14、夏8-大脑与节律-包爱民

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,大脑与节律,包爱民,浙江大学神经生物学系,Biological Rhythms,Almost all land animals coordinate their behavior according to,circadian rhythms-t,he daily cycles of daylight and darkness that result from the spin of the Earth.(The term is from the Latin,circa,“approximately,”and,dies,“day,.”),In humans,there is an approximately,inverse relationship between the propensity,（,倾向、习性,）,to sleep and body temperature,Human Circadian Time Structure,Environmental,Light Intensity,Plasma Melatonin,Concentration,Sleep-Wake Cycle,Time of Day,Body Temperature,Plasma,Cortisol,Concentration,When the cycles of daylight and darkness are removed,-circadian rhythms continue on more or less the same schedule,-the primary clocks for circadian rhythms are not astronomical(the sun and earth)but biological,in the brain.,Brain clocks are imperfect and,require occasional resetting,External stimuli,such as light and dark,or daily temperature changes,help adjust the brains clocks to keep them synchronized with the coming and going of the sunlight.,Brain clocks are an interesting example of,the link between the activity of specific neurons and behavior.,Biological clock,The first evidence,a brainless organism-the mimosa plant that raises leaves during the day and lowers them at night:reacts to sunlight,in some kind of reflex movement?,In 1729,French physicist de,Mairan,:mimosa plants continued to raise and lower their leaves in darkness,still sensing the suns movements?,100 yrs later,Swiss botanist de Candolle:a similar plant in the darkness moved its leaves up and down every 22,rather than 24,hrs,implying the plant was not responding to the sun and very likely had an internal biological clock.,Zeitgeber,:,a stimulus that resets the biological clock(,eg,.,bright light,exercise,temperature,),What sets the Clock?,The strongest,zeitgeber,for both plants and animals,is,light.,Non-,photic,zeitgebers,include,temperature,social interactions,pharmacological manipulation,exercise,and eating/drinking patterns.,Biological clock,Biological clock overrides,（,重写、无视,）,most environmental cues.,To maintain clock-environment synchrony,zeitgebers,induce changes in the concentrations of the molecular components of the clock,to levels consistent with the appropriate stage in the 24-hr cycle,a process termed,entrainment,.,In the presence of,zeitgebers,animals become,entrained,to the day-night rhythm and maintain an activity cycle of exactly 24 hrs.,Deprivation of,zeitgebers,:free-running,Even small,consistent errors of timing could not be tolerated for long-a 24.5-hour cycle would,within 3 weeks,completely shift an animal from daytime to nighttime activity.,When mammals are,completely deprived of,zeitgebers,they settle into an activity-rest rhythm often with a period more or less than 24 hrs,in which case their rhythms are said to,freerun,（,自由运转,）,In mice,the natural free-running period is about 23 hrs;hamsters,24 hrs;humans,24.525.5 hrs,Biological clock,Difficult to separate a human from all possible,zeitgebers,:deep caves,the sites for several isolation studies,When people in caves are allowed to set their own schedules of activity for months on endwaking and sleeping,turning lights on and off,and eating when they,choosethey initially settle into roughly a 25-hr rhythm.,But after days to weeks,their activity may begin to free-run with a surprisingly long period of 3036 hrs:,staying awake for about 20 hrs straight,then sleep for about 12 hrs,and this pattern seems perfectly normal to them at the time.,Biological clock,In isolation experiments,behavior and physiology,do not always continue to cycle together:the rhythms of temperature and sleeping-waking,which are normally synchronized to a 24-hr period,become,desynchronized.,One implication of this,desynchronization,is that the body has,more than one biological clock,uncoupled from one another.,Desynchronization,may occur temporarily -when we travel and force our bodies suddenly into a new sleep-wake cycle:,jet lag,and the best cure is bright light,which helps resynchronize our biological clock.,Light,Because behavior is normally synchronized with light-dark cycles,there must also be,a photosensitive mechanism for resetting the brain clock.,Mammals have a tiny pair of,neuron clusters,in the hypothalamus that serves as a biological clock:,the,suprachiasmatic,nuclei,(,SCN,),retinal ganglion cells send direct projections to the SCN,i.e.,the,retinohypothalamic,tract,which provides information about light to the SCN,light can also alter blood-borne factors,SCN is highly,vascularized,What Reset the Clock?,Melatonin,secreted from the pineal gland,increased levels of melatonin:one feels sleepy,melatonin can act on receptors in the SCN to phase-advance the,biological clock,What Reset the Clock?,The circadian timing system,Superior cervical,ganglion,Axons from,ganglion cells in the retina,synapse directly on the dendrites of