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Chapter 9 Wakefulness and Sleep. Rhythms of Waking and Sleep. Animals generate endogenous 24 hour cycles of wakefulness and sleep. Some animals generate endogenous circannual rhythms, internal mechanisms that operate on an annual or yearly cycle.
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Rhythms of Waking and Sleep • Animals generate endogenous 24 hour cycles of wakefulness and sleep. • Some animals generate endogenous circannual rhythms, internal mechanisms that operate on an annual or yearly cycle. • Example: Birds migratory patterns, animals storing food for the winter.
Rhythms of Waking and Sleep • All animals produce endogenous circadian rhythms, internal mechanisms that operate on an approximately 24 hour cycle. • Regulates the sleep/ wake cycle. • Also regulates the frequency of eating and drinking, body temperature, secretion of hormones, volume of urination, and sensitivity to drugs.
Rhythms of Waking and Sleep Circadian rhythms: • Remains consistent despite lack of environmental cues indicating the time of day • Can differ between people and lead to different patterns of wakefulness and alertness. • Change as a function of age. • Example: sleep patterns from childhood to late adulthood.
Rhythms of Waking and Sleep • Experiments designed to determine the length of the circadian rhythm place subjects in environments with no cues to time of day. • Results depend upon the amount of light to which subjects are artificially exposed. • Rhythms run faster in bright light conditions and subjects have trouble sleeping. • In constant darkness, people have difficulty waking.
Rhythms of Waking and Sleep • Human circadian clock generates a rhythm slightly longer than 24 hours when it has no external cue to set it. • Most people can adjust to 23- or 25- hour day but not to a 22- or 28- hour day. • Bright light late in the day can lengthen the circadian rhythm.
Rhythms of Waking and Sleep • Mechanisms of the circadian rhythms include the following: • The Suprachiasmatic nucleus. • Genes that produce certain proteins. • Melatonin levels.
Rhythms of Waking and Sleep • The suprachiasmatic nucleus (SCN) is part of the hypothalamus and the main control center of the circadian rhythms of sleep and temperature. • Located above the optic chiasm. • Damage to the SCN results in less consistent body rhythms that are no longer synchronized to environmental patterns of light and dark.
Rhythms of Waking and Sleep • The SCN is genetically controlled and independently generates the circadian rhythms. • Single cell extracted from the SCN and raised in tissue culture continues to produce action potential in a rhythmic pattern. • Various cells communicate with each other to sharpen the circadian rhythm.
Rhythms of Waking and Sleep • Two types of genes are responsible for generating the circadian rhythm. • Period - produce proteins called Per. • Timeless - produce proteins called Tim. • Per and Tim proteins increase the activity of certain kinds of neurons in the SCN that regulate sleep and waking. • Mutations in the Per gene result in odd circadian rhythms.
Rhythms of Waking and Sleep • The SCN regulates waking and sleeping by controlling activity levels in other areas of the brain. • The SCN regulates the pineal gland, an endocrine gland located posterior to the thalamus. • The pineal gland secretes melatonin, a hormone that increases sleepiness.
Rhythms of Waking and Sleep • Melatonin secretion usually begins 2 to 3 hours before bedtime. • Melatonin feeds back to reset the biological clock through its effects on receptors in the SCN. • Melatonin taken in the afternoon can phase-advance the internal clock and can be used as a sleep aid.
Rhythms of Waking and Sleep • The purpose of the circadian rhythm is to keep our internal workings in phase with the outside world. • Light is critical for periodically resetting our circadian rhythms. • A zeitgeber is a term used to describe any stimulus that resets the circadian rhythms. • Exercise, noise, meals, and temperature are others zeitgebers.
Rhythms of Waking and Sleep • Jet lag refers to the disruption of the circadian rhythms due to crossing time zones. • Stems from a mismatch of the internal circadian clock and external time. • Characterized by sleepiness during the day, sleeplessness at night, and impaired concentration. • Traveling west “phase-delays” our circadian rhythms. • Traveling east “phase-advances” our circadian rhythms.
Rhythms of Waking and Sleep • Light resets the SCN via a small branch of the optic nerve known as the retinohypothalamic path. • Travels directly from the retina to the SCN. • The retinohypothalamic path comes from a special population of ganglion cells that have their own photopigment called melanopsin. • The cells respond directly to light and do not require any input from the rods or cones.
Stages of Sleep And Brain Mechanisms • Sleep is a specialized state that serves a variety of important functions including: • conservation of energy. • repair and restoration. • learning and memory consolidation.
Stages of Sleep And Brain Mechanisms • The electroencephalograph (EEG) allowed researchers to discover that there are various stages of sleep. • Over the course of about 90 minutes: • a sleeper goes through sleep stages 1, 2, 3, and 4 • then returns through the stages 3 and 2 to a stage called REM.
Stages of Sleep And Brain Mechanisms • Alpha waves are present when one begins a state of relaxation. • Stage 1 sleep is when sleep has just begun. • the EEG is dominated by irregular, jagged, low voltage waves. • brain activity begins to decline.
Stages of Sleep And Brain Mechanisms • Stage 2 sleep is characterized by the presence of: • Sleep spindles - 12- to 14-Hz waves during a burst that lasts at least half a second. • K-complexes - a sharp high-amplitude negative wave followed by a smaller, slower positive wave.
Stages of Sleep And Brain Mechanisms • Stage 3 and stage 4 together constitute slow wave sleep (SWS) and is characterized by: • EEG recording of slow, large amplitude wave. • Slowing of heart rate, breathing rate, and brain activity. • Highly synchronized neuronal activity.
Stages of Sleep And Brain Mechanisms • Rapid eye movement sleep (REM) are periods characterized by rapid eye movements during sleep. • Also known as “paradoxical sleep” because it is deep sleep in some ways, but light sleep in other ways. • EEG waves are irregular, low-voltage and fast. • Postural muscles of the body are more relaxed than other stages.
Stages of Sleep And Brain Mechanisms • Stages other than REM are referred to as non-REM sleep (NREM). • When one falls asleep, they progress through stages 1, 2, 3, and 4 in sequential order. • After about an hour, the person begins to cycle back through the stages from stage 4 to stages 3 and 2 and than REM. • The sequence repeats with each cycle lasting approximately 90 minutes.
Stages of Sleep And Brain Mechanisms • Stage 3 and 4 sleep predominate early in the night. • The length of stages 3 and 4 decrease as the night progresses. • REM sleep is predominant later in the night. • Length of the REM stages increases as the night progresses. • REM is strongly associated with dreaming, but people also report dreaming in other stages of sleep.
Stages of Sleep And Brain Mechanisms • Various brain mechanisms are associated with wakefulness and arousal. • The reticular formation is a part of the midbrain that extends from the medulla to the forebrain and is responsible for arousal.
Stages of Sleep And Brain Mechanisms • The pontomesencephalon is a part of the midbrain that contributes to cortical arousal. • Axons extend to the thalamus and basal forebrain which release acetylcholine and glutamate • produce excitatory effects to widespread areas of the cortex. • Stimulation of the pontomesencephalon awakens sleeping individuals and increases alertness in those already awake.
Stages of Sleep And Brain Mechanisms • The locus coeruleus is small structure in the pons whose axons release norepinephrine to arouse various areas of the cortex and increase wakefulness. • Usually dormant while asleep.
Stages of Sleep And Brain Mechanisms • The basal forebrain is an area anterior and dorsal to the hypothalamus containing cells that extend throughout the thalamus and cerebral cortex. • Cells of the basal forebrain release the inhibitory neurotransmitter GABA. • Inhibition provided by GABA is essential for sleep. • Other axons from the basal forebrain release acetylcholine which is excitatory and increases arousal.
Stages of Sleep And Brain Mechanisms • The hypothalamus contains neurons that release “histamine” to produce widespread excitatory effects throughout the brain. • Anti-histamines produce sleepiness.
Stages of Sleep And Brain Mechanisms • Orexin is a peptide neurotransmitter released in a pathway from the lateral nucleus of the hypothalamus highly responsible for the ability to stay awake. • Stimulates acetylcholine-releasing cells in the forebrain and brain stem to increase wakefulness and arousal.
Stages of Sleep And Brain Mechanisms • Decreased arousal required for sleep is accomplished via the following ways: • Decreasing the temperature of the brain and the body. • Decreasing stimulation by finding a quiet environment. • Accumulation of adenosine in the brain to inhibit the basal forebrain cells responsible for arousal. • Caffeine blocks adenosine receptors.
Stages of Sleep And Brain Mechanisms (cont’d): • Accumulation of prostaglandins that accumulate in the body throughout the day to induce sleep. • Prostaglandins stimulate clusters of neurons that inhibit the hypothalamic cells responsible for increased arousal.
Stages of Sleep And Brain Mechanisms • During REM sleep: • Activity increases in the pons (triggers the onset of REM sleep), limbic system, parietal cortex and temporal cortex. • Activity decreases in the primary visual cortex, the motor cortex, and the dorsolateral prefrontal cortex.
Stages of Sleep And Brain Mechanisms • REM sleep is also associated with a distinctive pattern of high-amplitude electrical potentials known as PGO waves. • Waves of neural activity are detected first in the pons and then in the lateral geniculate of the hypothalamus, and then the occipital cortex. • REM deprivation results in high density of PGO waves when allowed to sleep normally.
Stages of Sleep And Brain Mechanisms • Cells in the pons send messages to the spinal cord which inhibit motor neurons that control the body’s large muscles. • Prevents motor movement during REM sleep. • REM is also regulated by serotonin and acetylcholine. • Drugs that stimulate Ach receptors quickly move people to REM. • Serotonin interrupts or shortens REM.
Stages of Sleep And Brain Mechanisms • Insomnia is a sleep disorder associated with inability to fall asleep or stay asleep. • Results in inadequate sleep. • Caused by a number of factors including noise, stress, pain medication. • Can also be the result of disorders such as epilepsy, Parkinson’s disease, depression, anxiety or other psychiatric conditions. • Dependence on sleeping pills and shifts in the circadian rhythms can also result in insomnia.
Stages of Sleep And Brain Mechanisms • Sleep apnea is a sleep disorder characterized by the inability to breathe while sleeping for a prolonged period of time. • Consequences include sleepiness during the day, impaired attention, depression, and sometimes heart problems. • Cognitive impairment can result from loss of neurons due to insufficient oxygen levels. • Causes include, genetics, hormones, old age, and deterioration of the brain mechanisms that control breathing and obesity.
Stages of Sleep And Brain Mechanisms • Narcolepsy is a sleep disorder characterized by frequent periods of sleepiness. • Four main symptoms include: • Gradual or sudden attack of sleepiness. • Occasional cataplexy - muscle weakness triggered by strong emotions. • Sleep paralysis- inability to move while asleep or waking up. • Hypnagogic hallucinations- dreamlike experiences the person has difficulty distinguishing from reality.
Stages of Sleep And Brain Mechanisms (Insomnia cont’d) • Seems to run in families although no gene has been identified. • Caused by lack of hypothalamic cells that produce and release orexin. • Primary treatment is with stimulant drugs which increase wakefulness by enhancing dopamine and norepinephrine activity.
Stages of Sleep And Brain Mechanisms • Periodic limb movement disorder is the repeated involuntary movement of the legs and arms while sleeping. • Legs kick once every 20 to 30 seconds for periods of minutes to hours. • Usually occurs during NREM sleep.