The retinohypothalamic tract is a photic input pathway involved in the circadian rhythms of mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells

Photosensitive ganglion cell

Photosensitive ganglion cells, also called photosensitive Retinal Ganglion Cells , intrinsically photosensitive Retinal Ganglion Cells or melanopsin-containing ganglion cells, are a type of neuron in the retina of the mammalian eye.They were discovered in the early 1990sand are, unlike other...

 (ipRGC), which contain the photopigment melanopsin

Melanopsin

Melanopsin is a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light. In structure, melanopsin is an opsin, a retinylidene protein variety of...

. The axons of the ipRGCs belonging to the retinohypothalamic tract project directly, monosynaptically, to the suprachiasmatic nuclei via the optic nerve

Optic nerve

The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...

 and the optic chiasm

Optic chiasm

The optic chiasm or optic chiasma is the part of the brain where the optic nerves partially cross...

. The suprachiasmatic nuclei receive and interpret information on environmental light, dark and day length, important in the entrainment

Entrainment (chronobiology)

Entrainment, within the study of chronobiology, occurs when rhythmic physiological or behavioral events match their period and phase to that of an environmental oscillation. A common example is the entrainment of circadian rhythms to the daily light–dark cycle, which ultimately is determined by...

 of the "body clock". They can coordinate peripheral "clocks" and direct the pineal gland

Pineal gland

The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...

 to secrete the hormone melatonin

Melatonin

Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...

.