Intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are a type of neuron in the retina of the mammalian eye. Unlike the rods and cones, they are not primarily used for vision, but provide a more general measure of light intensity. They are involved in a variety of non-image-forming (NIF) visual functions, including circadian photoentrainment and the pupillary light reflex.

Anatomy

File:Retina-diagram.png

Diagram of the mammalian retina, showing the position of the ipRGCs

ipRGCs are found throughout the retina, but are most densely concentrated in the ganglion cell layer. They are distinguished by their expression of the photopigment melanopsin, which is sensitive to blue light. The cells have long, branching dendrites that allow them to integrate light information over a large area of the retina.

Function

ipRGCs are involved in a variety of non-image-forming (NIF) visual functions. They provide a measure of overall light intensity, and are involved in the regulation of circadian rhythms, sleep, and pupil size.

Circadian photoentrainment

ipRGCs are the primary photoreceptors for the entrainment of circadian rhythms to the light-dark cycle. They send signals to the suprachiasmatic nucleus (SCN) in the hypothalamus, which is the body's master circadian clock.

Pupillary light reflex

ipRGCs also play a key role in the pupillary light reflex, the automatic adjustment of pupil size in response to changes in light intensity. They send signals to the olivary pretectal nucleus (OPN) in the midbrain, which controls the muscles that constrict and dilate the pupil.

Clinical significance

Because of their role in regulating circadian rhythms and sleep, ipRGCs are thought to be involved in a number of sleep and mood disorders, including seasonal affective disorder (SAD) and non-24-hour sleep-wake disorder. They may also play a role in certain types of blindness.

See also

• Retina
• Ganglion cell
• Melanopsin
• Circadian rhythm
• Pupillary light reflex