Endogenous Pacemakers & Exogenous Zeitgebers
The suprachiasmatic nucleus -
> In mammals, the main endogenous pacemaker is a tiny cluster of nerve cells called the suprachiasmatic nucleus (SCN), which lies in the hypothalamus.
> The SCN obtains information about light from the eye via the optic nerve - this happens even if our eyes are shut because light penetrates the eyelids.
> If our SCN is running slow (e.g. sun rises earlier than day before), morning light automatically shifts the clock ahead, putting the rhythm in step with the outside world.
> In 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.
> ALBUS ET AL. found that the ventral SCN is relatively quickly reset by external cues, whereas the dorsal SCN is much less affected by light and therefore more resistant to being reset.
The pineal gland and melatonin -
> 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 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 is the dominant zeitgeber in humans.
> Light can reset the body's pacemaker, the SCN.
> Light can reset the other oscillators located throughout the body because the protein CRY (cryptochrome), which is part of the protein clock, is light-sensitive.
(The ticking of the biological clock)
> The basis of the circadian rhythm lies in interactions between certain proteins, creating the 'tick' of the biological clock; it's an ingenious negative feedback loop.
Social cues -
> Until fairly recently, biologists thought that social cues were the main zeitgebers for human circadian rhythms (we eat meals at socially determined mealtimes, and go to bed at times appropriate for our age, etc.).
> We now know light is the dominant zeitgeber & it's now understood that all parts of the body produce their own oscillating rhythms & some of these aren't primarily reset by light.
> DAVIDSON says the zeitgeber for cells in the liver and heart is likely to be mealtimes because these cells are reset by eating.
> In cold-blooded animals variation in external temperature affects the setting of the circadian rhythms - cold temperature…