Foundations Of Biopsychology 2nd Edition – WICKENS A

At first sight the existence of circadian body rhythms may not appear to be too surprising. After all, it is easy to imagine that they are directly caused by the world around us – possibly by the alternating periods of light and dark, which, somehow, affect the biological processes of the body. But this is not the case. In fact, instead of being passive responders to events around us, we have our own internal clocks that time and control the body’s rhythms.

And we are not alone as nearly every life-form has its own time-keeping mechanisms. For example, in 1729, the French astronomer DeMarian was intrigued when he noted that his heliotrope plant opened its leaves during the day and shut them at night. To examine this further, he shut his plant away in a dark cupboard and found that it continued to open and shut its leaves in time with the light and dark cycles outside. Thus, the leaf rhythm of the plant was controlled by its own innate mechanism.

Similar circadian patterns have been found in creatures as simple as single-celled algae (Palmer 1975). In 1832, the Swiss botanist de Candolle performed an experiment similar to the one by DeMarian, but this time he noted something odd – when placed in the dark, his plant opened and shut its leaves every 22 hours, not every 24 hours.

The plant therefore appeared to have an internal ‘clock’ that did not have a very accurate timing mechanism. In fact, this was the first demonstration of a ‘free- running’ rhythm – that is, a rhythm which was running at a speed that didn’t quite match the outside world. Cut off from the normal cues of the environment, the plant was relying on its own, less than precise, timing mechanism. This also suggested that the plant was keeping accurate time (i.e. entraining itself to the rhythms of the outside world) by ‘resetting’ its clock by using external time cues such as periods of light and dark, temperature or humidity.

Indeed, these stimuli are now known to be important regulators of circadian rhythms and are collec- tively called zeitgebers (from the German for ‘time-givers’). Free-running rhythms in humans What evidence is there that humans have internal biological clocks with their own time-keeping mechanism? To show that such clocks exist it is necessary to prove that circadian rhythms can still operate in the absence of time cues.

Visit the Foundations of Biopsychology, second edition Companion Website at www.pearsoned.co.uk/wickens to find valuable student learning material including: • Learning objectives for each chapter • Multiple choice questions to help test your learning • Essay writing guide • Internet search guide • Mind Maps material focusing on the various parts of the brain Front cover image: Coloured 3-dimensional MRI scan of the white matter pathways of the brain, side view.

White matter is composed of myelin-coated nerve cell fibres that carry information between nerve cells in the cerebrum of the brain (top half of image) and the brain stem (bottom centre). This image was created by an MRI scanner sensitised to the movement of water around the brain. Blue represents neural pathways from the top to the bottom of the brain, green represents pathways from the front (left) to the back (right), and red shows pathways between the right and left hemispheres of the brain.

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