Science: How does the brain recognize day and night?

The human body has normal circadian rhythm, once disturbed by the strong light at night, it is easy to cause insomnia.for those night shift families, their circadian rhythm is disrupted, and the incidence rate of cancer, heart disease, obesity, depression and type 2 diabetes will increase.therefore, it is important to understand how humans perceive light, produce "smart" lights, and maintain a healthy circadian rhythm.DOI:10.1126/science.aaz0898 Recently, scientists from the Salk Institute published relevant research in science. They found three types of intrinsic photosensitive retinal ganglion cells (iprgcs), which can detect light and keep the circadian rhythm of the big brain consistent with the surrounding light.to evaluate the intrinsic light response of iprgcs cells in the human body After that, a small piece of retina was placed on a multi electrode array, and synaptic blockers were added to the recording medium to prevent the excitation output of rod cells and cone cells.the results showed that photosensitive cells appeared in all 5 retinas under 30 seconds of blue light pulse (470 nm).it can be concluded that cells are most sensitive to blue light, which is widely used in cold white LED lights and many electronic devices.the intrinsic response of these cells to light is relatively slow, lasting for 30 seconds, and after the light disappears, the stimulation lasts for several seconds.at the same time, it was found that opsinamide, a specific inhibitor of melanopsin, could inhibit the internal photoresponse of retina, which confirmed that the intrinsic photosensitivity was mediated by melanin.Figure 1: iprgcs are essentially light sensitive. There are three types of iprgcs Three cell types of iprgcs were found: type 1 responded to light relatively quickly, but it took a long time to turn off; type 2 had a long time to turn on and off; type 3 only responded to strong light and switched on and off quickly.among them, type 3 cells are the most abundant.human iprgcs show different subtypes. This new study also helps to explain the circadian rhythm of the blind.although they can't see, they can still make their sleep wake cycle, circadian rhythm consistent with the circadian cycle of time.light signals are sent to the brain via iprgcs, even for those who lack the rod and cone cells needed to transmit images to the brain.research prospects panda, a senior author, said that understanding the functions of each iprgcs type adds a new dimension to the design of TV, computer monitors and smartphone screens.he also mentioned that the next step will be to study the net output of these cells at different light colors, intensities and durations - for example, comparing their responses to short and long pulses of light, and also to exploring the cell's response to light sequences, such as the transition between blue and orange light, mimicking the various lights we encounter in nature at dawn and dusk.left to right: Ludovic MURE and satchin panda In addition, these experiments were repeated in donor retinas of different ages to find out whether there are significant differences in iprgcs function between young and old people, which may help design indoor light for better day night synchronization, and may even contribute to emotional improvement in the elderly and dementia patients.end reference: [1] three types of cells help the brain tell day from sight [2] "functional diversity of humanistically photoresistive final ganglia cells" Science (2019)