神经生物学：14-大脑与节律

1、大脑与节律大脑与节律Biological RhythmsAlmost all land animals coordinate their behavior according to circadian rhythms - the 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 i

2、nverse relationship between the propensity（倾向、习性倾向、习性） to sleep and body temperatureHuman Circadian Time StructureEnvironmentalLight IntensityPlasma MelatoninConcentrationSleep-Wake CycleTime of DayBody TemperaturePlasma CortisolConcentrationWhen the cycles of daylight and darkness are removed - cir

3、cadian 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 resettingExternal stimuli, such as light and dark, or daily temperature changes

4、, 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 clockThe first evidence, a brainless organism - the mimosa plant that raises leave

5、s 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 th

6、e 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

7、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、无视重写、无视）most environmental cues. To maintain clock-environment synchr

8、ony, 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 cyc

9、le of exactly 24 hrs. Deprivation of zeitgebers: free-runningEven 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 sett

10、le into an activity-rest rhythm often with a period more or less than 24 hrs, in which case their rhythms are said to freerun （自由运转自由运转）In mice, the natural free-running period is about 23 hrs; hamsters, 24 hrs; humans, 24.525.5 hrsBiological clockDifficult to separate a human from all possible zeit

11、gebers: deep caves, the sites for several isolation studiesWhen 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, t

12、heir 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 clockIn isolation experiments, behavior and physiology do not always continue

13、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 temporar

14、ily - 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.LightBecause behavior is normally synchronized with light-dark cycles, there must also be a photosensitive mechanism for resettin

15、g 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 ligh

16、t can also alter blood-borne factors SCN is highly vascularizedWhat 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 clockWhat Reset the Clock?The circadian timing system

17、Superior cervicalganglion Axons from ganglion cells in the retina synapse directly on the dendrites of SCN neurons. This input from the retina is necessary and sufficient to entrain sleeping and waking cycles to night and day. Pineal gland (to birds?) is unimportant to humans, but melatonin may be i

18、mportant.Biological clockWhen the SCN is stimulated electrically, circadian rhythms can be shifted in a predictable wayRemoval of both nuclei abolishes the circadian rhythmicity of physical activity, sleeping and waking, and feeding and drinking.In hamsters, the transplantation of a new SCN can rest

19、ore rhythms within 24 weeks. The brains internal rhythms never return without an SCN. SCNBiological clockA biological clock producing circadian rhythms consists of several components:Light-sensitive input pathway Clock Output pathwayOne or more input pathways are sensitive to light and dark; the clo

20、ck itself continues to run and keep its basic rhythm even when the input pathway is removed. Output pathways from the clock allow it to control certain brain and body functions according to the timing of the clock.Biological clockThe retinal cells synchronizing the SCN are neither rods nor cones -Ey

21、eless mice cannot use light to reset their clocks, but mutant mice with intact retinas that lack rods and cones can! David Berson et al. discovered a new photoreceptor in the retina that was not at all like rods and cones but was, remarkably, a very specialized type of ganglion cell. The light-sensi

22、tive ganglion cells express a newly recognized type of photopigment called melanopsin（视黑素视黑素）, which is not present in rods and cones, and are very slowly excited by light, their axons sending a signal directly to the SCN that can reset the circadian clock that resides there.?Biological clockOutput

23、axons of the SCN mainly innervate nearby parts of the hypothalamus, but some also go to the midbrain and other parts of the diencephalon Because almost all SCN neurons use GABA as their primary neurotransmitter, presumably they inhibit the neurons they innervate.In addition to the axonal output path

24、ways, SCN neurons may rhythmically secrete the peptide neuromodulator vasopressinHow do neurons of the SCN keep time? no complete answer at the molecular level, but its clear that each SCN cell is a minuscule（极小的） clock.Biological clockRemove SCN neurons of an animal and grow them alone in a tissue

25、culture dish - their rates of action potential firing, glucose utilization, vasopressin production, and protein synthesis continue to vary with rhythms of about 24 hrs, just as they do in the intact brain. Circadian rhythms of the SCN isolated from the rest of the brain. The activity of a clock gene

26、 was monitored in 100 individual SCN neurons maintained in tissue culture. Each neuron generates a strong circadian rhythm that is well coordinated with the neurons. (Fig 19.22)Biological clockSCN cells in culture can no longer be entrained to light-dark cycles (input from the eyes is necessary for

27、this), but their basic rhythmicity remains intact and expresses itself just as it does when an animal is deprived of zeitgebers.SCN cells communicate their rhythmic message to the rest of the brain through efferent axons, using action potentials (APs) in the usual way, and rates of SCN cell firing v

28、ary with a circadian rhythm.However, APs are unnecessary for SCN neurons to maintain their rhythm - When tetrodotoxin (TTX，河豚毒河豚毒), a blocker of sodium channels, is applied to SCN cells, it blocks their APs but has no effect on the rhythmicity of their metabolism and biochemical functions. Biologica

29、l clockWhen the TTX is removed, APs resume （重新开始重新开始）firing with the same phase and frequency they had originally - the SCN clock keeps running even without APs. SCN APs are like the hands of a clock; removing the clocks hands does not stop the clock from working, but it does make it very difficult

30、to read the time. It is a molecular cycle based on gene expression, which in humans is very similar to those found in mice, fruit flies (Drosophila), and bread mold: in Drosophila and mice, the system involves clock genes known as period (per), timeless (tim), and clock. Biological clockAlthough the

31、 details vary across species, the basic scheme is a negative feedback loop. Joseph Takahashi et al. at Northwestern University named the clock gene (an acronym for circadian locomotor output cycles kaput). A clock gene is transcribed to produce mRNA that is then translated into proteins. After a del

32、ay, the newly manufactured proteins send feedback and somehow interact with the transcription mechanism, causing a decrease in gene expression - less protein is produced, and gene expression again increases to start the cycle anew. This entire cycle takes about 24 hrs, and thus it is a circadian rhy

33、thmBiological clockClock genes. In the SCN, clock genes produce proteins that inhibit further transcription.Gene transcription and the firing rate of individual SCN neurons cycle up and down over 24 hrs.The cycles of many cells are synchronized by light exposure (input from theretina) and by interac

34、tions of the SCN neurons.(Figure 19.23).Biological clockThere must be a mechanism to coordinate the thousands of cellular clocks so that the SCN as a whole transmits a single, clear message about time to the rest of the brain. Light information from the retina serves to reset the clocks in the SCN n

35、eurons each day, but the SCN neurons also communicate directly with each other and seems to be independent of APs and normal synaptic transmission: TTX does not block them. Also, the SCN of the very young rat brain coordinates circadian rhythms perfectly well, even before it has developed any chemic

36、al synapses. In addition to classical chemical synapses, it may include other chemical signals, gap junctions, or the participation of glia.Types of Rhythms Circadian fluctuate daily Sleep-wake, temperature, hormones, urine production, gastrointestinal activity Cognitive and motor performance levels Infradian less than once a day Hibernation, ovulation Ultradian more than once a day Sleep cycles (REM and other sleep stages