Organization of behavior and biological rhythms

1. How do animals organize their behavior through time?
   1. circadian
   2. circannual
   3. ultradian (Fig. 3.3)
   
   
   
2. What is organized?
   1. sleep/wake
   2. body temperature
   3. hormone secretion
   4. growth
   5. cognitive function
   6. feeding
   
   
   
3. Are the rhythms exogenous or endogenous?
   
   Central question is whether rhythms persist in the absence of time cues.
   
   
4. Is the clock a single tissue or is every tissue a clock?
   1. Location of the master clock: suprachiasmatic nucleus (SCN)
   2. Evidence for a master clock - Hamster SCN transplant experiment
      1. Lesion SCN of 20 hour period mutant - results in unorganized activity (see Fig. 1E)
      2. Transplant fetal SCN tissue of 22 hour period mutant
      3. Circadian rhythm is restored with period of 22 hours
   3. Other tissues (all tissues?) show circadian rhythmicity in culture
   4. Molecular basis of circadian rhythms is now understood to be the result of a complex system of transcriptional-translational autoregulatory loops
   5. SCN coordinates the output of cellular clocks to produce synchronous activity throughout all tissues of the body
   
   
   
5. Entrainment
   1. Zeitgeber = German for "time giver"
      1. Light is primary signal
         • Photoreceptors in retina (pigment=melanopsin)
         • Non-mammals also have extraretinal photoreceptors (e.g., pineal gland)
         • Why don't mammals have extraretinal photoreceptors? Nocturnal bottleneck theory
           
           
      2. Other types of signals may influence other tissues
         • Food -> liver
   2. Exposure to zeitgeber changes onset of next cycle depending on when it occurs
      1. Phase advance (shortens current cycle; next "day" starts sooner)(see Fig. 4B)
      2. Phase delay (lengthens current cycle; next "day" starts later)(see Fig. 4 A)
      3. Video on how phase response curves are created and how they enable us to predict the effect that a brief exposure to a zietgeber with have on activity rhythms.
   3. Why have entrainment? Read more here.
      • Timing of sunrise and sunset varies with
        • Latitude
        • Longitude
        • Time of year
   
   
   

6. Classic paper on endogenous activity rhythms in humans