Sleep (Physiology 2)

Brain rhythms refer to distinct patterns of neuronal activity.

These patterns are associated with specific behaviours, arousal levels, and sleep states.

Brain rhythmicity is measured through various systems that monitor rhythmic brain activity, such as EEG.

The earth's environment is rhythmic, with variations in temperature, precipitation, and daylight.

To survive effectively, animals must adjust their behaviour and brain rhythms to match these environmental changes.

The sleep/wake cycle is one of the most striking examples of how brain rhythms are controlled and synchronized with environmental factors.

An EEG is a measurement of electrical activity generated by the brain and recorded from the scalp.

Hans Berger described the first human EEG in 1929.

He showed that waking and sleep EEGs are distinctly different.

Non-invasive electrodes are placed on standard positions on the head.

These electrodes are connected to amplifiers and a recording device to measure electrical activity.

The EEG is primarily used to diagnose certain neurological disorders, such as seizures in epilepsy.

An EEG measures the combined activity of a large number (thousands) of similarly oriented neurons.

Synchronous activity across groups of cells is required for EEG measurement.

The EEG reflects the summed post-synaptic activity of large cell ensembles.

The amplitude of an EEG signal depends on how synchronous the activity of a group of cells is.

When a group of cells are excited and synchronous, the tiny signals sum to generate a large surface signal.

If the same amount of excitation occurs at irregular intervals, it results in a small summed signal.

EEG rhythms can be categorized by their frequency range.

A high-frequency, low-amplitude EEG rhythm is associated with alertness and waking.

A low-frequency, high-amplitude EEG rhythm is associated with non-dreaming sleep.

Synchronous rhythms can be led by a central clock or pacemaker, such as the thalamus.

Synchronous rhythms can arise from the collective behavior of cortical neurons themselves.

The thalamus acts as a pacemaker by coordinating rhythms and passing them to the cortex via thalamocortical axons.

A relatively small group of thalamic neurons can influence a much larger group of cortical neurons, forcing individual neurons to conform to the rhythm through synaptic connections between excitatory and inhibitory thalamic neurons.

These rhythms rely on the collective interactions of cortical neurons themselves, where excitatory and inhibitory interconnections produce a coordinated, synchronous pattern of activity that can remain localized or spread across larger regions of the cortex.

Some hypotheses suggest brain rhythms may not have a direct function but instead could be by-products of neural activity.

However, even if they lack a specific function, brain rhythms provide insight into the functional states of the brain, such as in epilepsy.

Brain circuits are strongly interconnected, and the presence of excitatory feedback in these circuits could make rhythmic activity an inevitable consequence.

Sleep is a reversible state of reduced responsiveness and interaction with the environment.

Prolonged sleep deprivation can severely impair functioning.

Sleep appears to be universal among animals, including even fruit flies (e.g., Drosophila).

While sleep can be delayed, it cannot be avoided indefinitely.

Wakefulness: Active state where the brain processes sensory input and controls body movements.

Non-REM Sleep: Body can move involuntarily, but there are rarely vivid dreams. Described as an "idling brain in a moveable body."

REM Sleep: Body is immobilized, but vivid and detailed dreams occur. Known as "an active, hallucinating brain in a paralysed body."

EEG Rhythms: These can be sub-divided to indicate different stages of sleep (Stages 1-4).

Non-REM Sleep: Each night starts with a period of non-REM sleep.

Sleep Cycle: Sleep stages cycle throughout the night, repeating approximately every 90 minutes.

Shift to REM Sleep: As the night progresses, there is a shift from non-REM to REM sleep.

Non-REM Sleep:

Brain activity is low ("idling brain in a moveable body")

Temperature, heart rate, and breathing are all decreased

Brain energy consumption is low

REM Sleep:

Brain activity is high ("an active, hallucinating brain in a paralyzed body")

Temperature, heart rate, and breathing are irregular

Brain energy consumption is significantly increased

Restoration Theory:

Sleep allows the body and brain to rest, recover, and prepare for wakefulness again.

Adaptation Theory:

Sleep serves to protect us (e.g., hiding from predators) and conserve energy.

Increased Brainstem Activity:

During wakefulness, brainstem activity increases. Several sets of neurons increase firing rates in anticipation of waking and enhance the wake state.

Neurotransmitters Involved:

Acetylcholine (ACh), serotonin (5-HT), norepinephrine, and histamine are key neurotransmitters that promote wakefulness.

Effects on Thalamus and Cortex:

These neurons synapse directly with brain regions like the thalamus and cerebral cortex. Increased excitatory activity suppresses rhythmic firing patterns seen during sleep in both regions.

Decreased Brainstem Activity: During sleep, brainstem activity decreases.

Neurotransmitters Involved: Several sets of neurons decrease their firing rate during sleep, including acetylcholine (ACh), serotonin (5-HT), and norepinephrine.

Cholinergic Neurons in Pons: Cholinergic neurons in the pons increase their firing rate to induce REM sleep, which is linked with dreaming.

Thalamus Firing Patterns: Rhythmic firing in the thalamus blocks the flow of sensory information to the cortex, promoting sleep.

Nitric Oxide (NO): NO is a potent vasodilator, which decreases smooth muscle tone and lowers blood pressure. It also stimulates the release of adenosine.

Adenosine: Adenosine is a building block for DNA, RNA, and ATP. Its receptor activation reduces heart rate, respiratory rate, and smooth muscle tone, lowering blood pressure.

Adenosine's Inhibitory Effects: Adenosine inhibits acetylcholine (ACh), norepinephrine, and serotonin (5-HT), which promote wakefulness.

Adenosine Receptor Antagonists: Compounds like caffeine act as adenosine receptor antagonists and promote wakefulness.

Melatonin: A hormone secreted by the pineal gland at night that helps initiate and maintain sleep. It is commonly used as an over-the-counter medication for insomnia and jet lag.

Inflammatory Factors: Sleepiness is a common symptom of infection (e.g., cold, flu).

Cytokines: Cytokines like interleukin-1 stimulate the immune system to fight infections.

Interleukin-1 and Non-REM Sleep: Elevated levels of interleukin-1 promote non-REM sleep, which may support the adaptation theory of sleep.

Circadian Rhythms: A circadian rhythm refers to any rhythm with a period of approximately 24 hours.

Persistence in the Absence of Light: If cycles of daylight and darkness are removed, circadian rhythms continue to operate in animals.

Coordinated Behaviour: Almost all land animals coordinate their behavior based on circadian rhythms, which are linked to daily cycles of daylight and darkness caused by the Earth's rotation.

Physiological Processes: Most physiological processes, such as temperature and hormone levels, also rise and fall with these daily rhythms.

Definition of Zeitgebers: Environmental time cues like light-dark cycles, temperature, and humidity are collectively called zeitgebers.

Difficulty of Separation: It's challenging to completely separate humans from all zeitgebers, even in controlled environments such as laboratories, as factors like people coming and going provide time cues.

Isolation Studies: The most effective studies isolating humans from zeitgebers are conducted in deep caves.

Free-running State: When humans are removed from all zeitgebers, they enter a "free-running" state, where their internal biological clock operates on a cycle of approximately 24.5-25.5 hours.

Location and Function: The SCN is a small nucleus in the hypothalamus that synchronizes circadian rhythms with the daily light-dark cycle by receiving retinal innervation.

Role in Sleep Coordination: Inhibition of the SCN does not abolish sleep, as animals will continue to coordinate sleep with light-dark cycles if those cycles are present.

Light-Dark Synchronization: The SCN helps to maintain synchronization between internal biological rhythms and external environmental cues such as light.

Clock Genes and Feedback: SCN clock genes produce proteins that send feedback to the SCN, inhibiting further production of those proteins. This process happens over a 24-hour cycle.

Role of Light: Light information from the retina serves to reset the SCN neuron clocks each day, ensuring synchronization with the light-dark cycle.

Control of Body Clocks: The SCN controls circadian clocks throughout the body, including in organs like the liver, coordinating the body's overall circadian rhythm.

A method of measuring electrical activity in the brain.

It records brain wave patterns through electrodes placed on the scalp.

Occurs at intervals during the night.

Characterized by rapid eye movements, more dreaming, and bodily movement.

Associated with faster pulse and breathing.

Period of sleep with decreased metabolic activity.

Slowed breathing and heart rate.

Absence of dreaming.

The ‘body clock’ regulating sleep, rise, and eating patterns.

Controls many physiological processes.

Operates on a roughly 24-hour cycle.

A rhythmically occurring natural phenomenon.

Acts as a cue in the regulation of the body's circadian rhythms.

A pair of small nuclei in the hypothalamus of the brain.

Located above the optic chiasma.

Regulates physiological circadian rhythms in the body.