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The suprachiasmatic nucleus (SCN) is a pair of pinhead-size structure of the hypothalamus, a part of the brain located above the optic chiasm, the region where the optic nerves cross. It controls the circadian rhythm, the rhythmic repetition of certain phenomena at about the same time each day. Also known as the biological clock, it contains about 20,000 neurons that generate neuronal and hormonal signals that regulate the different functions of the body in an approximately 24-hour cycle. Some signals generated by the neurons of suprachiasmatic nucleus include the gamma-aminobutyric acid, vasopressin, vasoactive intestinal peptide, and somatostatin. The different functions of the body synchronized with the signals generated are the sleep-wake cycle, body temperature, changes in blood pressure, urine production, and hormone secretion.
Circadian rhythms can be entrained by the light and dark cycle of the environment. Once light coming from the environment reaches the eyes, specialized light-sensitive retinal cells known as photoreceptors convert the light into electrical signals then transmit it to the brain. These signals that travel along the retinohypothalamic tract produce entrainment. Entrainment is shifting of the phase of suprachiasmatic nucleus activity so that it synchronizes with the light and dark cycle of the environment of the same frequency. During subjective daytime, entrainment is mediated by cyclic adenosine monophosphate and pituitary adenyl cyclase-activating peptide, whereas during subjective nighttime, it is mediated by cyclic guanosine monophosphate and acetylcholine.
Aside from the environmental light signals, the suprachiasmatic nucleus is influenced by melatonin. Melatonin is a hormone synthesized from the neurotransmitter serotonin by the pineal gland. It is both an agent and regulator of the suprachiasmatic nucleus. Its synthesis is circadian, wherein it is inhibited by light and stimulated by darkness. As its level increases during environmental darkness, melatonin sends information to the brain that nighttime has arrived, and body functions entrainment is evoked accordingly.
Several genes have been identified that regulate the circadian rhythm. These are the timeless, period, dclock, and cycle genes. They control the protein expression, leading to a complex intracellular negative feedback system. The negative feedback system accounts for the periodicity of the suprachiasmatic nucleus.
Variation in the rhythm of neuronal and hormonal signaling occurs seasonally. It may reflect the influence of variation in the length, tides, temperature, and sunshine of daylight. In jet lag, the biological clock is disrupted due to a sudden artificial change in clock time from traveling to a different time zone, resulting in an abrupt shift of the neuronal and hormonal signals. Oftentimes, it requires several days to reset back the body functioning to a normal 24-hour cycle.