endogenous pacemakers and exogenous zeitgebers

• Endogenous pacemakers 


  -internal biological clocks.

  -They regulate functions of cells, tissues and organs throughout the body at certain times to perform specific functions and maintain homeostasis.

  -The main E.P is the suprachiasmatic nucleus (SCN).

  -SCN is located within the hypothalamus and is the master EP that influences when we eat, sleep and are alert. It stimulates and inhibits the release of hormones from glands.

  -SCN regulates a circadian rhythm of 24 hour sleep/wake cycle.
• Exogenous Zeitgebers 


  -are external time givers. External cues as to the time of day. e.g. light, meal times and social interactions.

  -Each morning the SCN receives information as to the time of day which helps it maintain a fairly accurate 25 hour cycle.

  -Entrainment (process of light affecting EP's) refers to an individuals chronology. Exogenous zeitgebers are responsible for entraining EP's which helps them stay to a 24 hour rhythm. They are able to shift the body clock forwards/backwards. This can explain jet lag and shift work.
• The SCN - Pineal system

  -Light enters the eye and hits the retina and detects brightness/light intensity.

  -SCN receives the info about changing light levels and uses it to co-ordinate biological processes.

  -E.g. During periods of low light the SCN stimulates pineal gland to secrete melatonin, induces sleep. When light levels are high info is passed to optic nerve and in high light intensity SCN inhibits secretion of melatonin and secretes cortisol, induces wakefulness.

  -This shows how EZ's can influence EP's in regulating our body's circadian rhythm.
• Evaluation - practical application

  -use of light therapy to improve sleep/wake cycles amongst night shift workers.

  -Zamanian et al studied 34 shift workers who are exposed to bright light during two break times for 4 consecutive weeks.

  -Body temperatures were measured during and after night shift.

  -Subjective alertness and fatigue were evaluated with a sleepiness scale. - self report. It was found that bright light administration supressed night-time melatonin levels during night shift.

  -These results show that light simulation in hospitals can have an influence on the adjustment of CR
• Issues with animal research

  -ethical issues: brain surgery and genetically engineered animals raises ethical issues. The animals suffer harm as a result of damage to their SCN and disrupted CR. However, there is cost vs benefit analysis.

  -Issues with extrapolation: caution must be taken when extrapolating the findings about endogenous pacemakers from animals to humans. Human anatomy and physiology is different to animals as is our internal rhythms. Questions external validity.
• Research - exogenous zeitgebers

  -Campbell and Murphy. 15 PP's were woken at various times and a light shone onto the back of their knee.

  -In some cases this shifted the sleep/wake cycle of the PP's by up to 3 hours.

  -This suggests that light doesn't have significant influence of the sleep wake cycle and is detected by other parts of the body.

  -However, failed replication which questions reliability of findings.
• Animal studies investigating the role of EP

  -Ralph et al bred genetically engineered hamsters with a 20h CR.

  -Transplanted SCN cells found in foetal tissue into the brains of another group of hamsters.

  -This caused their CR to change to 20h.

  -This supports the role of the EP in maintaining the sleep/wake cycle.

  -Issues with extrapolation and ethical issues.