Lecture 24 - What is a brain system and how do we identify one? Flashcards
What is a system?
A collection of structures in the brain that work together to perform a common function
What is a network?
a network loosely refers to the structure of circuits that connect areas of neurons together
System vs network?
A network is like the wiring/how things are pieced together whereas the system is more of a collective function/functional unit
Examples of systems
Examples: Visual system, auditory system, vestibular system, somatosensory system, motor system, reward system
Function of systems in general
Systems can work alone or in concert with other systems to bring about changes in behaviour – analysis of how this is achieved is called systems neuroscience
Sensory information can lead to
motor output
Core theme of systems neuroscience
We have many input sensory ‘systems’ but essentially ONE behavioural output motor ‘system’
Lots of information coming in but the way we respond/act is caused by only one output system because we only have one set of muscles that can work at any one particular time
Example of Sensory information and how it might be processed to produce a behavioural output
Sensory information comes in via the thalamus from the effectors out in the body such as limbs and it comes in and ends up via various pathways to the thalamus and then come from the thalamus and end up on the primary sensory cortex and from here it is just raw information so you need to process it so you send it backwards in your brain to the parietal lobe which is behind the primary sensory cortex which is in the post central gyrus and you send it back and then you can put things together from other inputs of sensory information such as visiaul information coming in via the thalamus and then to other visual parts or auditory information coming in via the thalamus to part of the auditory cortex on the temporal lobe and all of these come together to form the idea of a perception and memory which all occurs in the temporal and parietal cortices, from there when this sensory information comes in and forms/triggers a memory or triggers the wish for a particular response then this gets passed to the motor area so first to the frontal cortex (planning) which is preparing for an action and then to the primary motor cortex to relay that movement and then it will go down the different tracts to the muscles and drive them to bring about the behaviour that is required / respond to the sensory stimulus (very generic)
How do we identify the components of a brain system?
Neuroimaging
e.g. XRAY, MRI, CT, PET scan etc
What neuroimaging technique uses X-ray radiation?
CT
What neuroimaging technique uses Radio Waves?
MRI
What neuroimaging technique uses Radiolabelling?
PET
CT summary
CTs are reliant on X-rays which are impeded via very dense structures but are not impeded by non-dense structures so non-dense structures get lots of X-rays going through it
structural (density) imaging
MRI summary
High fidelity structural images where you can look diffusion of water molecules through tracts to pick up white matter tracts etc or functional where you can actually look at the brain while it is functioning and see what parts of the brain light up based on what you are thinking and doing
Types
- Structural (high fidelity)
- Diffusion
- Functional
PET summary
PET requires you having a radiolabelled molecule floating around inside that can be picked up based on metabolic activity in the brain or wherever you are messaging and can also look at molecular processes
Types
- Metabolic
- Molecular
CT full name
Computerised Tomography
CT summary of process behind it (orange)
- Tomography – imaging by creating ‘slices’ using any kind of penetrating wave
- Principles of X-ray radiation: x-rays absorbed to different degrees by tissues of different densities – dense tissues like bone absorb most X-rays so photographic film would be minimally exposed -> ‘white’; whereas low density fat/water passes X-rays easily and exposes the film to appear dark
- CT - rotation of a source of X-rays and a detector separated 180 deg from the source – very thin slices - computer integrates to form an image (lots of slices are bought together to form an image)
Big machine that generates x-rays usually by a rotating anode and blasts x-rays that hit the body and if they get through they are met on the other side by a detector
X-ray source that is rotating around head an emitting x-rays and the detector is always right on the other side
Way of telling it is a CT scan
Ways of telling it is a CT scan - very light and bright because it is the most dense and the X-rays have not gotten through the skull, least dense thing is air so the black around the head shows air, slightly more dense than air is fat so is slightly less dark than air, CSF is less dark than fat, white matter looks grey (slightly more dense) therefore starting to block the X-rays making it a lighter colour, grey matter even lighter because more dense, acute bleeding appears almost as dense as blood because globin in the blood is quite dense
Relative densities of parts of the head on CT
air (‘darkest’=least dense) < fat < CSF < white matter < grey matter < blood from haemorrhage (dense globin) < bone (‘brightest’=most dense)
Advantages of CT
Good for showing ‘acute’ bleeding or fracture of the skull
Relatively quick
Cheap
Less ‘scary’ for people (not enclosed or as noisy as MRI)
Disadvantages of CT
Structure only, not function
Not good for detail in the brain because of minimal contrast between areas (not very good demarcation of Frey and white matter)
dose of radiation
PET scanning summary of principles (ORANGE)
- Principles of Positron Emission Tomography (PET): radiation emitted from a radioisotope injected intravenously is registered by external detectors. ‘Positrons’ from isotope travel short distance (2-3 mm), combine with electrons and ‘annihilation’ results in energy release picked up by detectors = Just need to know than an isotope is injected and emits something which can be picked up by a detector
positrons are ions that come from the radioisotope
- Used to highlight areas of increased metabolism of glucose
(e. g. by cancers) – commonly use F18 radiolabelled fluoro-2- deoxyglucose (FDG) injected intravenously and enters organs where it highlights area of high glucose transport/phosphorylation
Radioisotopes in PET scan will go to
areas of high utilisation therefore can do functional measures with PET scanning
PET metabolism -
functional
PET molecular -
gene therapy
can also use PET to measure gene therapy/measure a molecule
PET advantages
Functional
Can identify and characterise tumours as benign or malignant
PET disadvantages
Poor resolution of brain tissue (can combine with CT) (poor resolution means that you cannot really resolve anything)
But also area localised is only approximate (within 5mm)
Requires radiation dose in the form of radio isotopes
MRI stands for
Magnetic Resonance Imaging
