Abstract
Endogenous biological clock optimizes a function of a biological system, enables a prediction of changeable environmental influences, and facilitates a proper reaction. On the other hand, environmental factors may regulate the biological clock.
The most important center of endogenous biological clock in mmammals is suprachiasmatic nucleus (SCN) ofthe anterior hypothalamu. SCN receives information on the actual astronomic time (light) directly from the eye via retino-hypothalamic neural pathway.
SCN influences melation secretion in the pineal gland as well as function of a number of peripheral biological clocks in tissues outside the brain. Periodic feedback expressíon or suppression of clock genes (Per1, Per2, Cry1, Cry2,Npas2 etc.) in the SCN represents the biological background for the SCN's biological rhytms.
Intracellular biochemical changes show themselves on the outside of SCN as neural firing or inactivity.