Endogenous pacemakers and exogenous zeitgebers

Mechanisms w/in body that govern the internal, bio bodily rhythms.

Main endogenous pacemaker in mammals, SCN. Tiny cluster nerve cells, in hypothalamus. Plays important role in generating body's CR. Acts as 'master clock', links to other brain regions that control sleep + arousal, has control over other bio clocks in body.

Neurons w/in SCN spontaneously sync, target neurons in sites elsewhere receive correctly time-coordinated signals. Peripheral clocks can maintain CR, not for long, must be controlled by SCN

This is poss b/c of SCN's built-in CR, only needs resetting when external light levels change. SCN receives info about light levels via optic nerve. Light penetrates eyelids, morning light adjusts clock, puts rhythm in step w/ world. SCN regulates manufacture + secretion of melatonin in pineal gland via interconnected neural pathway.

SCN sends signal to pineal gland, directs it to increase production + secretion of melatonin at night, decrease as light levels increase in morning. M induces sleep, inhibits brain mechanisms that promote wakefulness. 

Pineal + SCN function jointly as endogenous pacemakers in brain. Sensitivity of P + SCN too light, role of melatonin means that despite endogenous nature of clocks, activity must be sync'd w/ light-dark rhythm of world outside.

Environmental cue, eg light, helps regulate bui clock in organism.

Receptors in SCN sensitive to changes in light levels during day, use this info to sync activity of organs + glands. 

Light resets internal bio clock every day, keeps it on 24 hour cycle. Rods + cones in retina detect light to form visual images. 3rd type light-detecting cell in retina, guages overall brightness to help resent bio clock - protein called melanopsin, sensitive to natural light - critical. Small number retinal cells contain melanopsin + carry signald to SCN to set daily body cycle.

Eg mealtimes.

Aschoff et al (1971) - individuals able to compensate for absence of zeitgebers eg natural light by responding to social zeitgebers instead.

Klein + Wegmann (1974) - jet lag, found CR of air travelers adjusted more quickly if when outside more at destination. Thought to be b/c more exposed to social cues of new time zone, acted as zeitgeber.

CR of blind people no diff to sigheted as exposed to same social cues. Sleep/wake cycle of blind people still influenced by light during day, even tho no visual perception. B/c connections exist b/ween eye + SCN that don't involve those parts of visual system that depends on perception of light.

EP - role of SCN - Morgan (1995) - bred strain of hamsters so had abnormal CR 20 hours. SCN neurons transplanted into brains of normal - normal hamsters displayed same abnormal 20 hour CR. Reversed, went back to normal.

EZ - role of SCN - some blind people still able to reliably entrain CR in response to light despite lack of image-forming visual perception.

Using light exposure to avoid jet lag - Burgess et al (2003) - exposure to bright light before eat-west flight decreased time needed to readjust to local time on arrival.

Separate rhythms - some circumstances, rhythms can become out of step. Folkard (1996 - studied uni student, volunteered to spend 25 days in controlled environment of lab. During time in lab, no access to daylight or other zeitgebers might have reset SCN. End of 25 days, core temp rhythm still at 24 hours. Sleep/wake cycle extended to 30 hours, periods of sleep as long as 16 hours.