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Rhythms All Biological biological clocks are adaptations to life on a rotating planet. Colin Pittendrigh, undated. Physiological Pacemakers Environmental Synchronizers Biological Clocks: Internal timing mechanisms Biological Rhythms Activities and behaviors Biological rhythms are a great way to study proximate (mechanistic) questions (nervous system, sensory systems, hormonal release etc.) as well as ultimate (fitness, etc.) questions (e.g., peak feeding activities, timing of reproduction). General patterns of rhythms (these concepts and labels are universally applicable to other phenomena such as sound) Period Amplitude Phase shift Biological rhythms can be categorized based on their frequency and periods. Epicycles (ultradian, variable lengths of time): An ultradian rhythm is defined as a regular (usually physiological) cycle or oscillation (e.g., of hormone levels) that takes less than a day and sometimes a very short period of time to complete. This is from a book and website suggesting that we all take 20 minutes to relax every 90 minutes. (Wish I had that kind of time!) Tidal Rhythms (12.4 hours): The ebb and flow of tides affects the behavior of some animals occupying the tidal zone. These occur on a daily basis, monthly bases, as well as yearly cycles. Lunar rhythms (28-day cycle): are related to tidal rhythms, but can affect animals that are not necessarily in the tidal zones. Circadian rhythms (24 hours): are probably the best known. Diurnal: Active in daylight Nocturnal: Active at night Crepuscular: Active at dusk and/or dawn Circadian rhythms can be altered over a yearly period. Example: Bird species that live in northern temperate habitats throughout the year can switch from crepuscular activity in the spring and summer to diurnal activity in the winter. Circadian rhythms can also be altered over the lifetime of an individual animal. Example: Young woodchucks are active in early evening, but adults are more diurnal in their activity patterns. A typical human circadian rhythm. Circannual rhythms (12 months) are behavioral and physiological patterns that are governed by self-sustaining internal pacemakers and that occur within a period of about 1 year. Examples: Hibernation (some vertebrates) and diapause (a period of dormancy in insects, some fish) Ground squirrels Possible human circannual cycles: Mood (Seasonal Affective Disorder; SAD) Diastolic blood pressure and heart rate (?) Hair growth (Randall, 1991; greatest in summer) T, FSH and LH: In the controls, annual rhythms were validated for the secretion of T (annual crest time in late fall and winter), LH (annual crest time in February), FSH (annual crest time in January) and sexual activity (crest time September) This data for humans in North America strongly suggests circannual patterns in reproduction. A critical component of a biological rhythm is the existence of an internal self-sustaining, endogenous pacemaker or chronometer. We see evidence of this when we place an animal in constant environmental conditions such as constant darkness. Activity level is measured by an actograph that functions similarly to a seismograph in recording movements of the animal. Actograph When actographs from several cyclical periods are put together, the endogenous activity levels are revealed, and they are not always a perfect match to a 24 hour day cycle. Free-running and entrained are terms to describe the patterns of activity. Aschoffs Rule states that when animals are kept in constant darkness, their activity rhythm continues at approximately a 24 hour cycle, but drifts slightly. Nocturnal animals will show a slightly SHORTER than 24 hour period of activity. Diurnal animals will show a slightly LONGER 24 hour period of free-running activity. Other evidence for the endogenous nature of pacemakers within animals comes from isolation studies (lizard eggs maintained under different lighting regimens), genetics (mutant hamsters showing shorter free-running periods), translocation (moving animals from 1 time zone to another), individual variation in period. It is very apparent that endogenous rhythms are important in the lives of animals, however they are rarely perfect in the sense that they are unvarying in matching the environmental cycle. Endogenous rhythms must be synchronized with the external stimulus. This process is called entrainment. Cues that provide information to animals about the periodicity of environmental variables are called Zeitgebers (time givers). Photoperiod is the most common Zeitgeber used by endothermic animals. It is both predictable and reliable. Ectotherms can use temperature also. Other Zeitgebers include: cycles of food (goldfish, house sparrows), social cues (humans) Zeitgebers do not have to last long. Example: A 15 minute exposure to light at the appropriate time is enough to entrain flying squirrels to a 24 hour activity cycle. Model of mammalian pacemaker system Environmental Input (e.g., light) Entrainment pathway and clock Output pathway Receptor system (e.g., photosensitive cells, photoreceptors) Nervous system (e.g., brain or ganglion) Circadian oscillator generates endogenous self-sustained rhythm (e.g., suprachiasmatic nucleus, SCN) Message carried to other neural structures via neurotransmitters (e.g., serotonin) Gene expresssion affected in target cells Output system via messenger to effector tissue and organs (e.g., feeding observed behavior) Overt rhythm (e.g., circadian pattern of activity) Many pacemaker systems have characteristics in common. Where are these endogenous pacemakers located? Cockroaches and sea hares (Aplysia) seem to have rhythmic tissues located in their optic lobes and eyes. Work in Drosophila has identified some genes that are responsible for rhythmic activity. Another view of Drosophila pacemaker control. Avian Mammalian a | The neuroendocrine-loop model of avian pacemaker organization. This representation of a generalized avian brain shows the locations of the pineal gland, retina and suprachiasmatic nucleus (SCN; consisting of the visual SCN (vSCN) and the medial SCN (mSCN)) each of which are damped circadian pacemakers that rely on mutual interactions to maintain rhythm stability and amplitude. For simplicity, the vSCN and the mSCN are shown as a single unit. The roles of the pineal gland and retina in circadian organization vary between species; the pineal gland is crucial to circadian rhythms in passerine (perching) birds, such as the sparrow, whereas the retina has a more important role than the pineal in chickens and quails. In sparrows and chickens, the SCN is active during the subjective day and inhibits melatonin biosynthesis in the pineal gland, so that it is only produced during the night. Therefore, neither the vSCN or mSCN secretes melatonin directly, but lesions of the vSCN affect pineal secretion of melatonin. In addition, humoral and neural outputs from the SCN affect the CNS and peripheral sites to which the CNS projects. During the night the pineal gland secretes melatonin into the bloodstream. Among other targets, melatonin inhibits activity within the SCN through specific melatonin receptors and restricts the SCN's output to the subjective day. This output coordinates downstream oscillators in peripheral tissues that are responsive to melatonin. In chickens and quails, the retina secretes melatonin into the bloodstream at night to inhibit SCN activity and regulate melatonin-responsive peripheral oscillators. The vSCN, but not the mSCN, receives light signals from the retina through the retinal hypothalamic tract (RHT). b | The mammalian pacemaker system differs from that in birds primarily in the number of tissues that make up the centralized pacemaker. In mammals, the SCN alone serves as a pacemaker that receives light signals from the retina through the RHT (whereas the light-perceptive pineal gland and retina, together with the SCN, form the pacemaker system in birds), and directly regulates pineal melatonin biosynthesis as an output of the clock. In both mammals and birds, pineal melatonin secretion is restricted to the night and, through melatonin receptors expressed in the SCN, inhibits nighttime SCN activity. Similar to birds, rhythmic melatonin levels regulate sleepwake cycles, and along with other neural and humoral outputs from the SCN, is thought to coordinate peripheral oscillator function. In both panels, interactions show overall effects only, as not all steps in the pathways involved are shown. In mammals, evidence suggests that there are at least 2 pacemaker systems: one in the suprachiasmatic nucleus (SCN) and the other (possibly) in the ventro-medial hypothalamus (VMH). Adaptive value of biological rhythms Many animals live in environments in which light, temperature, and humidity can be critical for their survival and that can fluctuate on a daily or yearly basis, such as deserts, arctic or temperate conditions. Woodlice are naturally nocturnal (photonegative), but can show photopositive tendencies if the humidity is low. The pervasive presence of biological rhythms in the lives of animals brings up a question. This is: Why do animals have endogenous rhythms at all? Why not just respond directly to external cues? The answer seems to be that with an endogenous rhythm, an animal is prepared for changes in its environment. An endogenous rhythm allows it to anticipate potentially dangerous conditions and prepare for them through physiological and behavioral change. Hibernation in harsh temperate or arctic regions is a circannual behavior that shows clear ecological significance. Animals prepare for hibernation in several ways: 1. increasing food intake 2. digging extra deep burrows or sleeping chambers Arctic ground squirrels can super-cool their bodies to -2.9oC. Some animals do not hibernate, but prepare for winter in other ways: 1. Mice produce large numbers of progeny, some of which will survive the winter and reproduce the next summer. 2. Other rodent species show communal nesting. 3. Some mammals, such as raccoons, do not actually hibernate, but will remain torpid in their dens during the coldest days and nights of winter. Surviving the Winter In all cases, animals anticipate the coming of winter and prepare appropriately. Hibernating animals must mate soon after emerging from their winter den. They are able to do this because hormones are timed for release during the hibernation period, awakening the physiological systems necessary for reproduction before hibernation ends. 13-lined ground squirrels are ready to reproduce 1-2 days after emerging from hibernation and mate successfully within the first week. Garter snakes also mate shortly after emergence from their hibernacula. Migration is another dramatic circannual cycle that we will be exploring in the future when we get to chapter 13. ... View Full Document

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