Suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is a nucleus in the hypothalamus and is so named because it resides immediately above the optic chaism (OX). It consists of two nuclei each of which lies on either side of the hemisphere separated by the third ventricle (3V). Its principal function is to create the circadian rhythm, which regulates the body functions over the 24-hour period. It consists of various cell types, principally ventrolatral (vlSCN) and dorsomedial (dmSCN) types. The vlSCN neurons possess ability for light-induced gene expression. Melanopsin-containing cells in the retina have a direct connection to suprachiasmatic nucleus via the so-called retino-hypothalamic tract (RHT). The SCN sends information to other hypothalamic nuclei and the pineal gland to modulate body temperature and hormone production such as cortisol and melatonin. Rats with damage to the SCN sleep "erratically" (i.e. they do not show night-day rhythm).

Gene Expression

The circadian rhythm in the SCN is generated by the gene expression cycle in the nucleus. A so-called clock gene is made into a protein that inhibits its own transcription. This negative feedback mechanism gives a 24-hour rhythm in the expression of the clock genes. The genes encode transcription factors which trigger expression of various proteins.

Electrophysiology

The firing rate of the spontaneous action potentials in the SCN gets modulated with 24-hour period as well. During the mid-day the firing rate reaches the maximum (above 7 Hz at room temperature) and during the night it goes near zero or at a rate below 4 Hz with irregular patterns. How the gene expression cycle (so-called the core clock) connects to the neural firing remains unknown.

Calcium Dynamics

Two contradictory reports exist about circardian variation of the cell calcium content. However, both reports agree the resting calcium level is slightly higher during the day compared to night.