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Endogenous CyclesEndogenous circannual rhythm: generated within for about a yearBirds generate a rhythm that prepares it for seasonal changesEven if it is kept in a cage with no clues to the season, it will become restless in the spring and will fly North if it was released.Endogenous circadian rhythms: lasts about a dayLight from the sun helps you feel less sleepyYour urge to sleep depends on the part of day it is, not just how many hours you have been awakeexample: college student that feels sleepier and sleepier as the night goes on, but as morning arrives not so muchNo one has succeeded in producing a rhythm far from the 24-hour normMammals and humans have circadian rhythms in their waking and sleeping, eating and drinking, urination, secretion of hormones, sensitivity to drugs, and other variablesDiffer among individuals…Older rats reach their best performance shortly after awakening, younger rats improve performance as the day progressesSetting and Resetting the Biological ClockCircadian rhythm is not perfect but generates a period close to 24 hours.We readjust our internal workings daily to stay in phase with the outside world.We expose ourselves to lights, noises, and activity at night and then awaken late the next morningLight is critical for resetting the rhythmFree Running Time: rhythm that occurs when no stimuli reset or alter it.the stimulus that resets the rhythm is called zeitgeberZeitgeberlight is the dominant zeitgeber for land animalsThe tides are important for marine animalsAlso, exercise, noise, meals, and the temperature but these effects are generally weak on their own without lightExample: in Antarctica without sunlight, people try to maintain the 24-hour rhythm but different people generate different free running rhythms until it’s difficult for them to work togetherJet LagJet Lag: disruption of circadian rhythms due to crossing time zonestravelers complain of sleepiness during the day, sleeplessness at night depression, and impaired concentrationthese problems stem from mismatch between internal circadian clock and external timeFlight attendants on average have smaller than average volumes of the hippocampus and surrounding structures, and they showed some memory impairmentsThese results suggest a danger from repeated adjustments of circadian rhythmShift WorkPeople who sleep irregularly find that their duration of sleep depends on what time they go to sleepWhen people go to sleep in the morning or early afternoon, they sleep only briefly, even if they have been awake for many hoursThe Suprachiasmatic Nucleus (SCN)Suprachiasmatic nucleus (SCN): biological clock that depends on apart of the hypothalamusProvides the main control of the circadian rhythms for sleep and body temperatureAfter damage to the SCN, the body’s rhythms are less consistent and no longer synchronized to environmental patterns of light and darkSCN generates circadian rhythms itself in a genetically controlled, unlearned mannerIf SCN neurons are disconnected from the rest of the brain or removed from the body and maintained in tissue culture, they continue to produce a circadian rhythm of action potentialsA mutation in one gene causes hamsters’ SCN to produce a 20-hour rhythmHow Light Resets the SCNThe SCN is located just above the optic chiasmA small branch of the optic nerve, known as the retinohypothalamic path, extends directly from the retina to the SCNaxons of that path alter the SCN’s settingsMelanopsin: special ganglion cells respond directly to light even if they do not receive any input from rods or conesRespond to overall average amount of light, not to instantaneous changes in lightSo how does it work?A branch of the optic nerve, the retinohypothalamic path, conveys information about light to the SCN. The axons comprising that path originate from special ganglion cells that respond to light by themselves, even if they do not receive input from rods or cones.The Biochemistry of the Circadian RhythmGenes generate a circadian rhythmTwo genes, period(per) and timeless(tim) produce the proteins Per and TimPer and Tim increase in amount as the day persistsBy evening, they reach a high level that causes sleep feeds back to the genes to shut them down so they don’t continue producing Per and TimWhen Per and Tim are high, they interact with a protein called ClockClock induces sleepinessLow Per and low Tim = wakefulnessExtra light decreases the Tim protein therefore decreasing sleepiness and resetting the biological clockDamage to the clock causes less than normal sleepMice with a mutation in one of the modifier genes, known as overtime, produce circadian rhythms lasting 26 hoursany people with this mutation would have a hard time waking up at a normal timea mutation of the period gene makes people get sleepy early in the evening and awaken early in the morningMelatoninSCN regulates waking and sleeping by controlling activity levels in other brain areas, including the pineal glandpineal gland=endocrine gland located just posterior to the thalamusThe pineal gland releases the hormone melatoninmelatonin influences the circadian and circannual rhythmsthe pineal gland secretes melatonin mostly at night to make us sleepymelatonin secretion starts to increase about 2 or 3 hours before bedtimeSleep and Other Interruptions of ConsciousnessSleep is a state that the brain actively producesCharacterized bymoderate decrease in brain activitydecreased response to stimuliVegetative state: person alternates between periods of sleep and moderate arousalperson shows no awareness of surroundings in moderate arousalMinimally Conscious State: one stage higher, occasional brief periods of purposeful actions and limited amount of speech comprehensionBrain Death: condition of no sign of brain activity and no response to any stimulusphysicals usually wait 24hours before pronouncing deathThe Stages of SleepEEG records rises or falls when the cells do the same thing at the same timePolysomnograph: combination of EEG and eye movement records, for a college student during various stages of sleepAlpha waves: characteristic of relaxation, not of full wakefulnessStage 1: irregular, jagged, low voltage waves. brain activity is less than in relaxed wakefulness but higher than other stages of sleepStage 2: sleep spindles and k complexessleep spindle: consists of 12-14 Hz waves during a burst that lasts at least half a secondresult from oscillating interactions between cells in the thalamus and the cortexK


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FSU PSB 2000 - Circadian Rhythms 9.1

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