How does the circadian input system work?
Photic information is transmitted from the retina directly to the oscillator network in the SCN via the RHT. However, the photic information is not communicated by retinal rod and cone cells.
Within the retinal there are non-rod, non-cone cells, the so-called retina ganglion cells (RGCs). These cells contain a photopigment, melanopsin, which is preferentially excited by the short wavelengths, especially blue light.
The RGC/RHT system then can be thought of as a transducer and tract, respectively, for communication of light–dark (LD) information to the SCN.
Normally, the SCN is most active during the day and most inactive at night. SCN activity inhibits the tonic activity of the paraventricular hypothalamic (PVH) nuclei, which normally is a stimulus for melatonin synthesis from the pineal.
So, by SCN inhibition of the PVH, daytime melatonin production decreases. In the absence of light, the PVH stimulates the pineal gland to allow melatonin secretion. It would be easier to understand if the RHT went directly to the pineal gland.
But this would omit levels of integration which are associated with this melanopsin–RHT signal. This chemical LD signal is not just destined for the pineal gland; it also reaches non-SCN circadian centers as well as sleep–wake centers.