Sleep patterns, C3.1.11

• What is a circadian rhythm?

  Circadian rhythms are the physiological and behavioural changes of an organism, over a roughly 24-hour cycle. They dictate multiple body processes, including alertness, appetite and body temperature.

  Circadian rhythms exist in life forms from unicellular bacteria to all multicellular organisms.

  Although circadian rhythms are synchronised with light and darkness, they will continue even if an organism is placed in continuous darkness or light.
• What is the suprachiasmatic nucleus?

  In mammals, the pacemaker of the circadian rhythm is two groups of cells in the hypothalamus, known as the suprachiasmatic nucleus (SCN).

  Neurons in the SCN produce a circadian rhythm of neuron firing frequency that allows them to synchronise other cells in the body. They maintain circadian rhythm even when grown in vitro culture, with no external cues

  In the brain, visible light synchronises the rhythm of the SCN to the day-night cycle. Cells in the eye's retina sense wavelengths of light and send a neural impulse to the SCN, that regulates the circadian rhythm.
• How are melatonin and light synchronised?

  When the retina detects darkness, the SCN stimulates melatonin release from the pineal gland, which is a small endocrine gland that is located in the centre of the brain. When it detects light, it inhibits secretion of melatonin from the pineal gland.

  Melatonin is an amine hormone that integrates the whole body response to the circadian rhythm. Due to the synchronisation between the circadian cycle and light/dark cues, the amount of melatonin circulating in the blood is low in the day, and high at night. Teenagers have a natural shift in their timing of melatonin release, typically around two hours delayed in comparison to adults.
• What are the effects of high melatonin in humans?

  Reduces blood pressure

  Reduces kidney production of urine

  Drops core body temperature

  Enhances helper-T immune responses, and reduces inflammation responses

  Effects sexual maturation through regulation of gonadotropin-releasing hormone (GnRH) gene expression

  Alters blood flow to different tissues
• What does melatonin secretion trigger?

  Melatonin secretion triggers a signalling pathway, which is process in which binding of an extracellular chemical to a receptor is translated into changes in the cell.

  The signalling pathway has three main steps:

  Reception - cell detects signal

  Transduction - Activating a change

  Response- change that occurs
• What is melatonin reception?

  Melatonin is hydrophilic, so the transmembrane melatonin receptor is found on the surface of target cells. The two primary melatonin receptors in humans are a type of G-protein coupled receptor. Many cells have these, including those in the heart, kidney, lung, etc.

  Melatonin receptor activation occurs when melatonin bind to the receptor, causing it to change shape.

  This change in shape causes GDP to detach from the alpha subunit of the receptor.

  GTP binds in its place, causing the G-protein subunits (on the receptor) to dissociate into two parts: The GTP-bound alpha subunit and the beta-gamma dimer.

  Both of these parts remain anchored to the plasma membrane, but they are no longer bound to the receptor, so they can diffuse and interact with other membrane proteins.
• What is melatonin transduction?

  Melatonin signal transduction occurs when the activated GTP- alpha subunit inhibits the adenylate cyclase enzyme on the cell membrane.

  The inhibits adenylate cyclase cannot catalyse the conversion of ATP in the cytoplasm to cyclic AMP (cAMP). As a result, intracellular levels of cAMP decrease. As cAMP increases cell activity, its decreased formation results in decreased cell activity.