Endogenous Pacemakers and Exogenous Zeitgebers Revision Notes

Endogenous Pacemakers

·         The Superchiasmatic Nucleus (SCN) is an endogenous pacemaker. The SCN is a cluster of nerve cells in the hypothalamus. It receives information about light above the optic chiasma, even when our eyes are closed.

·         Stephan and Zucker (1972) found that when the SCN is lesioned, the circadian rhythm of sleep/waking, body temperature and feeding becomes irregular.

·         The retinohypothalamic tract is the main neural pathway from the retina at the back of the eye eventually runs to the visual cortex at the back of the brain. There is a SCN in each hemisphere:  the ventral SCN is relatively quickly reset by external cues and the dorsal SCN is less affected by light which helps to explain desynchronisation.

·         Morgan (1995) bred mutant hamsters so they had circadian rhythms of 20 hours and then transplanted their…