lecture_notes_37-38_(ta)_2 - Lecture 37-38 Central...

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Lecture 37-38: Central Endocrine Glands ( PLEASE SEE D R . D EBELLO OR I IF YOU HAVE ANY QUESTIONS ABOUT THIS MATERIAL ) Reading: chapter 18, pgs 662-677 (5 th edition 681-697) Circadian Rhythms - Regular fluctuations in physiological or performance parameters that occur with a roughly 24 hr period. - Rely on an endogenous generator and entrainment. - Focal ablation of the SCN eliminates circadian rhythms, indicating that it is the endogenous generator (i.e. “pacemaker” or “master clock”). Cells within the SCN express clock proteins , which are transcriptional regulators that suppress their own production at high concentrations. When transcription shuts down, protein levels fall, relieving the transcriptional suppression and starting the cycle anew. The native period of this cycle is individually variable, from about 22 to 28 hours. - The mammalian SCN integrates multiple inputs, but light is the dominant cue for entrainment (photoentrainment). When animals (including humans) are placed in continuous darkness or light conditions for extended periods (i.e. days to weeks in the absence of light/dark cues) they go into “free run,” which reflects the period of their endogenous generator. Where are the photoreceptors that guide photoentrainment? - Rod/cone knockout animals show normal photoentrainment. This misled scientists to believe that the photoentrainment photoreceptors were located elsewhere (e.g. bloodborne). - Within the past few years new data came forth. About 1 in 100 Retinal Ganglion Cells (neurons that provide output from the eye to the brain) were found to express a photopigment called melanopsin . Indeed these RGCs are intrinsically photosensitive . Moreover, this same sub-class of RGCs (ipRGCs) was found to project to the SCN.
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lecture_notes_37-38_(ta)_2 - Lecture 37-38 Central...

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