NEURO: Sleep Flashcards
What is an EEG?
EEG stands for electroencephalogram.
An amplified recording of the waves of electrical activity generated by the brain. These waves are measured by non-invasive electrodes placed on the scalp-connected to amplifiers and a recording device.
How are the electrodes of an EEG labelled?
Right hemisphere
-labelled evenly
Left hemisphere
-labelled oddly
Requirements for EEG to measure brain activity
There needs to be:
- the combined activity of a large number (1000s) of similarly orientated neurones
- synchronous activity across groups of cells
*thousands of neurones must be firing synchronously to detect signals because EEG can’t measure the activity of individual neurones or small groups of neurones
What does the EEG measure?
the post-synaptic activity of a group (1000s) of synchronous neurones is summed to generate a large surface signal, which is then read on an EEG
Importance of synchronous firing for EEG measurement
If synchronous post-synaptic firing:
-summed response detected on EEG
If irregular post-synaptic firing:
-only a small summed signal detected on EEG
Describe EEG rhythms.
EEG rhythms correlate with states of behaviours. They are categorised by their frequency range.
High-frequency low-amplitude is associated with alertness and waking.
Low-frequency high amplitude is associated with non-dreaming sleep or coma.
List the different EEG rhythm during different functional states of the brain.
AWAKE WITH MENTAL ACTIVITY: β 14-30 Hz
AWAKE AND RESTING: α 8-13 Hz
SLEEPING: θ 4-7 Hz
DEEP SLEEP: <3.5 δ Hz
How is synchronous firing achieved?
1) Pacemaker:
>synchronous rhythms can be led by a central clock or pacemaker (e.g. thalamus)
2) Collective Behaviour of Cortical Neurones:
>cortical neurones can coordinate themselves and collectively generate synchronous brain rhythms (mutual excitation/inhibition)
Thalamic pacemaker
Synaptic connections in the thalamus between excitatory and inhibitory thalamic neurones force each individual neurone to conform to the rhythm of the group
Co-ordinated rhythms are then passed to the cortex by thalamocortical axons
Collective Behaviour of Cortical Neurones
Some rhythms don’t depend on the thalamic pacemaker but this:
- excitatory and inhibitory interconnections of cortical neurones result in a co-ordinated synchronous pattern of activity
- this can remain localised to a certain region in the cortex or can spread to encompass larger regions of the cortex, depending on the cortical network involved
What is the function of these brain rhythms?
The answer is, we don’t really know. However, there are hypotheses going around:
The hypothesis for slow-frequency high-amplitude rhythms during sleep: the thalamus acts as a gate-keeper for information transmission.
During wakefulness, information is transmitted. During sleep, there are synchronous rhythms that block information transmission.
The hypothesis for fast-frequency low-amplitude rhythms during wakefulness: the brain is ‘attention grabbing’ to ‘bind together’ regions needed for task execution.
Hypothesis:
-No direct function, by-products of strongly interconnected circuits.
However, even if brain rhythms don’t have a function, they provide us with a convenient therapeutic window on the functional states of the brain. For example, we can detect seizures in epilepsy patients by looking at EEGs.
What is sleep?
It is ‘a readily reversible state of reduced responsiveness to, and interaction with, the environment ’.
What are the different functional states of the brain?
- Wakefulness
- REM Sleep
- Non-REM Sleep
Wakefulness (EEG activity)
AWAKE:
- EEG: low-amplitude, high frequency
- Sensation: vivid, externally generated
- Thought: logical, progressive
- Movement: continuous, voluntary
- REM: often
- alpha, beta and gamma rhythms
REM Sleep (EEG activity)
- EEG: low-amplitude, high frequency
- Sensation: vivid, internally generated
- Thought: vivid, illogical, bizarre, detailed dreams
- Movement: muscle paralysis, movement commanded by the brain but not carried out, body immobilised
- REM: often
- beta and gamma rhythms
Non-REM Sleep (EEG activity)
- EEG: high-amplitude, low frequency
- Sensation: dull or absent
- Though: logical repetitive, rarely accompanied by vivid, detailed dreams
- Movement: occasional, involuntary
- REM: rare
· Stage 1: Theta Rhythms
· Stage 2: Spindle and K-complex Rhythms
· Stage 3: Delta Rhythms
· Stage 4: Delta Rhythms (deep sleep-higher in amplitude)
Physiological changes in REM and non-REM sleep compared to when we are awake
Both have a decreased temperature, heart rate and breathing
However, brain energy consumption decreases in non-REM sleep, but increases in REM sleep.
Describe the sleep cycle.
The EEG stages can be sub-divided to
indicate the depth of sleep (Stages 1-4).
Each night begins with a period of
non-REM sleep, and as the night progresses, there is a shift from
non-REM to REM sleep.
Sleep stages are then cycled throughout the night, repeating ~90 minutes.
Why do we sleep?
We don’t really know.
Is it to:
>Restoration: to rest and recover and to prepare to be awake again
>Adaptation: to protect ourselves (e.g. hide from predators) and to conserve energy
Brainstem activity during wakefulness
Increase brainstem activity:
> several sets of neurones increase the rate of firing in anticipation of wakening to enhance the waking state (e.g. ACh, 5-HT, NE and histamine)
> synapse directly with regions e.g. thalamus and cerebral cortex
increasing excitatory activity in the thalamic pacemaker, which suppresses the rhythmic/synchronous form of firing in the thalamus and cortex present during sleep
Brainstem activity during sleep
decrease in brainstem activity
>several sets of neurones decrease rate of firing during sleep (e.g. ACh, 5-HT and NE).
We get thalamic driven θ and δ waves.
Theory of dreaming
Cholinergic neurones in the pons have been shown to increase the rate of firing to induce REM sleep. There is a theory of this increased firing and why we dream:
> there is semi-random firing in the pons during REM sleep that activates regions of our brain associated with memories and emotion, translating into vivid dreams
Why are dreams often bizarre, vivid and sometimes delusional?
> because the pattern of cholinergic firing from pons isn’t particularly synchronised and is happening in various regions of the brain randomly
What are some sleep-promoting factors?
- Adenosine
- Nitric Oxide (NO)
- Inflammatory Factors
- Melatonin
Adenosine
-receptor activation causes decreased heart rate, respiratory rate and smooth muscle tone (decreasing blood pressure)
Adenosine antagonists
Caffeine
-promote wakefulness
Nitric Oxide
potent vasodilator which decreases smooth muscle tone (decreasing blood pressure)
-stimulates adenosine release, therefore promoting sleep
Inflammatory Factors
cytokines (e.g. interleukin-1) released during infection (e.g. cold, flue) have been shown to promote non-REM sleep
>linked to adaptation theory-sleeping to protect ourselves
Melatonin
a hormone secreted by pineal gland at night, shown to initiate and maintain sleep
Circadian rhythm?
A physiological cycle of about 24 hours is present in all eukaryotic organisms and that persists even in the absence of external cues (e.g. daylight/darkness) due to us having biological/brain clocks for circadian rhythms.
What is a zeitgeber?
Environmental cues (e.g. light-dark cycle, temperature, humidity) that entrain circadian rhythms
Where are isolation circadian studies best performed?
in deep caves, where there are no environmental cues that affect the circadian rhythm:
If humans are separated from all possible zeitgebers, they are said to be in a “free-running” state- internal biological clock of approximately 24.5-25.5 hours.
> Length of the circadian cycle remains constant compared to the natural situation
> But, there are fluctuations in the time of the circadian rhythms, due to the absence of environmental cues (free-running)
> If we were to reintroduce the natural situation, we can become entrained again to display the initial natural situation circadian cycle
What is our biological clock?
suprachiasmatic nucleus
Suprachiasmatic Nucleus
small nucleus of hypothalamus directly above optic chiasm that receives retinal innervation and synchronises circadian rhythms with the daily light-dark cycle
What are the retinal cells synchronising the SCN?
not rods or cones, but speciaised photoreceptor cells expressing the photopigment melanopsin
SCN mechanism in circadian rhythms
· Photoreceptors expressing melanopsin are slowly excited by light and can detect changes in luminosity.
· Melanopsin receptors project directly to the SCN, inhibiting the production of melatonin by the pineal gland
· Therefore, in a light environment, we stay awake
Importance of SCN in circadian rhythms
In a control environment, animals were kept in a constant light environment. The animals showed a normal circadian clock around 24-25 hours, with temperature dropping during sleep.
If we were to abolish or inhibit the SCN in these animals, this rhythm is completely disrupted. There are fluctuations in the sleep/wake cycle and body temperature. Therefore, it seems that the SCN is important in the absence of environmental cues.
