Lecture 19: Cortical States and sleep

Sleep

A naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles

Governed by brainstem nuclei with diffuse projections to brain and spinal cord

Brain remains active

Measuring Sleep

• Electroencephalography (EEG) = scalp recordings of electrical activity

• Low spatial resolution = frontal, temporal, central, parietal, occipital leads

Ecological adaptation

Hunting/avoiding predators difficult when relying on vision at night

Energy conservation and restoration

• Sleep deprivation compromises immune system

• Glycogen levels fall during day = replenish brain glycogen

• Metabolism decreases during sleep = reduce heat loss in colder night

Memory consolidation

• Increase strength of synaptic connections induced by experiences during waking hours

• Studies of remembered spatial location in rodents = ensembles of hippocampal neurons activated during a spatial memory task are reactivated during sleep

• Humans show sleep dependent improvement in learning

Stage 1

drowsy period

→ decrease in frequency, slight increase in amplitude of EEG waves

Stage 2

light sleep

→ decrease frequency, increase amplitude, intermittent spike clusters (sleep spindles)

Stage 3

moderate to deep sleep

→decrease frequency, increase amplitude + (sleep spindles)

Stage 4

deep sleep

low frequency, high amplitude delta waves

Wave Activity

• Alpha and beta waves = wake

• Theta and delta waves = sleep

• Deep sleep characterized by synchronous activity in pyramidal cortical neurons

• Synchronization = increased amplitude of EEG waves as in deep sleep delta waves

REM sleep

• Follows non-REM sleep

• First period of REM sleep lasts about 10 minutes then cycles back through the four nonREM stages

• Usually about 5 periods of REM sleep/night, each period gets longer as sleep continues

• EEG recordings of REM sleep are remarkably similar to the awake state

Dreaming

During REM sleep → elaborate, vivid, emotional (sometimes bizarre and emotional)

During non-REM sleep → fewer dreams, more conceptual, less vivid, less emotional, related to everyday activities, more thought-like

→ (Sigmund Freud’s “day residue”: events and conflicts of the day)

Activation synthesis theory

Dreams are caused by the random firing of neurons in the cerebral cortex during the REM period.

The forebrain then “creates a story” in an attempt to make sense of the nonsensical sensory information → explains irrationality of the mind during REM sleep.

melanopsin

Photosensitive retinal ganglion cells that contain ____ are depolarized by light

→ Encode illumination to set the “biological clock”

Suprachiasmatic Nucleus

A part of the hypothalamus that is the central site for circadian control

Have an endogenous 24-hour rhythm that can persist when external time cues are removed

In darkness, it turns on the pineal gland to produce melatonin

Melatonin

A sleep-promoting hormone released by the pineal gland

REM sleep activity

Increased limbic activity

→ Amygdala, hippocampus, pontine tegmentum, anterior cingulate cortex activity increased during REM sleep

Decreased frontal cortex activity

→ Dorsolateral prefrontal cortex and posterior cingulate cortex inactivated during REM sleep

reticular activating system

Stimulation of cholinergic neurons here wakes up the cat

Low-frequency pulse stimulation of the thalamus causes the cat to sleep

Cholinergic Nuclei

Located near the pons-midbrain junction

Key element of wakefulness and REM sleep → project to thalamus and cortex

• Show high activity during waking

• Low activity during non-REM sleep

• Activity gradually increases during REM sleep

Serotonergic Neurons

Located in the raphe nuclei

• Active during wakefulness

• Decreased activity or inactive during sleep

Noradrenergic neurons

Located in the locus coeruleus

• Active during waking and arousal/stress

• Inactive during sleep

Histamine neurons

Located in the tuberomammilary nucleus (TMN) of the hypothalamus

• Fire rapidly during waking

• Fire slowly during periods of relaxation/tiredness

• Stop firing during sleep

• Modulates LC and raphe

Tuberomammilary Nucleus (TMN)

Activated by the peptide orexin from lateral hypothalamus

→ Orexin and histamine promote wakefulness

Antihistamines cause drowsiness

Ventrolateral preoptic nucleus (VLPO)

Part of the hypothalamus that inhibits wake circuits

• Onset of sleep

• Lesions of this produce insomnia

Thalamocortical neurons

Have 2 states:

• Intrinsic bursting (oscillatory) state = sleep; thalamocortical neurons synchronized with cortex (disconnect cortex from outside world)

→ Oscillation decreased by activation of ascending systems = diminishes high amplitude, low frequency slow waves of sleep

• Tonically active state = awake → thalamocortical neurons depolarized by reticular activating system; transmitting information encoding peripheral stimuli to cortex

Insomnia

Inability to sleep a sufficient length of time (or deeply enough)

Causes → stress, jetlag, caffeine, depression, age altering neurotransmitters controlling sleep

Sleep apnea

Interrupted breathing during sleep causes little to no slow-wave sleep and less time in REM sleep (figure)

Causes → problems with pharynx musculature or compressed by obesity, rise in CO2 causes reflexive inspiration and wakes person up

Narcolepsy

Frequent 30 sec to 30 min REM sleep attacks during day, enter REM from wake without going through non-REM sleep

Causes → animal studies indicate mutation to orexin-2 receptor gene (Orx2)

Jet lag

(desynchronosis) → Internal vs. external clock

• Headaches

• Fatigue, irregular sleep patterns, insomnia

• Disorientation, grogginess, irritability

• Mild depression

• Constipation or diarrhea

Delayed sleep phase syndrome (DSPS)

out-of-phase with the 24 cycle → later every day

Advanced sleep phase syndrome (ASPS)

out-of-phase with the 24 cycle → earlier every day.

Non-24-hour sleep wake-syndrome

short- or long-circadian rhythm.

Situational circadian rhythm disorder (shift work disorder)

changing circadian rhythm disorder

Night terror (Pavor nocturnus)

Abrupt terrified/agitated awakening from sleep.

Somniphobia

Dread of going to sleep.

Hypopnea syndrome

Abnormally shallow breathing or slow respiratory rate while sleeping.

Parasomnia

Disruptive inappropriate actions during sleep

Bruxism

Involuntarily grinding or clenching of the teeth while sleeping

Periodic limb movement disorder (PLMD)

Involuntary movement of arms and/or legs

Restless legs syndrome (RLS)

Irresistible urge to move legs, without other PLMD.

Sleep paralysis

Temporary paralysis before or after sleeping.

Nocturia

Frequent need to get up to urinate at night.

Enuresis

Bed wetting (without waking).