Retinohypothalamic tract

The retinohypothalamic tract consists of retinal ganglion cells.[3] 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.

Summary of pervasive effects of light

The SCN of the hypothalamus contains an endogenous pacemaker that regulates circadian rhythms.[4] The zeitgeber found to have the most profound effect on the SCN is light, which is the form of stimulation of which conversion is needed for it to be processed by the brain. Neurotransmitters that travel the RHT are responsible for delivering this message to other parts of the brain. If damage is done to this important pathway, alterations in circadian rhythms including phase shifts may occur. Studies done on rats show that even with severely degenerated photoreceptors (blind, no visible light perception), they have the ability to entrain to the light/dark cycle because they have intact RHTs.[5]

A study was conducted to observe the differences in three groups of Sprague-Dawley rats: ones that had part of the RHT pathway cut when it was an adult (AE), ones that had part of the pathway cut within 24 hours of their birth (NE), and a control group.[6] Further development of the brains of those in the NE group showed that the two suprachaismatic nuclei (SCN) have nearly equal inputs shortly after the pathway is cut. This was shown to drastically slow down the re-synchronization of internal biological rhythms to the external time cues, primarily light. Rats in the AE and NE groups similarly reduced the amount of fluid intake during the study during the hours they were exposed to constant light. This may indicate that the intake of water is affected by the number of connections in this pathway and affect the further development of other parts of the brain that are dependent on light.