endogenous pacemakers + exogenous zeitgebers

• the term 'endogenous' refers to anything whose origins are within the organism, these pacemakers are most probably the products of inherited genetic mechanisms and allow us to keep pace w/ changing cycles in the environment
• the most important pacemaker in human beings is the suprachiasmatic nucleus
• the suprachiasmatic nucleus: in mammals the main endogenous pacemaker is a tiny cluster of nerve cells called the suprachiasmatic nucleus (SNC) which lies in the hypothalamus
• the suprachiasmatic nucleus: the SCN plays an important role in generating the body's circadian rhythm, it acts as the 'master clock' w/ links to other brain regions that control sleep and arousal and has control over other biological clocks throughout the body
• the suprachiasmatic nucleus: neurons within the SCN spontaneously synchronise w/ each other so that their target neurons in sites elsewhere in the body receive correctly time-coordinated signals
• the suprachiasmatic nucleus: these peripheral clocks can maintain a circadian rhythm but not for very long which is why they must be controlled by the SCN
• the suprachiasmatic nucleus: this is possible because of the SCN's built in circadian rhythm which only needs resetting when external light levels change, the SCN receives information about light levels via the optic nerve
• the suprachiasmatic nucleus: this happens even when our eyes are shut because light penetrates the eyelids, if our biological clock is running slow (e.g. the sun rises earlier than on the previous day), then morning light automatically adjusts the clock putting its rhythm in step w/ the world outside
• the suprachiasmatic nucleus: the SCN also regulates the manufacture and secretion of melatonin in the pineal gland via an interconnecting neural pathway
• the pineal gland: 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
• the pineal gland: melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness, the pineal and the SCN function jointly as endogenous pacemakers in the brain
• the pineal gland: the sensitivity of the pineal gland and the SCN to light and the role of melatonin is controlling sleep and activity mean that despite the endogenous nature of these clocks their activity must be synchronised w/ the light-dark rhythm of the world outside
• the term 'exogenous' refers to anything whose origins are outside the organism, the term 'zeitgeber' comes from the German words zeit and geber meaning 'time giver'
• exogenous zeitgebers are environmental events that are responsible for entraning the biological clock of an organism, the most important zeitgeber for most animals is light
• light: receptors in the SCN are sensitive to changes in light levels during the day and use this information to synchronise the activity of the body's organs and glands
• light: light resets the internal biological clock each day keeping it on a 24 hour cycle, Rod and cones in the retina of the eye detect light to form visual images
• light: however there is a 3rd type of light-detecting cell in the retina that gauges overall brightness to help reset the internal biological clock, the protein called melanopsin which is sensitive to natural light is critical in this system
• light: a small number of retinal cells contain melanopsin and carry signals to the SCN to set the daily bodily cycle
• social cues: social stimuli such as mealtimes and social acitvities may also have a role at zeitgebers
• social cues: Aschoff et al (1971) showed that people are able to compensate for the absence of zeitgebers such as natural light by responding to social zeitgebers instead
• social cues: one of the earliest studies on jet lag (Klein and Wegmann, 1974) found that the circadian rhythms of airtravelers adjusted more quickly if they went outside more at their destination
• social cues: this was thought to be because they were exposed to the social cues of their new time zones which acted as a zeitgeber
• social cues: likewise the circadian rhythms of blind people were thought to be no different to sighted people as both groups were exposed to the same social cues
• social cues: we now know that both examples can be better explained in terms of light exposure acting as a zeitgeber
• social cues: the sleep-wake cycle of most blind people is still influenced by light during the day even though they have no visual perception, this is because connections exist between the eye and the SCN that do not involve those parts of the visual system on which the perception of light depends
• evaluation S: research in this area has led to the development of strategies to reduce the impact of shift work and jet lag, the manipulation of light levels e.g. allows shift workers to slepe when they would normally be awake
• evaluation S: Siffre's (1975) cave study shows that the body clock maintains a regular sleep-wake cycle in the absence of all exogenous cues supporting the view that a biological mechanism underlies this cycle
• evaluation L: free-running body clock studies such as Siffre's have few participants thus limiting the generalisability of such studies
• evaluation L: these studies are further criticised for using artificial light which could affect the way the body clock works and so lacks ecological validity