The Biological Clock's Mastermind
The main cog in the human biological clock is the
suprachiasmatic nucleus (SCN), a group of nerve cells in a region at the base of
Michael Young, "The Tick-Tock of the Biological Clock," Scientific American, 2000.
Circadian timing mechanisms are based on chemical oscillations within cells. But
how does this mechanism work? And how do organisms use it to tell time? There
appear to be two systems at work a master clock or pacemaker as well as local
clocks in the body's various organs. The master clock, which is reset via light
and other environmental cues, synchronizes the local clocks. The main "cog" of
the mammalian master clock is located in the brain. It is a paired structure
known as the suprachiasmatic nucleus (SCN),which lies at the base of the
Human Brain Slice.
The SCN, composed of two pinhead-sized clusters of nerve cells in the
hypothalamus, sits in the midline of the brain directly above the optic nerve.
Each cluster is composed of thousands of neurons that together control
biochemical, physiological, and behavioral rhythms.
Anatomical Model of the Human Brain
This model, which dates to the early 20th century, shows the left lobe of the
human brain. The gross structure of the human brain has been known for centuries.
The SCN, which is found in both the left and right lobes, is a relatively recent
The SCN as Mastermind.
Photoreceptors in the retina respond to light and transmit information to the SCN
along a specialized group of fibers within the optic nerve. The SCN processes
that information about day length and sends a message to the pineal gland, which
secretes the hormone melatonin and triggers various endocrine responses.
"Seeing" the Light From Input to Output.
This schematic flow chart depicts the pacemaker system. The precise mechanisms
and pathways that allow cells to act in concert are not yet understood.
Caught in the Act A Newborn's Circadian Clock Starts Up.
This actogram shows a baby's circadian system maturing and synchronizing to the
external environment, as evidenced in its ability to sleep through the night.
The process takes approximately four months. Produced by Nathaniel Kleitman, of
Mammoth Cave (station 3) fame, this actogram is based on data recorded by mothers
observing their babies' sleep patterns.