In synchrony with the rotation of the earth, all biological organisms have an internally controlled or endogenous cycle referred to as a circadian rhythm. During this approximately 24-hour period, organisms undergo a cyclical regulation of physiological events and behaviors. In mammals, this internal clock is driven by a nerve bundle in the hypothalamus called the suprachiasmatic nucleus (SCN). To synchronize the body with the local environment, the SCN uses external cues, such as light and temperature. An adaptive mechanism called temperature compensation prevents variation in external temperature from disrupting the body’s daily cycle. Using information from the eyes about the length of day and night, the SCN regulates the sleep-wake cycle. Additionally, the SCN acts as a pacemaker for daily rhythms in peripheral cells and tissues, resulting in the regulation of many other physiological processes including release of hormones at specific times, changes in body temperature, peak times for cell regeneration, and digestion.
In humans, the circadian rhythm forms during the first few months of life and starts to deteriorate in advanced age. Without changes in influencing external cues or disease to disrupt the rhythm, it should remain unchanged.
In the late 1960s, scientists discovered that vision and its pathways had a substantial role in the metabolic processes of the hypothalamus, a region in the brain that regulates metabolism through the autonomic nervous system. These scientists discovered that in rodents, the retinohypothalmic tract (RHT) projected from the eye directly to the SCN. Upon further research, lesions performed in mice to this part of the hypothalamus resulted in arrhythmia of the circadian clock. With this function established, follow-up research focused on understanding the function of the SCN in circadian rhythm.
In one critical set of experiments, neuroscientists found that with a total lesion of the SCN, the endogenous clock ceases to function. However, when new SCN tissue from another animal species with a shorter circadian period was grafted to the animal with the lesion, the rhythm was restored. Additionally, the period exhibited was that of the donor animal and not of the receiving animal. These experiments confirmed that the SCN acts to synchronize all biorhythms of the organism and that the period of the daily cycle is determined by this group of cells.
One of the defining factors of circadian rhythms is the ability to be entrained, or adapt to the local environment, particularly light. As light enters the eye, several types of photoreceptive cells in the retina are activated, including rods, cones, and photosensitive retinal ganglion cells that contain the light-sensitive pigment melopsin. Activation of these cells sends information about the amount of environmental light along the RHT, which directly synapses to the SCN via the optic nerve. The SCN passes this information to other parts of the hypothalamus and the pineal gland, which modulates the secretion of a hormone called melatonin throughout the day. Increased amounts of melatonin are released during the nighttime hours. Melatonin inhibits activation of cells in the SCN, ultimately resulting in the modulation of the sleep-wake cycle.
Circadian Rhythms and the Sleep-Wake Cycle
The sleep-wake cycle includes approximately 8 hours of nocturnal sleep and 16 hours of daytime wakefulness. The circadian system works with the homeostatic system to maintain this cycle via mutually inhibited groups of neurons in the hypothalamus and brainstem. From the moment a person wakes, adenosine levels rise in the blood. As adenosine blood levels continue to increase throughout the day, the need for sleep significantly increases. This occurs regardless of the time of day when that person wakes up. The secretion of melatonin from the pineal gland keeps the circadian system synchronized with external light cues and promotes wakefulness during the daylight hours. However, both photic (light) and nonphotic (food availability and temperature) cues can promote a sleep-wake schedule most adaptive for the organism. A normal circadian rhythm is characterized with increased desire for sleep between midnight and dawn, but this can be artificially affected with indoor lighting and blackout conditions. Artificial changes to light conditions can change the circadian rhythm but seem most disruptive to the sleep-wake cycle when they occur in the morning hours.
B. Dnate’ Baxter
See also: Chronoception; Cones; Optic Nerve; Retina; Rods; Sensory Receptors; Visual System
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