Chapter 19: Brain Rhythms and Sleep Neuroscience: Exploring the Brain, 4e Chapter 19: Brain Rhythms and Sleep

Introduction Rhythmic activities of the brain Sleeping and waking, hibernation, breathing, walking, electrical rhythms of cerebral cortex Cerebral cortex: range of electrical rhythms correlated with interesting behaviors EEG: classical method of recording brain rhythms, essential for studying sleep Circadian rhythms: changes in physiological functions according to brain clock

The Electroencephalogram (EEG) Measurement of generalized activity of cerebral cortex Helps diagnose neurological conditions, such as epilepsy and sleep disorders, and for research

Recording Brain Waves Noninvasive, painless Electrodes on scalp with low-resistance connection Connected to banks of amplifiers and recording devices Voltage fluctuations measured (tens of microvolts) Electrode pairs: measure different brain regions Amplitude of the EEG signal a measure of synchronous activity of underlying neurons

Generation of Electrical Fields Recorded by EEG

Generating Large EEG Signals by Synchronous Activity

Magnetoencephalography (MEG) Recording of miniscule magnetic signals generated by neural activity Compared with EEG, fMRI, PET MEG localizes sources of neural activity better than EEG. MEG cannot provide detailed images of fMRI. EEG and MEG measure neuron activity. fMRI and PET measure changes in blood flow or metabolism.

Magnetoencephalography (MEG)—(cont.)

EEG Rhythms Often correlate with particular states of behavior Categorization of rhythms based on frequency Beta: 15–30 Hz, activated or attentive cortex Alpha: 8–13 Hz, quiet, waking state Theta: 4–7 Hz, some sleep and waking states Delta: less than 4 Hz, deep sleep Spindles, ripples Deep sleep: high synchrony, high EEG amplitude

A Normal EEG

EEG Rhythms Across Species

Mechanisms of Synchronous Rhythms Rhythms can be led by a pacemaker or arise from collective behavior of all participants.

Mechanisms of Brain Rhythms Synchronized oscillation mechanisms Central clock/pacemaker and/or collective methods Thalamus  massive cortical input as pacemaker Neuronal oscillations Voltage-gated ion channels

Functions of Brain Rhythms Hypotheses Sleep as brain’s way of disconnecting cortex from sensory input Some rhythms may have no direct function—by- products of strongly interconnected circuits Walter Freeman: Neural rhythms coordinate activity of regions of the nervous system. By synchronizing oscillations from different regions, brain may bind together a single perceptual construction.

Seizures and Epilepsy Epilepsy causes repeated seizures. Causes: tumor, trauma, genetics, infection, vascular disease, many cases unknown Generalized seizure: entire cerebral cortex, complete behavior disruption, consciousness loss Partial seizure: circumscribed cortex area, abnormal sensation or aura Absence seizure: less than 30 seconds of generalized 3 Hz EEG waves

An EEG of a Generalized Epileptic Seizure

Sleep A readily reversible state of reduced responsiveness to, and interaction with, the environment Universal among higher vertebrates Sleep deprivation is devastating to proper functioning. One-third of our lives spent in sleep state Purpose of sleep still unclear

Three Functional Brain States

Physiological Changes During Non-REM and REM Sleep

EEG Rhythms during Sleep Stages

Why Do We Sleep? All mammals, birds, and reptiles appear to sleep— apparently needed by the brain Two main categories of theories of sleep function Restoration Sleep to rest and recover, and prepare to be awake again Adaptation Sleep to keep out of trouble, hide from predators

Functions of Dreaming and REM Sleep Unclear why we dream—but body requires REM sleep Sigmund Freud: dream functions—wish fulfillment, conquer anxieties Hobson and McCarley: activation–synthesis hypothesis Karni: Certain memories require strengthening time period  REM sleep.

Neural Mechanisms of Sleep Critical neurons  diffuse modulatory neurotransmitter systems Noradrenergic and serotoninergic neurons: fire during and enhance waking state Cholinergic neurons: Some enhance REM events, others active during waking. Diffuse modulatory system control rhythmic behaviors of thalamus  controls cortical EEG  sensory input flow to cortex blocked by slowed thalamic rhythms Activity in descending branches of diffuse modulatory systems (e.g., inhibitory neurons)

Wakefulness and the Ascending Reticular Activating System Brain stem lesions cause sleep, coma Moruzzi’s research Lesions in midline structure of brain stem cause state similar to non-REM sleep. Lesions in lateral tegmentum do not cause non-REM state sleep. Electrical stimulation of midline tegmentum of midbrain changes cortex from slow, rhythmic EEGs of non-REM sleep to alert and aroused state.

Key Components of the Waking/Sleeping Modulatory Systems

Falling Asleep and Non-REM State Sleep: progression of changes ending in non-REM state Non-REM sleep: decrease in firing rates of most brain stem modulatory neurons using NE, 5-HT, ACh Stages of non-REM sleep EEG sleep spindles Spindles disappear Replaced by slow, delta rhythms (less than 4 Hz)

PET Images of Waking and Sleeping Brain

Control of REM Sleep by Brain Stem Neurons

Sleep-Promoting Factors Adenosine: released by neurons; may have inhibitory effects of diffuse modulatory systems Nitric acid (NO): triggers release of adenosine Muramyl dipeptide: isolated from the CSF of sleep- deprived goats, facilitates non-REM sleep Interleukin-1: synthesized in brain, stimulates immune system, induces fatigue and sleepiness Melatonin: released at night, inhibited during daylight; helps initiate and maintain sleep—used to treat symptoms of jet lag and insomnia

Gene Expression during Sleeping and Waking Cirelli and Tononi compared gene expression in brains of awake and sleeping rats. 0.5% of genes showed differences of expression levels when awake or asleep. Genes that increased in awake rats: intermediate early genes and mitochondrial genes Genes that increased in sleeping rats: genes that contribute to protein synthesis and plasticity mechanisms These changes specific to the brain, not other tissues

Circadian Rhythms Daily cycles of light and dark Schedules of circadian rhythms vary among species. Most physiological and biochemical processes in body rise and fall with daily rhythms. If daylight and darkness cycles are removed, circadian rhythms continue. Brain clocks require occasional resetting.

Circadian Rhythms of Physiological Functions

Evidence for Biological Clocks Jacques d'Ortous de Mairan Mimosa plant leaf movement continues circadian rhythm even in the dark. Augustin de Candolle A plant in the dark responds to internal biological clock. Zeitgebers (German for “time givers”) Environmental time cues For mammals: primarily light–dark cycle

Light sensor  Clock  Output pathway Biological Clocks Free-run: Mammals completely deprived of zeitgebers settle into rhythm of activity and rest but drift out of phase with 12-hour day/light cycle. Behavior and physiology do not always cycle together. Components of biological clock Light sensor  Clock  Output pathway

Circadian Rhythms of Sleep and Wakefulness

The Suprachiasmatic Nucleus (SCN) Intact SCN produces rhythmic message: SCN cell firing rate varies with circadian rhythm. Retinal input necessary to entrain sleep cycles to night

Retinal Ganglion Cells Berson and colleagues discovered specialized type of ganglion cell in retina. Photoreceptor but not a rod or cone Expresses melanopsin, slowly excited by light Synapses directly onto SCN neurons to reset circadian clock SCN output axons to parts of the hypothalamus, midbrain, diencephalons; use GABA as primary neurotransmitter

SCN Mechanisms Molecular clocks similar in humans, mice, flies, mold Clock genes: period (per), cryptochrome, clock Takahashi: regulation of transcription and translation, negative feedback loop

Concluding Remarks Rhythms ubiquitous in the mammalian CNS Some from intrinsic brain mechanisms Some from environmental factors Some from interaction of neural processes and zeitgebers (like SCN clock) Function of many neural rhythms unknown—may arise as a secondary consequence of other functions Sleep research: Still little is known about why we sleep and the function of dreams and sleep.