Circadian Rhythms 9 1 10 29 2013 Endogenous Cycles year Endogenous circannual rhythm generated within for about a o Birds generate a rhythm that prepares it for seasonal changes o Even 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 day o Light from the sun helps you feel less sleepy o Your urge to sleep depends on the part of day it is not just how many hours you have been awake o example college student that feels sleepier and sleepier as the night goes on but as morning arrives not so much o No one has succeeded in producing a rhythm far from the 24 hour norm o Mammals and humans have circadian rhythms in their waking and sleeping eating and drinking urination secretion of hormones sensitivity to drugs and other variables o Differ among individuals o Older rats reach their best performance shortly after awakening younger rats improve performance as the day progresses Setting and Resetting the Biological Clock Circadian rhythm is not perfect but generates a period close to 24 hours outside world alter it Zeitgeber We readjust our internal workings daily to stay in phase with the We expose ourselves to lights noises and activity at night and then awaken late the next morning Light is critical for resetting the rhythm Free Running Time rhythm that occurs when no stimuli reset or o the stimulus that resets the rhythm is called zeitgeber Jet Lag o light is the dominant zeitgeber for land animals o The tides are important for marine animals o Also exercise noise meals and the temperature but these effects are generally weak on their own without light Example 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 together Jet Lag disruption of circadian rhythms due to crossing time zones travelers complain of sleepiness during the day sleeplessness at night depression and impaired concentration o these problems stem from mismatch between internal circadian clock and external time Flight attendants on average have smaller than average volumes of the hippocampus and surrounding structures and they showed some memory impairments o These results suggest a danger from repeated adjustments of circadian rhythm Shift Work People who sleep irregularly find that their duration of sleep depends on what time they go to sleep When people go to sleep in the morning or early afternoon they sleep only briefly even if they have been awake for many hours The Suprachiasmatic Nucleus SCN Suprachiasmatic nucleus SCN biological clock that depends on apart of the hypothalamus Provides the main control of the circadian rhythms for sleep and After damage to the SCN the body s rhythms are less consistent and no longer synchronized to environmental patterns of light and SCN generates circadian rhythms itself in a genetically controlled body temperature dark unlearned manner If 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 potentials A mutation in one gene causes hamsters SCN to produce a 20 hour rhythm How Light Resets the SCN The SCN is located just above the optic chiasm A small branch of the optic nerve known as the retinohypothalamic path extends directly from the retina to the SCN axons of that path alter the SCN s settings Melanopsin special ganglion cells respond directly to light even if they do not receive any input from rods or cones Respond to overall average amount of light not to instantaneous changes in light So how does it work o 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 Rhythm Genes generate a circadian rhythm Two genes period per and timeless tim produce the proteins Per and Tim and Tim Per and Tim increase in amount as the day persists By 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 When Per and Tim are high they interact with a protein called Clock o Clock induces sleepiness Low Per and low Tim wakefulness Extra light decreases the Tim protein therefore decreasing sleepiness and resetting the biological clock Damage to the clock causes less than normal sleep Mice with a mutation in one of the modifier genes known as overtime produce circadian rhythms lasting 26 hours o any people with this mutation would have a hard time waking up at a normal time a mutation of the period gene makes people get sleepy early in the evening and awaken early in the morning Melatonin SCN regulates waking and sleeping by controlling activity levels in other brain areas including the pineal gland o pineal gland endocrine gland located just posterior to the thalamus The pineal gland releases the hormone melatonin o melatonin influences the circadian and circannual rhythms the pineal gland secretes melatonin mostly at night to make us melatonin secretion starts to increase about 2 or 3 hours before sleepy bedtime Stages Of Sleep and Brain Mechanisms 9 2 10 29 2013 Sleep and Other Interruptions of Consciousness Sleep is a state that the brain actively produces Characterized by o moderate decrease in brain activity o decreased response to stimuli Vegetative state person alternates between periods of sleep and moderate arousal arousal o person shows no awareness of surroundings in moderate Minimally Conscious State one stage higher occasional brief periods of purposeful actions and limited amount of speech comprehension Brain Death condition of no sign of brain activity and no response to any stimulus The Stages of Sleep same time o physicals usually wait 24hours before pronouncing death EEG records rises or falls when the cells do the same thing at the Polysomnograph combination of EEG and eye movement records for a college student during various stages of sleep Alpha waves characteristic of relaxation not of full wakefulness Stage 1 irregular jagged low voltage waves brain activity is less than in relaxed wakefulness but higher than other stages of sleep Stage 2 sleep spindles and k complexes o sleep spindle consists of 12 14 Hz waves
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