Organisation of the cerebral cortex Flashcards
what percentage of neurones in the grey matter are hidden?
70%
what makes up the central white matter?
myelinated neuronal axons that make the tracts
what are the 6 layers of the neocortex from superficial to deep?
- molecular
- external granular (association fibres)
- external pyramidal (association fibres)
- internal granular (thalamic input)
- internal pyramidal (input)
- multiform (output)
what does the molecular level contain?
layer 1–>neuropil: dense network of interwoven nerve fibres and glia
what do the external granular and external pyramidal layers contain?
small pyramidal neurones and their primary cortico-cortical connections
what does the internal granular layer contain?
layer 4: stellate neurones with local axons in primary sensory cortices receiving input from the thalamus
what do the internal pyramidal and multiform layers contain?
pyramidal neurones whose axons leave the cortex
contain large Betz cells
what are Brodmann’s areas based on?
histology and function
area that responds to the same stimuli
what are association fibres?
connect areas within the same hemisphere
there are short and long fibres connecting structures of varying distance apart
what fibres connect the right and left hemisphere?
commissural fibres
what do projection fibres connect?
cortex to lower brain structures like the thalamus, brain stem and spinal cord
example of commissural fibre?
corpus callosum
example of projection fibre?
internal capsule
what association cortex is found in the occipital lobe?
vision
what are the ventral and dorsal pathways of the VAC responsible for?
ventral- form and colour
dorsal- spatial relationship and movement
what’s the result of injury to the parietal lobe?
examples of spatial mapping problems
disorientation and inability to map or understand spatial relationships
apraxia and neglect
what lobe is responsible for language, object recognition, memory and emotion?
temporal
what is the result of injury to the temporal lobe?
agonsia- inability to interpret sensations
receptive aphasia- inability to comprehend (Wernickes)
what is the function of frontal lobe?
judgement
foresight
personality
appreciation of self in relation to the world
injury to the frontal lobe leads to-
deficits in planning
inappropriate behaviour e.g. hyper sexuality
which regions are responsible for voluntary skeletal muscle movements?
primary motor cortex (precentral gyrus)
premotor cortex (motion association area) Brodmanns 6
what are the primary somatosensory cortex and sensory association area responsible for?
where are they located?
Recognition and interpretation of sensory information from skin, muscles and taste buds
located in the parietal lobe
where are the primary visual cortex and association area located?
in the occipital lobe
where is the primary auditory cortex and association area located?
in the temporal lobe
which area coordinates information with the other association areas?
where is it located?
prefrontal association area
located in the frontal lobe
consequences of lesion of PMC
o Paralysis – full or partial loss of fine voluntary movements contralateral.
o Paresis – muscular weakness
consequence of lesion of pre-motor cortex i.e. the motor association area?
o Apraxia – difficulty in motor planning to perform voluntary tasks
consequences of lesion of the prefrontal association area in the frontal lobe?
o Deficits in planning and inappropriate behaviour.
This area is often involved in personality, self-control, attention, planning, emotions, motivation, decision making and reasoning.
o Aphasia – inability to create speech but preserved comprehension.
- Broca’s area is in the frontal lobe.
consequences of lesion in primary somatosensory cortex?
sensory deficits in perception of basic sensory information
consequences of lesion in sensory association area
o Sensory deficits – in interpretation of sensory information.
E.G. Spatial neglect – unawareness of contralateral side.
Area is involved in – tactile recognition, flavour recognition, spatial orientation, ability to read maps, reading, writing and calculations.
- Alexia – inability to recognise or read words.
- Agraphia – inability to write words.
- Acalculia – inability to perform calculations.
consequence of lesion in primary visual cortex?
in visual association area?
blindness
visual deficits
- interpretation of visual information
- prosopagnosia: inability to recognise familiar faces (ventral pathway)
consequence of lesion in primary auditory cortex?
in auditory association cortex?
deafness
Wernicke’s/receptive aphasia – impaired comprehension but preserved speech function.
consequence of lesion in temporal lobe?
anterograde amnesia- inability to form new memories
remember: hippocampus is in the medial temporal lobe
differences between primary cortices and association areas?
Primary cortices – function predictably
organised topographically have left-right symmetry.
Association cortices – function is less predictable, not organised topographically
left-right symmetry is weak or absent.
what is Transcranial Magnetic stimulation (TMS)?
measuring effect of interference of normal information processing
benefits of TMS over lesion/patient studies?
o Effects of stimulation are generally more precise.
o Healthy participants can be used as their own control – as they have a lesion that can be on/off.
o Insufficient time for functional reorganisation to take place so recovery processes of the brain are unlikely to confound the results.
o Allows investigation of temporal dynamics of on-line neuronal processing.
what is Positron Emission Tomography (PET) used for?
measures changes in amount of flow directly to a brain region
Uses a radioactive tracer attached to a molecule to locate brain areas where that molecule is being absorbed in the brain.
Expensive but good spatial resolution and specificity
what is functional MRI used for?
measures changes in amount of blood oxygen in a brain region.
what is an EEG?
Electroencephalography – measures electric signals generated by the brain by voltage fluctuations from ionic current
what is an MEG?
Magnetoencephalography – measures magnetic fields generated by the electric currents in the brain.
Diffusion Tensor Imaging
movement of water molecules in the brain can be used to infer underlying structure of the white matter which can estimate connections.
measure effects of lesions in white matter- TBI as cause of disruptions in white matter tracts
hemispheric specialisation
One or the other hemisphere is more active during certain tasks:
E.G. language is dominant left side – does not mean the other hemisphere is not working.
hemispherical differences
Left is more verbal and analytical.
Right is more non-verbal.
effect of callosotomy
if you see something that is on the right, the input is processed on the opposite side.
the hemispheres cannot communicate. Therefore seeing a word on the right, means the verbal left can process it and you can say it, but when the word is on the left, the right word can only process it in terms of a drawing but not verbally due to the missing bridge between the hemispheres.
Layer 1 of neocortex
Contains NEUROPIL:
Unmyelinated axons, dendrites and glial cell processes
Synaptically dense region + low number of cell bodies
Layer 2 and 3 of neocortex
external granular and external pyramidal layers respectively:
- Contain small PYRAMIDAL NEURONES
- use association fibres
- mostly used for CORTICOCORTICAL connections
Layer 4 of neocortex
Rich in STELLATE NEURONES
Receive input from thalamus
Layer 5 of neocortex
Contain PYRAMINDAL NEURONES:
- internal pyramidal layer
- connect the cortex to extra-cortical structures using pyramidal neurones
What is the occipital dorsal pathway for?
analysing spatial relationships and movements
what is the occipital ventral pathway for?
analysing form and colour
what is the inability to recognise or learn new faces?
PROSOPAGNOSIA
what is contained in the parietal lobe?
posterior parietal association cortex
what is the effect of a lesion in the posterior parietal association cortex?
- disorientation
- inability to read and understand spatial relationships
- apraxia
- hemi-spatial neglect
what is hemi-spacial neglect?
Deficit of attention
patient can be reminded to focus on other side
what is apraxia?
Inability to make skilled movements with accuracy
what is the role of the posterior parietal association cortex?
cortex creates spatial map of body in surroundings from multiple senses
receives the Dorsal pathway from VSC
what is the effect of a temporal lesion?
Agnosia
Receptive Aphasia
Short-Term Anterograde Amnesia
what is agnosia?
Inability to interpret sensations with fully working sensory organs
auditory, tactile, visual
what is receptive aphasia?
Inability to understand language in written or spoken form
comprehension (Wernicke’s)
what is short-term anterograde amnesia?
Inability to form new short-term memories
where does the prefrontal cortex receive its inputs from?
receives input from somatosensory, visual and auditory
sensory association cortices
and dorsomedial nucleus of the thalamus
what are the effects of a lesion to the prefrontal cortex or dorsomedial nucleus of the thalamus?
- lack of impulse control
- loss of abstract reasoning
- inappropriate behaviour
- planning deficits
where is the processing centre for smell?
piriform and orbitofrontal cortices
in the temporal lobe
what does the right hemisphere specialise in?
artistic and creative
what does the left hemisphere specialise in?
logical and scientific
what is a callosotomy?
Splits the Corpus Callosum to avoid spread of epileptic activity
What is the role of the corpus callosum?
inter hemispheric communication
example of commissural connection
what effect will a callosotomy have on reading and drawing taking into consideration the hemispheric specialisations?
lateralised deficits:
they could either only read the word or only draw the word
read–> left hemisphere
draw–> right hemisphere
they can’t do both as the same time do to lack of inter hemispheric communication
what is tractography? how is it done?
analysing white matter structure and paths
diffusion tensor imaging
what is diffusion tensor imaging?
the movement of water molecules is used to estimate the location and connection between different white matter pathways
what is the purpose of brain stimulation?
test what functions are specific region of the brain is involved in
what are 3 methods of imaging brain activity?
1) positron emission tomography (PET)
2) magnetoencephalography (MEG) and electroencephalography (EEG)
3) functional magnetic resonance imaging (fMRI)
what is the difference between an MEG and EEG?
MEG measures magnetic fields
EEG measures electric fields
what are the benefits and disadvantages of using MEG and EEG?
- non invasive
- signals are noisy therefore a large number of trials need to be done to calculate an average activity
the average is substrates from the captured signals to identify the underlying activity
what does fMRI depend on to function?
that cerebral blood flow and neuronal activation are coupled
when a region in the brain is in use, blood flow to that region increases
what happens in Transcranial Magnetic Stimulation?
- External magnetic field induces current in brain, causing neurones to fire
- Results in FOCAL stimulation of the targeted area
- Briefly activates the brain area, which can then be tested
what happens in Transcranial Direct Current Stimulation?
- Changes LOCAL EXCITABILITY of neurones
- Does NOT directly induce neuronal firing, only affects the threshold required
- Anode will INCREASE excitability (decrease threshold)
- Cathode will DECREASE excitability (increase threshold)
what is an application of Transcranial Direct Current Stimulation?
Could be used to reduce motion sickness by decreasing excitability of Vestibular Cortex
what is the difference in TMS and TDCS?
TMS uses an external magnetic field to induce neuronal firing to induce focal stimulation
- more focused
TDCS uses electric fields (a cathode and anode) but does NOT induce neuronal firing but adjusts the threshold for firing and causes location stimulation
- across an area
