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The Biological Basis of Non-Image-Forming Vision
Published in Agnieszka Wolska, Dariusz Sawicki, Małgorzata Tafil-Klawe, Visual and Non-Visual Effects of Light, 2020
Agnieszka Wolska, Dariusz Sawicki, Małgorzata Tafil-Klawe
The paired suprachiasmatic nuclei (SCN) located in the anterior hypothalamus, dorsal to the optic chiasm, are the primary circadian oscillator in the brain. Each nucleus is composed of about 10,000 interconnected small neurons, expressing circadian rhythmicity in the rate of action potential firing and in the gene expression. Several animal studies confirmed that SCN is the site of a circadian oscillator whose neurons rhythmically alter their metabolism and activity/firing rate [Sollars et al. 2015]. Retinal projection terminating in the SCN (the retinohypothalamic tract) enables direct synchronization with the light/darkness cycle and entrainment to the environmental day/night cycle (entrainment means more than synchronization – it allows for great plasticity and adaptation) (Figure 3.3). The ipRGCs – melanopsin photoreceptors located at the beginning of the retinohypothalamic tract – have spectral characteristics: blue light (480 nm) is the strongest stimulus, whereas red light (≥600 nm) has a minimal effect on melanopsin response. Daytime sunlight contains more blue wavelengths than sunset. As the sun reaches the horizon, short wavelengths are scattered in the atmosphere and longer, redder wavelengths more easily reach the surface of the earth [Bedrosian et al. 2017]. These authors suggest that “the sensitivity spectrum of melanopsin may be an adaptation to the natural solar cycle, so that ipRGCs are tuned to discriminating daylight from evening, better entraining the circadian rhythm”. Light detected by ipRGCs sets the molecular clock in the SCN.
Circadian System and Diurnal Activity
Published in Anthony N. Nicholson, The Neurosciences and the Practice of Aviation Medicine, 2017
At the same time, Moore (Moore and Lenn, 1972; Moore and Eichler, 1972) was using a different approach to finding the clock. Many studies had shown that there was an intimate relationship between the clock and light. The approach was to follow the light path as it came in through the eye. Moore and his co-workers injected radioactive amino acids into the eyes of rats. They followed the tracer molecules as they travelled from the eye, through the optic nerves, and concentrated in the suprachiasmatic nucleus (SCN) as well as the known visual structures. The newly found pathway from the eye to the SCN was called the retinohypothalamic tract (RHT). When the SCN was lesioned, there was a loss of the circadian rhythm of the hormone corticosterone. Essentially, Richter, Moore, Stephan and Zucker had established that removing the whole of the SCN (but not just a part of it) destroyed circadian rhythms in both behavioural and endocrine outputs. Just 20,000 or so cells seemed to be responsible for controlling the timing of the endogenous rhythms of mammals. Confirmation came from studies on the metabolism of the SCN. Schwartz and Gainer (1977) injected 2-deoxyglucose, and found that the SCN was metabolically active during the light phase of a 12h:12h light–dark cycle and relatively inactive during the dark phase. Significantly, no other brain region showed such a dramatic rhythm.
Contributing Factors of Accident Occurrence
Published in Koji Fukuoka, Safer Seas, 2019
Circadian mechanism is a daily cycle of alertness and sleep and works as an internal body clock, regulating circadian rhythms. Internal body clock is regulated by the superchiasmatic nucleus (SCN), a small cluster of nerve cells near hypothalamus in the brain. Information of light and darkness from the eyes are conveyed through the Retinohypothalamic tract to the SCN, which synchronizes our body’s rhythms to a 24-hour cycle (Moore-Ede 1993).
A Review of Human Physiological Responses to Light: Implications for the Development of Integrative Lighting Solutions
Published in LEUKOS, 2022
Céline Vetter, P. Morgan Pattison, Kevin Houser, Michael Herf, Andrew J. K. Phillips, Kenneth P. Wright, Debra J. Skene, George C. Brainard, Diane B. Boivin, Gena Glickman
From the retina, light information is transmitted to multiple targets in the human brain via two major pathways. The visual pathway employs the optic nerve, chiasm and tract, which sends information to structures involved in image formation, including the lateral geniculate nucleus (LGN), intergeniculate leaflet (IGL) and visual cortex of the occipital lobe. The retinohypothalamic tract (RHT) is responsible for carrying light information from the retina to the suprachiasmatic nuclei (SCN) in the hypothalamus. The SCN serves as the biological clock in mammals and has numerous downstream connections with other central nervous system structures, including the spinal cord and brain (e.g. septum, thalamus, midbrain and other regions of the hypothalamus). The RHT also projects to other nonvisual nuclei and regulatory centers of the brain that are independent of the circadian pacemaker (Gooley et al. 2003; Hattar et al. 2006).