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Animal Anatomy and Physiology

Biological Rhythms and Clocks

Seasons of the year, phases of the moon, high and low tides, and alternation between night and day areexamples of cyclic changes in the environment. Each presents a different set of challenges to the animals.

For instance, day and night are two different environments that force animals to evolve two different sets

of physiological and behavioral adaptations.

Homeostasis implies a relatively stable defense of a set-point value of a physiological parameter. Differenttimes (of day, year etc.) demand different levels of physiological activity.

Rheostasis addresses the controlled change of a set-point. Almost every biochemical, physiological and behavioral function shows a daily cycle. Types of cycles: circadian (about a day), infradian (e.g., circalunar, circannual), and ultradian (e.g.,

circatidal, circahoral).

It is advantageous for animals to predict, and not merely react to changes in the environment.Geophysical cycles are highly predictable (in contrast with meteor hits), so organisms evolved timing

mechanisms

biological clocks (and calendars). Thus, physiological and behavioral states are

synchronized to the outside world.

Along with synchronization to the outside world, biological clocks also synchronize events within thebody, e.g,. ensure that time of hormone release coincides with the time when the hormone receptor is

available at the cell membrane of the target tissue, etc.

Another function of the biological clock is to put a time-stamp on memories. If an event happens to ananimal that impacts its potential fitness, the animal will remember not only what happened and where,

but also when.

Continuously Consulted Clocks are used for sun-compass orientation and navigation, e.g., in migratorybirds and in honeybees (time-sense).

Diurnal rhythms in daily environmental cycles. Circadian rhythms in constant conditions. Monitoring an output of the clock (hands, not the gears) Entrainment synchronization of the internal time to the external time. The cues from the environment

are called Zeitgebers. Light-dark cycles are the most potent cues in almost all organisms. A variety of

other cues, e.g., temperature cycles and social cues can entrain biological rhythms in animals in which it is

an ecologically relevant cue.

Freerunning rhythms expressed in constant conditions in complete isolation from all time cues.Biological rhythms are endogenous, i.e., generated by the organism itself and inherent (have a genetic

basis).

Temperature compensation the period of the freerunning rhythm is independent of temperature. Clocks are inherent based on genetic/molecular mechanisms in the cell. Biological clocks behave like physical oscillators. Phase Response Curve (PRC) a measure of

responsiveness of the clock to environmental cues (Zeitgebers).

Almost every cell has a clock. Pacemakers are specialized tissues: - responsive to environmental cues

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o send neural or chemical signals to other clocks, - cycle indefinitely in isolation Circadian organization: one or more pacemakers drive the rhythms in all other cells. Pacemakers in invertebrates: eyes, optic lobes, brain. Pacemakers in vertebrates:

o suprachiasmatic nuclei (SCN)o pineal organo retina

Mammals:o SCN is the only pacemakero retina is the only source of light information, also contains a clock, makes melatonino pineal is not a pacemaker, secretes melatonin rhythmically

Non-mammalian vertebrates:o suprachiasmatic area often not sufficiento pineal organ is the pacemaker in many animalso retina is the pacemaker in some animalso photoreception via eyes, pineal and extraretinal photoreceptors found deep inside the brain

Circadian photoreception in mammals:o not through rods and coneso about 1000 retinal ganglion cellso photopigment melanopsino projections to SCN, centers for pupillary reflex and mood

Jet-lag: consequence of long-distance flights over multiple time zones Shift-lag: consequence of rotating work shifts Internal desynchronization, e.g., SCN shifts fats, clocks in liver and GI tract shift slowly. Result: greater incidence of ulcers, heart attacks, cancer Melatonin: indoleamine. Derivative of amino-acid tryptophan, via serotonin. Secreted by pineal, retina

and intestine at night, but not during the day. Found also in bacteria, protists, plants, fungi and

invertebrates.

Photoperiodism: measurement of daylength as a seasonal cue.o mammals: clock measures duration of nightly melatonin releaseo other animals: clock involved but not known how

Circannual rhythms: endogenous annual cycles, role of circadian system iffy at best Circalunar and circatidal rhythms very little known about the mechanism