What kinds of cells sense light?
Light-sensitive cells in the retina signal through the optic nerve to the lateral geniculate nucleus in the thalamus, which then sends visual signals to the visual cortex, which forms perceptions of light, color, and pattern. There are three classes of light-sensitive cells in the retina: rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs). Rods contain the photopigment rhodopsin (peak excitation at 498 nm) and rods are important for vision in dim light. Three types of cones are responsible for color vision: red light excites cones containing L-opsin (peak excitation at 564 nm), green light excites cones containing M-opsin (peak excitation at 534 nm), and blue light excites cones containing S-opsin (peak excitation at 420 nm). ipRGCs are very sparsely distributed throughout the retina, but contribute to perception of visual contrast (Allen et al., 2019). ipRGCs also receive signals from rods and cones.
Allen AE, Martial FP, Lucas RJ. Form vision from melanopsin in humans. Nat Commun. 2019 May 22;10(1):2274. https://doi.org/10.1038/s41467-019-10113-3
How does light control circadian rhythms?
In the retina, light excites the photopigment melanopsin in intrinsically photosensitive retinal ganglion cells (ipRGCs). There are at least 3 types of ipRGCs in humans and types 1 and 2 are preferentially sensitive to blue light (peak excitation at 459 nm and 457 nm, respectively). Type 3 cells respond to a broad range of wavelengths (447-560 nm) relatively equally, although also with a slight peak in the blue range around 470 nm (Mure et al., 2019). The ipRGCs initiate signalling to the hypothalamus and pineal gland that together regulate circadian rhythms, the coordination of the sleep/wake cycle with the dark/light cycle. An important aspect of this is the release of the hormone melatonin by the pineal gland in the dark.
As a hormone, melatonin travels through the bloodstream to other areas of the brain and body. In addition to regulating circadian rhythm, melatonin, a potent antioxidant, also decreases pain and decreases the transcription of the neuroinflammatory peptide, calcitonin gene-related peptide (CGRP) that plays a role in the pathology of migraine, dry eye, and concussion. For this reason, melatonin might be able to play a role in reducing inflammation caused by photophobia signalling and melatonin supplements are being studied for the treatment and prevention of migraine headaches (reviewed in Peres et al., 2019).
Mure LS, Vinberg F, Hanneken A, Panda S. Functional diversity of human intrinsically photosensitive retinal ganglion cells. Science. 2019 Dec 6;366(6470):1251-1255. https://doi.org/10.1126/science.aaz0898
Peres MF, Valença MM, Amaral FG, Cipolla-Neto J. Current understanding of pineal gland structure and function in headache. Cephalalgia. 2019 Nov;39(13):1700-1709. https://doi.org/10.1177/0333102419868187
What is the pupillary light reflex?
Light causes the constriction of both pupils. In the dark, the pupils dilate.
Light excites retina ipRGCs which signal through the midbrain's olivary pretectal nucleus to then signal through the oculomotor nerve to cause constriction of both pupils.
The accommodation-convergence reflex:
How do the eyes change focus?
In order to focus on near objects:
The pupil constricts, increasing the depth of focus by reducing light scatter. This limits the amount of light that reaches the fovea, the most sensitive part of the retina, so that the light that does reach the fovea is that of the image upon which the gaze is directed.
Ciliary muscles increase the curvature of the lens, shortening the focal length (accommodation)
Eyes turn inward (convergence)
This coordinated response is controlled by complex signaling that includes the retina, lateral geniculate nucleus in the thalamus, visual cortex, visual association cortex (detects when images are out-of-focus), and multiple regions of the midbrain.
Eye movements may be reflexive or intentional. Eyes turn inward to focus on close objects. Eye movement might compensate for head movement to maintain the gaze on a fixed object (vestibulo-ocular reflex) or might track a moving object. Gaze shift might be use a smooth pursuit or saccades to rapidly jump to a new position. Both eye and neck muscles can participate in gaze shift. These various eye movements employ the visual system, other sensory input, and motor control (reviewed in Straube & Büttner, 2007).
Straube, A. & Büttner, U., Neuro-Opthamology: Neuronal Control of Eye Movements. Developments in Opthamology, 40 (2007). https://www.karger.com/Book/Toc/232644