Endogenous Pacemakers

A01:

Endogenous pacemakers:

Internal body clocks that regulate many of our biological rhythms, such as the influence of the suprachiasmatic nucleus (SCN) on the sleep/wake cycle.

The suprachiasmatic nucleus (SCN):

SCN is a tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain. It is one of the primary endogenous pacemakers in mammalian species (including humans) and is influential in maintaining circadian rhythms such as the sleep/wake cycle. Nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the visual area of the cerebral cortex. The SCN lies just above the optic chiasm (thus ‘supra’, which means ‘above’). It receives information about light directly from this structure. This continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep.

Animal studies and the SCN:

The influence of SCN has been demonstrated in studies involving animals. DeCoursey et al (2000) destroyed the SCN connections in the brains of 30 chipmunks that were then returned to their natural habitat and observed for 80 days. The sleep/wake cycle of the chipmunks disappeared and by the end of the study a significant proportion of them had been killed by predators (presumably because they were awake and vulnerable to attack when they should have been asleep). Martin and Ralph et al (1990) bred ‘mutant’ hamsters with 20-hour sleep/wake cycle. When SCN cells from the foetal tissue of mutant were transplanted into the brains of normal hamsters. Both of these studies emphasise the role of SCN in establishing and maintaining the circadian sleep/awake cycle.

The pineal gland and melatonin:

The SCN sends signals to the pineal gland, directing it to increase production and secretion of the hormone melatonin at night and to decrease it as light levels increase in the morning. Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness.

The pineal and the SCN function jointly as endogenous pacemakers…