forwards the signals to your brainstem.
• Nerves in your brainstem contract a muscle in your iris.
• This muscle surrounds your pupil. When it tightens, your pupil gets smaller.

Sympathetic PLR

Your sympathetic nervous system is a group of nerves. These nerves manage your body’s reaction to stressful situations. One reaction makes your pupils bigger to let in more light and make it easier for you to see in dim light or darkness. When that happens:

1. Your hypothalamus activates your sympathetic nervous system.
2. It reacts by sending chemical messages to the muscle in your iris that makes your pupil open.
3. This muscle looks like a sun with rays that extend out from the middle. It tightens to open your pupil from the inside out.

What conditions affect this reflex?

There’s a long list of conditions that may affect your pupillary reflex. Remember, your brainstem, iris, pupil and optic nerve all need to work as they should. Issues with this reflex may be a symptom of conditions like:

• Brain tumors or stroke
• Detached retina
• Eye injuries
• Glaucoma that affects your optic nerve or pupil
• Horner’s syndrome
• Optic neuritis
• Traumatic brain injury
• Uveitis

Tests to check your pupillary light reflex

The most common test is part of a comprehensive eye exam. An eye care specialist shines a light into your eyes to see if both of your pupils shrink. A healthcare provider may check your pupillary light reflex if they suspect you have a condition that affects the reflex. For example, pupils that don’t shrink in light may be symptoms of serious conditions like stroke or a brain injury.