Our eyes are the window to our world. As it turns out, they are also critical for regulating our day-night clock. The human circadian rhythm is governed by a complex interaction between real world light and our brains internal clock setting mechanisms. Normal outdoor light, as well LED light, smartphones, TV’s, and other electronic devices stimulate our brain to stay awake and alert. This happens when the blue and green components of the light source stimulate specialized retinal cells in the eye, to message our brains that melatonin should be suppressed and body temperature maintained. These cells, called intrinsically photosensitive retinal ganglion cells (ipRGCs), comprise only 2% of all photoreceptors in the retina. Nevertheless, they are extremely active in controlling a wide array of non-image forming inputs to the brain.
The ipRGCs contain a photopigment, melanopsin, which is responsible for the detection of the light. In a process called phototransduction, a biochemical cascade takes place which results in the depolarization of the axon and transmission of the light signal to an area of the hypothalamus called the suprachiasmatic nucleus (SCN). As the axons exit the eye through the optic nerve, they then pass through the optic chiasm, and finally reach the SCN. This specialized monosynaptic pathway is called the retino-hypothalmic tract (RHT).
Once the SCN has received information about external light through the ipRGCs, It can then signal the melatonin secretion process in the pineal gland. This process, which establishes the rhythm of the daily 24 hour cycle, is known as circadian entrainment. The SCN is also directly involved in sleep temperature regulation and the release of cortisol. Exciting new research has not only validated this process, but has also shown there are additional neurological pathways that are impacted by the light transmitted through the ipRGCs.
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