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Circadian Rhythms – Our Internal Body Clock

Circadian Rhythm

This is often defined as an approximately-24-hour cycle in the physiological processes of living beings, including plants, animals, fungi and bacteria. The first evidence of circadian rhythms were observed in the fourth century BC, in the form of day and night (diurnal) movements of plant leaves. Circadian rhythms are believed to have begun in the earliest cells, for the function of protecting replicating DNA from high ultraviolet radiation during the daytime. For most of us, circadian rhythms are the reason we wake up at our usual workday hour, even on a sleepy Sunday.

Circadian rhythms follow certain patterns: the rhythm persists in constant conditions with a period of about 24 hours (following the diurnal, day-nighttime pattern); and the rhythm period can be reset by exposure to a light or dark pulse (when your body clock adjusts after a few days in a foreign country).

Circadian rhythms are important in determining the sleeping and feeding patterns of all animals and human beings, with biological patterns of brain wave activity, hormone production, cell regeneration connected to this daily cycle. Our circadian "clock" is located in the suprachiasmatic nucleus (SCN), a group of cells in the bilateral region of the hypothalamus. The clock acts on neural and endocrine pathways to regulate individual circadian rhythms so that internal state varies predictably over 24 hours. Disruption to diurnal rhythms usually has a negative effect such as the effect known as jet lag, with its symptoms of fatigue, disorientation and insomnia.

A series of findings over the past decade has begun to identify the brain circuitry and neurotransmitters that regulate our daily cycles of sleep and wakefulness. The neurotransmitters are interconnected with a network of cell groups that activate the thalamus and the cerebral cortex.

During sleep, a chemical switch in the hypothalamus turns off this arousal system during sleep. The absence of other hypothalamic neurons to stabilize this switch can result in an incorrect altering of behavioral states, such a narcolepsy.

Chemical Switch For Circadian Rhythms Found

Researchers at the University of California, Irvine announced that they have found an answer to how circadian rhythms are controlled in the body. Published in the scientific journal Nature, and to everyone’s surprise, it turns out that a single amino acid activates the genes that adjust circadian rhythms. By modifying the single amino acid in an organic protein molecule called BMAL1, the switch is turned on for adjusting the body’s “internal clock.”

Through a process called “the interplay of interconnected transcriptional–translational feedback loops,” the CLOCK–BMAL1 complex triggers crypto chromes (clock-controlled genes) that respond as repressors. Through the enzymatic chemistry of intrinsic histone acetyltransferase activity, chromatin-remodelling events occur to alter the circadian transcription of the crucial genes.

New Discoveries Lead to a Variety of Circadian Cycles

Just when you think you understand circadian rhythms, scientists discover these internal rhythms are far more complicated than first thought. It seems that now we really have three different types! The first type is the one we’ve already covered – the diurnal cycle. This cycle is regulated by a gene, which encodes proteins called PER (per mRNA). Drug companies are constantly working on ways to manipulate the level of PER in the body to treat sleep disorders.

Second Mode of Internal Clock

This is believed to function at the millisecond level. Research at the UCLA Brain Research Institute (published in the journal Neuron) is examining this sensitive mechanism for measuring time intervals as short as 10 or 20 milliseconds – less than the blink of an eye. Auditory, visual and touch centers of the brain seem to have this timing capacity. For example, to understand speech, we process the millisecond intervals in voice onset time to interpret the verbal timing, and ultimately, the real meaning (“reading between the lines”).

Third Mode of Internal Clock

Also called “interval timing,” was recently the subject of a new article in the journal Nature Reviews Neuroscience, published by researchers at Duke University. This brain mechanism is used to process functions that require seconds to minutes of attention. For example, to decipher the pacing of what was said, to organize our thoughts clearly and to respond back in a prompt manner.

Unlike the first two modes of internal clock, scientists theorize that interval timing does not reside in just one location of the brain, because it has to be relayed and distributed so it can combine information from all the senses. The current hypothesis is that the different parts of the brain oscillate and all these oscillations are monitored and integrated by certain circuits, perhaps in the basal ganglia, an area of the brain that integrates incoming data.

For now, the only substantial proof for this third mode of timing is through studying the brains of Parkinson’s patients. As one of the papers authors comments, when the patients are on their medications, their timing is normal. However, when the medication wears off, “we can see their clock slow down by recording their brain signals.”

Source: Wikipedia (en.wikipedia.org) What makes us tick? Functional and Neural Mechanisms of Interval Timing Nature Reviews Neuroscience 6, 755-765 (October M2005) Regulation of Spike Timing in Visual Cortical Circuits Nature Reviews Neuroscience 9, 97-107 (February 2008) Decoding Temporal Information: A Model Based on Short-Term Synaptic Plasticity The Journal of Neuroscience February 1, 2000, 20(3):1129-1141 How Does Your Brain Tell Time? University of California Newsroom (www.universityofcalifornia.edu/news/article/8874)

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