Endogenous Pacemakers, Exogenous Zeitgebers

The world we live in has cyclic changes, eg day and night - animals and plants need to be attuned to these changes and therefore they need to have endogenous pacemakers (internally-managed rhythms) that are similar to the likely cyclic changes in the environment. These rhythms also need to be fine-tuned by external cues (exogenous zeitgebers) with the external world which fluctuates in its rhythms (eg. shorter days in winter).

Biological organisms are complex systems where lots of different chemical processes are going on. To co-ordinate these processes you need something like a conducter of an ochestra to keep everything in time. Another reason for having an endogenous pacemaker.

A tiny cluster of nerve cells called the suprachiasmatic nucleus (SCN), which lies in the hypothalamus. It is located just above the place where the optic nerves from each eye cross over (called the optic chiasm).

The SCN obtains information about light from the eye via the optic nerve. This happens even when our eyes are shut because the light penetrates the eyelids.

If our endogenous clock is running slow (eg. the sun rises earlier than the day before), morning light automatically shifts the clock ahead, putting the rhythm in step with the world outside.

Internally managed rhythms, or an internal 'clock'.

External cues.

Each SCN is a pair of structures, one in each hemisphere of the brain, and each of these is divided into a ventral and dorsal SCN.

The ventral SCN is relatively quickly reset by external cues. The dorsal SCN is much less affected by light and therefore more resitant to being reset. (Albus et al)

The SCN sends signals to the pineal gland, directing it to increase production of the hormone melatonin at night. Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness.

The pineal gland lies just beneath the bone of the skull in birds and reptiles and is directly regulated by light; light inhibits the production of melatonin. In fact, many lizards have a 'third eye' near the pineal gland which actually protrudes through a small opening in the skull and receives information about light.

The process of resetting the biological clock with exogenous zeitgebers is known as entrainment.

The opposite of entrainment is 'free-running' - where the biological clock operates in the absence of any exogenous cues.

Light.

As a zeitgeber, light can reset the body's main pacemaker, the SCN. It can also reset the other oscillators located throughout the body because the protein CRY (cryptocrhome), which is part of the protein clock, is light sensitive.

This may explain why Campbell and Murphy found that if you shine light on the back of ppts' knees, this shifted their circadian rhythms.