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. 2021 Mar 12;15:642745. doi: 10.3389/fnins.2021.642745

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Copyright © 2021 Lorsung, Karthikeyan and Cao.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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A diagram illustrating key steps involved in photic entrainment of the circadian system. (1) Ambient light stimulates intrinsically photosensitive retinal ganglion cells (ipRGCs) in the retina. (2) The axons of ipRGCs travel via the retinohypothalamic tract (RHT) to form synaptic connections with the core neurons of hypothalamic suprachiasmatic nucleus (SCN). Glutamate and pituitary adenylate cyclase activating polypeptide (PACAP), among other neurotransmitters are released at the synapses of the RHT terminals to the SCN neurons. Synaptic activities induce clock gene expression and reset the SCN clock. (3) SCN sends rhythmic outputs to other brain regions and peripheral oscillators to reset their rhythms.