Discuss the role of endogenous pacemakers and exogenous zeitgebers in biological rhythms
A rhythm is something that is regularly repeated e.g. eating several times a day. These rhythms are repeated over different intervals and they are governed by both internal (endogenous) and external (exogenous) factors. An endogenous pacemaker is something internal that sets the rhythm, and an exogenous zeitgeber is something external that affects / sets the rhythm e.g. light.
A biological rhythm that depends upon endogenous pacemakers is the sleep/wake cycle circadian rhythm in humans. The Suprachiasmitic Nucleus (SCN) is a small group of cells in the hypothalamus; it lies just above the optic chasm so it can gain input directly from the eyes, and therefore the rhythm can be reset by the amount of light entering the eye. The SCN generates its own rhythms, probably as a result of protein synthesis (cells in the SCN produce a protein for a period of hours until the level stops further production, and then when the protein level drops below the threshold, the SCN starts producing the protein again – Kalat 1998). This generates the biological rhythm.This biological rhythm then effects the sleep-wake cycle via the pineal gland. Electrical stimulation of the pineal gland produces the hormone melatonin, which makes a person feel sleepy. In the absence of light, or when light levels are low, more melatonin is released (hence, why we sleep at night).
There are studies to support the functioning of the SCN in circadian rhythms. Silver et al showed that transplanted SCNs can restore circadian rhythm to an animal whose own SCN has been removed. Morgan also removed the SCN from hamsters and found that their circadian rhythms disappeared. He also transplanted the SCN cells from mutant hamsters (those who had been bred to have shorter cycles than normal) to ‘normal’ hamsters. He found that the hamster receiving the transplant took on the mutant circadian rhythms.
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