Global Brain Activity Flashcards
what rhythms are there in the brain?
sleeping and waking
breathing cycles
steps of walking
stages of night sleep
what does an electroencephalogram (EEG) measure?
generation of small fields in pyramidal cells
takes many thousands of underlying neurons to generate an EEG big enough to see
what are the basic requirements for signal detection?
a whole population of neurons must be active in synchrony to generate a large enough electrical field at the level of the scalp
neuron population must be aligned in parallel orientation
(so they summate, not cancel each other out)
what is does synchronous activity depend on?
number of active cells, total amount of excitation, timing of activity
in pyramidal cell layer, each neuron receives thousands of synaptic inputs
if inputs are irregular or out of phase, sum has small amplitude
if inputs are activated at the same time, EEG waves will tend to be in phase and amplitude larger
how are synchronous rhythms generated?
brainstem nuclei modulate the rhythm of interactions from collective behaviour of neurons
what is a one neuron oscillator?
thalamic cells have set of voltage-gated ion channels which allow each cell to generate rhythmic, self-sustaining discharge patterns, even in the absence of external inputs
the rhythmic activity of each thalamic pacemaker cell becomes synchronised with many other thalamic cells
what does the oscillatory mode of thalamocortical neurons correspond to?
sleep state
tonically active mode corresponds to awake state
bursts of APs are evoked when neuron is hyperpolarised to activate low threshold Ca2+ channels
bursts account for spindle activity of EEG recording (stage II sleep)
depolarisation of cell transforms oscillatory activity into tonically active mode (by injecting current or stimulating cholinergic reticular activation system
functions of brain rhythms
sensory input: thalamus-cortex
activity coordination (binding) or different cortical regions (synchrony, oscillations)
meaningless by-product of feedback circuits
EEG rhythms: window of functional states of the brain
behavioural criteria for sleep
reduced motor activity
decreased response to stimulation
stereotypic postures
relatively easy reversability
what are the functional states of sleep?
awake
non-REM
REM
which neurons control sleeping and waking?
part of diffuse modulatory neurotransmitter systems
control rhythmic activity of the thalamus and EEG rhythms of the cortex
diffuse modualtory systems in waking state
activation is high
modulation is aminergic (norarenaline, serotonin)
information source is external
diffuse modulatory systems in REM sleep
activation is high
modulation is cholinergic
information source is internal
histamine-containing neurons
help in control sleep and waking
found in tuberomammillary nucleus in the hypothalamus
what function does sleep/dreaming have?
conversion of metabolic energy cognition thermoregulation neural maturation mental health
imaging techniques
structural or functional
what is structural imaging used for?
measures the spatial configuration of types of tissue in the brain static maps)
CT, MRI
what if functional imaging used for?
measures movement-to-movement variable characteristics of the brain that may be associated with changes in cognitive processing (dynamic maps)
PET, fMRI
what are CT scans?
computerised tomography
based on amount of x-ray absorption in different types of tissue
bone absorbs most, CSF absorbs least, brain matter is intermediate
used in clinical settings - to diagnose tumours, haemorrhaging, other gross abnormalities
what are MRI scans?
magnetic resonance imaging
completely safe, people can be scanned many times
better spatial resolution than CT
better discrimination between white and grey matter
can be adapted for detecting changes in blood oxygenation associated with neural activity (fMRI)
sequence of events in an MRI scan
magnetic fields of protons = initially random
add external magnetic field and some protons align
brief radio wave pulse orients protons to 90 degrees, producing measurable MR
protons return back and new brains slice is scanned
physiology underpinning functional imaging
brain makes up 2% body weight but 20% oxygen uptake
oxygen and energy needs are constantly supplied by local blood supply
when neuron metabolic activity increases, so does blood supply to that area
what is the difference between PET and fMRI?
PET measures change in blood flow to a region
fMRI is sensitive to O2 concentration in the blood
experimental condition always compared to baseline
indirect measures of neuronal activity
what is a PET scan?
positron emission tomography
based on blood volume
involves radioactivity (radioactive tracer in blood)
participants scanned once or a few times
effective spatial resolution
sensitive to whole brain
can use pharmacological tracers
what is a (BOLD) fMRI scan?
functional magnetic resonance imaging based on blood oxygen concentration no radioactivity participants scanned many times some brain regions are hard to image - e.g. near sinuses
what is a BOLD signal?
blood oxygen level dependent contrast
measured in fMRI
relates to concentration of oxy and deoxyhaemoglobin
what does HRF mean?
haemodynamic response function
describes changes of BOLD signal over time
