Ch.14.2 The Hypothalamus Houses a Circadian Clock Flashcards
What is the suprachiasmatic nucleus (SCN)?
A small region of the hypothalamus above the optic chiasm that is the location of a circadian oscillator
What area is responsible for the endogenous clock?
The suprachiasmatic nucleus (SCN)
Lesions confined to the SCN portion of the hypothalamus interfered with…
Circadian rhythms of drinking and locomotor behavior.
What is the pineal gland
A secretory gland in the brain midline that is the source of melatonin release.
How does the pathway that entrains circadian rhythms to light-dark cycles vary in species (give a couple of examples)
- Pineal gland of some amphibians is sensitive to light because the skull is thin.
- Birds, too, the pineal possesses photoreceptors that can detect daylight through their thin skulls.
- In mammals cells in the eye tell the SCN when it is light out.
What is the retinohypothalamic pathway?
The projection of retinal ganglion cells to the suprachiasmatic nuclei.
Certain retinal ganglion cells send their axons along the retinohypothalamic pathway, splitting off at the optic chiasm to synapse directly within the SCN. This tiny pathway carries info about light to the hypothalamus to entrain behavior.
Do most retinal ganglion cells that extend their axons to the SCN rely on traditional photoreceptors (rods and cones) to learn about light?
No, they are sensitive to light because they contain a special photopigment called melanopsin.
What is melanopsin?
A photopigment found in some retinal ganglion cells (that go to the SCN)
Why does blue light (like that put out by LCD screens) have the largest effect on humain circadian systems?
Melanopsin is most sensitive to light frequencies in the blue range. This is the special photopigment that helps entrain circadian rhythms.
Explain how flies and mice have a molecular clock.
I don’t think you have to memorize this but maybe remember that light passes through the fly’s body into brain cells to degrade the Cry protein and activate a specialized photopigment to synchronize the clock.
How does light entrain the molecular clock to the light-dark cycle in humans?
We use the retinohypothalamic track to get light info to the SCN. Glutamatergic synapses trigger a chain of events in SCN cells that promotes the production of Per protein.
When the animal’s photoperiod is shifted, this light-mediated boosting of Per production shifts the phase of the molecular clock and therefore the animal’s behavior.
What is infradian rhythms?
A rhythmic biological event with a period longer than that of a circadian rhythm - that is, longer than a day.
What are examples of infradian rhythms?
A. 28 day human menstrual cycle
B. seasonal cycle in body weight (many animals)
What is an circannual clock?
Occurring on a roughly annual basis.
What are some examples of endogenous circannual clock?
A. animals breeding only during a particular season.
B. body weight
