Retinohypothalamic tract

In neuroanatomy, the retinohypothalamic tract (RHT) is a photic neural input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells (ipRGC), which contain the photopigment melanopsin. The axons of the ipRGCs belonging to the retinohypothalamic tract project directly, monosynaptically, to the suprachiasmatic nuclei (SCN) via the optic nerve and the optic chiasm. The suprachiasmatic nuclei receive and interpret information on environmental light, dark and day length, important in the entrainment of the "body clock". They can coordinate peripheral "clocks" and direct the pineal gland to secrete the hormone melatonin. The retinohypothalamic tract consists of retinal ganglion cells. A distinct population of ganglion cells, known as intrinsically photosensitive retinal ganglion cells (ipRGCs), is critically responsible for providing non-image-forming visual signals to the brain. Only about two percent of all retinal ganglion cells are ipRGCs, whose cell bodies are in mainly the ganglion cell layer (and some are displaced in the inner nuclear layer of the retina). The photopigment melanopsin is present on the dendrites of ipRGCs, giving ipRGCs sensitivity to light in the absence of rod or cone input. The dendrites spread outwards from ipRGCs within the inner plexiform layer. These dendrites can also receive more canonical signals from the rest of the neuroretina. These signals are then carried through the optic nerve, which projects to the suprachiasmatic nucleus (SCN), anterior hypothalamic area, retrochiasmatic area, and lateral hypothalamus. However, a major portion of the RHT ends in the SCN. Glutamate levels in the RHT are measured by means of immunoreactivity. Retinal nerve terminals display a significantly higher content of glutamate immunoreactivity than the postsynaptic dendrites and non-retinal terminals. The higher immunoreactivity in terminals shows that is readily available before transmission and is used up as the electrical signals travel along the RHT.