W10 - Neuro: Sleep Flashcards
What are brain rhythms?
Brain rhythms refer to distinct patterns of neuronal activity that are associated with
specific behaviours, arousal level and sleep state.
- The earth has a rhythmic environment that can vary with the seasons:
Temperature
Precipitation
Daylight - In order to compete effectively and survive, an animals behaviour must oscillate with its environment - varies within species, eg. hibernation.
- The brain has evolved a variety of systems for rhythmic control –
most striking example is our sleep/wake cycle.
What is an EEG?
The electroencephalogram (EEG) is a measurement of electrical activity generated by
the brain and recorded from the scalp.
- The first human EEG was described in 1929 –
Hans Berger showed that waking and sleep
EEGs are distinctly different - Involves non-invasive electrodes placed on standard positions on the head – connected to amplifiers and a recording device. The electrodes are names in accordance to their position and the lobe. C or Z refers to the line of electrodes placed in the midline of the head. Even numbers used for right and odd used for left hemisphere.
- Today, the EEG is used primarily to help diagnose certain neurological disorders (e.g. seizures in epilepsy)
How are EEG measures possible?
Electrical activity needs to be measured through bone like skull and non neuro tissue layers like the meninges.
* EEG measures the combined activity of a
large number (1000s) of similarly orientated neurons
* Requires synchronous activity across
groups of cells
* EEG reflects summed post-synaptic
activity of large cell ensembles
- The amplitude of an EEG signal depends upon 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
- However, timing is everything – the same amount of excitation can occur at irregular intervals and result in a small summed signal
How are EEGs read?
EEG rhythms can be categorised by their frequency range, with each range named after a Greek letter.
A high-frequency low-amplitude associated with alertness and waking
B = Awake with mental activity
a = Awake and resting
A low-frequency high-amplitude associated with non-dreaming sleep
Theta = Sleeping
Delta = Deep Sleep
How are synchronous brain rhythms made? What are the two types?
Pacemaker:
Synchronous rhythms can be led by a central clock or pacemaker (e.g. thalamus)
Collective behaviour:
Synchronous rhythms can arise from the collective behaviour of cortical neurons themselves
What are thalamus pacemakers?
- The thalamus, with its vast input to the cerebral cortex, can act as a pacemaker
- Synaptic connections between excitatory and inhibitory thalamic neurons force each individual neuron to conform to the rhythm of the group
- Co-ordinated rhythms are then passed to the cortex by thalamocortical axons
- Thus, a relatively small group of centralised thalamic neurons can compel a much larger group of cortical neurons
What is the collective behaviour of cortical neurons?
- Some rhythms of the cerebral cortex do
not depend on a thalamic pacemaker –
rely instead on collective interactions of
cortical neurons themselves - Excitatory and inhibitory interconnections
of neurons result in a co-ordinated,
synchronous pattern of activity - This can remain localised or spread to
encompass larger regions of the cerebral
cortex
What are the functions of brain rhythms?
CORTICAL RHYTHMS PARALLEL MANY INTERESTING HUMAN BEHAVIOURS…
BUT DO THEY SERVE A FUNCTIONAL PURPOSE?
- One plausible hypothesis is that most brain
rhythms have no direct function – instead they
are by-products - Brain circuits are strongly interconnected with
various forms of excitatory feedback – rhythms
may be an unavoidable consequence of such
circuitry - However, even if brain rhythms don’t have a
function, they provides us with a convenient
window on the functional states of the brain
(e.g. epilepsy)
What is sleep?
Sleep is a readily reversible state of reduced responsiveness to, and interaction with,
the environment.
* Sleep may be universal amongst all animals (e.g. fruit fly Drosophila sleeps)
* Prolonged sleep deprivation can be
devastating to proper functioning
* However, we can stave off sleep… but not
forever…
World record for the longest time
without sleep (264.4 hours)
What are the functional states of the brain?
WAKEFULNESS
NON-REM SLEEP
Body capable of involuntary movement, rarely
accompanied by vivid, detailed dreams
“Idling brain in a moveable body”
REM SLEEP
Body immobilised, accompanied by vivid,
detailed dreams
“An active, hallucinating brain in a paralysed
body”
What is the sleep cycle?
- EEG rhythms can be sub-divided to indicate
depth of sleep (Stages 1-4) - Each night begins with a period of non-
REM sleep - Sleep stages are then cycled throughout the
night, repeating approximately every 90
minutes - As night progresses, there is a shift from non-
REM to REM sleep
What is the difference between REM sleep and Non-REM sleep?
NON-REM SLEEP
“Idling brain in a moveable body”
Temperature ↓
Heart rate ↓ ↓
Breathing ↓ ↓
Brain energy consumption ↓ - oxygen consumption
REM SLEEP
“An active, hallucinating brain in a
paralysed body”
Temperature ↓ ↓ ↓
Heart rate ↓ (irregular)
Breathing ↓ (irregular)
Brain energy consumption ↑↑↑ - this is balanced by low energy consumption in muscle.
Why do we sleep?
No single theory of the function of sleep is widely accepted, although most reasonable ideas fall into two categories – theories of restoration and adaptation.
Restoration:We sleep to rest and recover and to prepare to be awake again
Adaptation:We sleep to protect ourselves (e.g. hide from predators) and to conserve energy
What are the natural mechanisms of wakefulness?
- During wakefulness, there is an increase in
brainstem activity - Several sets of neurons increase rate of firing in anticipation of wakening and enhance the wake state (e.g. ACh, 5-HT, norepinephrine and
histamine) - Locus coeruleus = norepinephrine
- Raphe nuclei = Raphe nuclei
- Midbrain = Histamine
- Lateral hypothalamus = Hypocretin (orexin)
- Basal forebrain = Acetylcholine
- Collectively, these neurons synapse directly
brain regions including the thalamus and
cerebral cortex. Shown with Thalamic recording. - Addition to these neurotransmitters lead to changes in rhythmicity. - Increase in excitatory activity supresses
rhythmic forms of firing in the thalamus and
cortex present during sleep
What happens to the activity of the brain while we sleep?
- During sleep, there is an decrease in brainstem
activity - Several sets of neurons decrease rate of firing
during sleep (e.g. ACh, 5-HT and
norepinephrine). However, ACh in the pons of midbrain increases to induce REM sleep. These activate several regions of cerebral cortex that can elicit memories and emotions - dreams are bizzare/nonsensical. - Rhythmic forms of firing in the thalamus
shown to block the flow of sensory information up to the cortex - However, cholinergic neurons in pons shown to increase rate of firing to induce REM sleep – linked with dreaming
- However, other sleep-promoting factors also
involved in promoting sleep…
