NEUR 2600 Lecture Notes - Lecture 13: Implicit Memory, Sensory Deprivation, Reticular Formation
CHAPTER 13: WHY DO WE SLEEP AND DREAM
● A clock for all seasons: origins of biological rhythms
○ Biorhythm
■ Inherent timing mechanism that controls or initiates various biological
processes
■ Linked to the cycle of days and seasons produced by the Earth’s rotation
around the sun
■ Animals living near the poles of the Earth are more affected by seasonal
changes than animals living in equatorial regions
■ Biorhythms are not unique to animals
● Plants display rhythmic behaviour: species whose leaves or
flowers open during the day and close at night
■ Humans largely evolved as equatorial animals, and our behaviour is
dominated by a circadian rhythm of daylight activity and nocturnal sleep
■ Daily cycles in humans
● Pulse rate, blood pressure, body temperature, rate of cell division,
blood cell count, alertness, urine composition, metabolic rate,
sexual drive, feeding behaviour, responsiveness to medications
○ Biological clocks
■ Behaviour is not simply driven by external cues from the environment
■ Rhythms are endogenous: control comes from within
■ Biological clock
● Neural system that times behaviour
● Allows animals to anticipate events before they happen
○ Example: birds migrate before it gets cold
■ Humans have a biological clock that synchronizes behaviour to the
temporal passage of a real day and makes predictions about tomorrow
■ Clock lets us anticipate events and prepare for them both physiologically
and cognitively
● Biological clock regulates feeding times, sleeping times, and
metabolic activity
● Regulates gene expression in every cell in the body
○ Measuring biological rhythms
■ Period
● Time required to complete a cycle of activity
■ Circannual rhythm
● Yearly (e.g., migratory cycles of birds)
■ Infradian rhythm
● Less than a year (e.g., human menstrual cycle)
■ Circadian rhythm
● Daily (e.g., human sleep cycle)
■ Ultradian rhythm
● Less than a day (e.g., eating cycle)
○ Free-running rhythms
■ Rhythm of the body’s own devising in the absence of all external cues
● Without input from external cues, our body has its own rhythms
with a period of 25 to 27 hours
● Sleep-wake cycle shifts an hour or so everyday
■ Animals expand and contract their sleep periods as the sleep-related
lighting period expands or contracts
■ Hamsters: nocturnal
● In constant darkness, free-running periods are shorter than 24
hours
● In constant light, free-running periods are longer than 24 hours
■ Sparrows: diurnal
● In constant darkness, free-running periods are longer than 24
hours
● In constant light, free-running periods are shorter than 24 hours
○ Zeitgebers
■ Zeitgeber
● Environmental event that entrains biological rhythms; a time setter
○ Example: light resets the biological clock
● The property that allows entrainment of a biological clock explains
how circadian rhythms synchronize with seasonal changes in
daylight
● Light pollution
○ Exposure to artificial lighting disrupts circadian rhythms
and accounts for much inconsistent behaviour associated
with accidents, daytime fatigue, alterations in emotional
states, obesity, diabetes, and other disorders
● Jet lag
○ Fatigue and disorientation resulting from rapid travel
through time zones and exposure to a changed light-dark
cycle
○ West-to-east traveler generally has a more difficult
adjustment than does the east-to-west traveler
● Persistent asynchronous rhythms generated by jet lag are
associated with
○ Altered sleep and temperature rhythms, fatigue, and
stress, even reduced success by sports teams
■ Entrainment
● Determines or modifies the period of a biorhythm
■ An entrained biological clock allows an animal to synchronize its daily
activity across these seasonal changes
● Neural basis of the biological clock: suprachiasmatic rhythms
○ Suprachiasmatic nucleus (SCN)
■ Main pacemaker of circadian rhythms; located just above the optic chiasm
○ SCN is the master clock
○ The intergeniculate leaflet and the pineal gland also display clocklike activity
○ Nearly every cell in the body has its own clock
○ If SCN is damaged, daily activities occur haphazardly
○ SCN cells increase metabolic activity during light period
○ Cells are more electrically active in light period
○ SCN neurons maintain rhythmic activity in absence of input and output
○ Cells in a dish retain periodic rhythm
● Keeping time
○ If SCN neurons are isolated from one another, each remains rhythmic, but the
rhythmicity of some cells is different from that of other cells
■ Timing of the rhythm must be set so that the cells can synchronize their
activity in relation both to each other and to Zeitgebers
○ Retinohypothalamic pathway
■ Neural route from a subset of cone receptors in the retina to the SCN
■ Allows lights to entrain rhythmic activity of SCN
■ Begins with specialized retinal ganglion cells (RGCs) that contain the
photosensitive pigment melanopsin
○ The retinohypothalamic tract activates core cells
Document Summary
Chapter 13: why do we sleep and dream. A clock for all seasons: origins of biological rhythms. Inherent timing mechanism that controls or initiates various biological processes. Linked to the cycle of days and seasons produced by the earth"s rotation around the sun. Animals living near the poles of the earth are more affected by seasonal changes than animals living in equatorial regions. Plants display rhythmic behaviour: species whose leaves or flowers open during the day and close at night. Humans largely evolved as equatorial animals, and our behaviour is dominated by a circadian rhythm of daylight activity and nocturnal sleep. Pulse rate, blood pressure, body temperature, rate of cell division, blood cell count, alertness, urine composition, metabolic rate, sexual drive, feeding behaviour, responsiveness to medications. Behaviour is not simply driven by external cues from the environment. Rhythms are endogenous: control comes from within. Allows animals to anticipate events before they happen. Example: birds migrate before it gets cold.
