The Nature and Function of Sleep Flashcards
What is the electroencephalogram? What do the oscillation and amplitude on the EEG represent?
EEG is a measurement of electrical activity from the surface of the scalp → glimpse of generalized activity of the cerebral cortex. EEG methods are usually noninvasive and painless.
Amplitude of the EEG signal depends on the synchronous activity of the underlying neurons.
You need to think about the number of activated cells and the total amount of excitation in terms of timing since they might not have changed only the timing did.
What is Magnetoencephalography and what are its benifits?
Another method to record rhythms of the cerebral cortex.
When neurons generate currents, they also produce a magnetic field which is recorded by the MEG.
MEG is better than EEG at localizing the sources of neural activity in the brain that are deep below the surface.
MEG also records rapid fluctuations of neural activity that are too fast to be detected by fMRI.
What are EEG rhythms and when do they vary?
EEG rhythms vary and correlate with particular states of behaviour and pathology.
Range from 0.05-400 Hz
High frequency, low amplitude rhythms: alertness and waking, or dreaming stages of sleep
When the cortex is most actively engaged in processing information, the activity level of cortical neurons is high but also unsynchronized. Low synchrony → low amplitude and dominant beta and gamma rhythms.
Low frequency, high amplitude rhythms: non-dreaming sleep states, drugged states, or pathology of coma
Cortical neurons are not engaged in information processing, and large numbers are phasically excited by a common, slow, rhythmic input. Synchrony is high → high EEG amplitude
How are synchronous rhythms generated?
1) neurons may take all of their cues from a pacemaker
2) neurons may share or distribute the timing function among themselves by mutually exciting or inhibiting one another.
What is the thalamus’ role as a powerful pacemaker?
The thalamus can act as a powerful pacemaker.
Under certain conditions, the thalamic neurons can generate very rhythmic action potential discharges.
Some thalamic neurons have certain voltage-gated ion channels that allow for rhythmic and self-sustaining discharge patterns to occur even without external input
Synaptic connections between excitatory and inhibitory thalamic neurons force each individual neuron to conform to the rhythm of the group
These rhythms then → cortex via thalamocortical axons → small groups of thalamic cells can compel the cortical cells to march to the thalamic beat.
What is the possible function of brain rhythm?
Hypothesis for sleep-related rhythm: it’s the brain’s way of disconnecting the cortex from sensory input. When you are asleep, the thalamic neurons enter a self-generated rhythmic state that prevents organized sensory information from being relayed to the cortex.
As in visual perception, there is also synchronous activity of cortical neurons when responding to an object.
Another hypothesis is that rhythms in fact don’t have a direct function. They may be byproducts of the brain’s tendency to have strong brain circuits and to be interconnected with various forms of excitatory feedback.
What are the brain activities associated with generalized seizures and partial seizures?
Seizures are the most extreme form of synchronous brain activity. Neurons within the affected area fire with a synchrony that never occurs in normal behaviour. → large EEG patterns
Generalized seizure: involves the entire cerebral cortex of both hemispheres;
partial seizure involves only a circumscribed area of the cortex.
What is epilepsy? And how is related to seizures?
Epilepsy: when a person experiences repeated seizures. Some forms of epilepsy show a genetic predisposition.
The genes implicated code for a diverse array of proteins, but some specifically for sodium channel proteins. The mutated Na channels tend to stay open longer than normal → hyperexcitable neurons.
Another group of mutations → impairment of synaptic inhibition of GABA by affecting the receptors and enzymes critical for its synthesis/transport. Or, the proteins involved in its release.
Drugs that block GABA receptors are potent convulsants (seizure promoting agents)
What does REM sleep look like in the brain and body?
EEG looks more awake than asleep. The body is immobilized.
The conjuring of vividly detailed illusions as dreams.
Dreaming sleep - it’s only found in small periods during our sleep
During REM sleep the brain is the most excited
The physiology of REM sleep (EEG) is indistinguishable from an active awake brain
Fast, low voltage fluctuations → paradoxical sleep
Oxygen consumption of the brain is highest in REM even when compared to being awake and being concentrated.
Paralysis of REM sleep is caused by atonia, the total loss of skeletal muscle tone. The body is incapable of moving.
Muscles that control eye movement and inner ear are very active
Physiological control systems are dominated by sympathetic activity during REM sleep.
Body temperature and it’s control system die off and core temperature begins to drop. Heart and resp rates increase but become irregular.
Clitoris and penis become engorged with blood and become erect.
What does non-REM sleep look like in the brain and body?
The brain doesn’t generate dreams.
Is also considered “slow-wave sleep”
Period for rest
Muscle tension is reduced and movement is minimal
The body is capable of moving but only rarely does the brain command it to
Temperature and energy consumption is lowered.
Increase in activity of parasympathetic division of the ANS
HR, resp, and kidney function slow
Digestive processes speed up
Brain rests - rate of energy use and firing rates of neurons are at their lowest point
Large amplitude of EEG rhythms indicate that neurons of the cortex are oscillating at high synchrony
Experiments suggest that sensory input can’t even reach the cortex at this phase.
Mental processes also hit their lowest point
What is the Sleep Cycle? What are the 4 stages of Non-REM sleep
Roughly 75% of total sleep time is spent in non-REM and 25% in REM sleep - as we move through these states in periodic cycles throughout the night.
The non-REM stages repeat their cycle roughly every 90 minutes. → ultradian rhythms
Stage 1 non-REM sleep
Transitional sleep; eyes make slow rolling movements; theta rhythms of relaxed waking become less regular and wane; lightest stage of sleep
Stage 2 non-REM sleep
Slightly deeper sleep; lasts around 5-15 minutes; occasional 8-14 Hz oscillations (sleep spindle generated by thalamic pacemaker); high-amplitude sharp wave (K complex); eye movements almost cease
Stage 3 non-REM sleep
EEG begins large-amplitude, slow delta rhythms; eye and body movements are decreased
Stage 4 non-REM sleep
Deepest stage of sleep; large EEG rhythms of 2Hz or less; this period initially lasts for 20-40 minutes but decreases progressively as more cycles ensue
Why do we sleep? 2 Theories.
Theories of Restoration
We sleep in order to rest and recover and to prepare to be awake again
Theories of adaptation
Sleeping to “keep us out of trouble”. As a mechanism to hide from predators when we are at our most vulnerable or from other features of the environment. It may also be to conserve energy.
Why do we need REM sleep?
Activation-synthesis hypothesis Dreams, or at least some of their features, are the associations and memories of the cerebral cortex that are elicited by random discharges of the pons during REM sleep Pontine neurons (via thalamus) → activation of areas of cerebral cortex → elicit well-known images or emotions → cortex attempts to synthesize disparate images into a “whole” story. However, this theory doesn’t explain how random activity can trigger the complex and fluid stories that dreams can contain or how dreams can be recurring.
Dreams and memory
Some evidence suggests that REM sleep aids in the integration or consolidation of memories.
Deprivation of REM sleep can impair abilities to learn different tasks.
After difficult learning experiences, the duration of REM sleep increases.
Karni study found that if you are deprived of REM sleep, learning a task doesn’t improve. Deprivation of non-REM sleep → enhanced performance.
What does sleep look like in the brain?
Neurons most critical to the control of sleeping and waking are part of the diffuse modulatory neurotransmitter systems.
Brain stem modulatory neurons using norepinephrine and serotonin fire during waking and enhance the awake state. Some neurons use Acetylcholine to enhance critical REM events. Other cholinergic neurons are active during waking.
The diffuse modulatory systems control the rhythmic behaviors of the thalamus → which controls EEG rhythms of the cerebral cortex. Slow and sleep related rhythms of thalamus block flow of sensory information to the cortex.
Sleep involves activity in descending branches of the diffuse modulatory systems e.g. inhibition of motor neurons during dreaming.
What is the process by which we transition from sleep to wakefulness?
A diffuse regulatory network known as the ascending reticular activating system is a set of nuclei in the brain that is responsible for wakefulness-sleep transitions Several sets of neurons increase their firing rate in anticipation of awakening and during various forms of arousal. These neurons synapse directly to the entire thalamus, cerebral cortex and other brain regions.
General effects of these transmitters → depolarization of neurons → increase in excitability → suppression of rhythmic forms of firing
