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.
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
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
