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Discuss the role of endogenous pacemakers and exogenous Zeitgebers in biological rhythms In relation to biological rhythms, endogenous factors are internally caused, as distinct from external causes. The main pacemaker for endogenous rhythms is the superchiasmatic nucleus. This is a small group of cells in the hypothalamus. It lies just above the optic chiasm. It can then receive input directly from the eye; rhythm can be reset by the amount of light entering the eye. It also generates its own rhythms, as a result of protein synthesis. The cells produce a protein for a period of hours, until the level inhibits further production. When the level drops below another threshold the SCN starts producing the protein again. This generates the biological rhythm. Kalat says it produces protein until you have enough. However, this is only one study and so cannot be used to generalise to everybody and therefore lacks validity. Morgan...

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