Cerebral cortex Flashcards
What is important to remember about the cerebral cortex
it is the cerebral cortex that makes us human, and we are only starting to learn how to study it and interact with it therapeutically.
No part of the brain is working in isolation- everything is interconnected and functional imaging is showing the interaction between different regions of the brain
What is still a largely experimental aspect of neuroscience but is beginning to have increasing therapeutic benefits
Brain stimulation
Compare the grey and white matter of the cerebral cortex
Grey matter – there are around 50bn neurones and 500bn glial cells and only 30% is visible as 70% is hidden in the sulci- ribbon of neuronal cell bodies around the outside in a ‘cortical ribbon’
§ White matter - includes myelinated neuronal axons forming white matter tracts.
What is important to remember about grey and white matter
The central nervous system is divided into regions of grey and white matter. In the cerebrum the grey matter may be superficial (cortical ribbon) or deep (deep grey nuclei). Both grey and white matter contain many cell types (astrocytes, oligodendrocytes, microglia, endothelial cells), but only grey matter contains neuronal cell bodies. White matter appears white due to the high myelin content.
Summarise tractography
Shows connections between the different parts of the brain.
How many different types of fibres does the cerebral white mater consist of
3 types of fibres:
Association fibres
Commisural fibres
Projection fibres
Summarise the 3 different types of fibres in the cerebral white matter
Association fibres: connect areas within the same hemisphere
Commissural fibres: connect left hemisphere to right hemisphere
Projection fibres: connect cortex with lower brain structures (e.g. thalamus), brain stem and spinal cord
Describe the association fibres
short fibres that run between adjacent parts of the cortex - connecting areas within same hemisphere
§ Short association fibres – connect short distance.
§ Long association fibres – connect more distant structures
Give some examples of long association fibres
Superior longitudinal fasciulus:
connects frontal and occipital lobes
arcuate fasciculus
connects frontal and temporal lobes
inferior longitudinal fasciculus
connects temporal and occipital lobes
uncinate fasciculus
connects anterior frontal and temporal lobes:
Describe the commissural fibres
interconnect corresponding cortical areas of 2 hemispheres
corpus callosum interconnects frontal, parietal, occipital and some temporal cortex
anterior commissure provides additional temporal links (limbic system function)
posterior commissure- function not known.
Describe the projection fibres
interconnect cortex with subcortical regions
incoming fibres mainly from thalamus, but also from hypothalamus & brainstem
outgoing fibres to corpus striatum, thalamus, brainstem, spinal cord most go through corona radiata and internal capsule:
corticospinal & corticobulbar pass through posterior limb, (of internal capsule) therefore posterior limb lesions result in motor deficits- go to alpha motor neurones.
thalamocortical, corticothalamic & cortico-pontine fibres pass through both limbs
sensory deficits can arise from any capsular lesion; modality depends on position
What do capsular lesions refer to
Injuries of the internal capsule
Describe the corpus striatum
part of the basal ganglia of the brain, comprising the caudate and lentiform nuclei.
Describe the corona radiata
The corona radiata is a bundle of nerve fibers located in the brain. Specifically, the nerves of the corona radiata carry information between the brain cells of the cerebral cortex and the brain cells in the brain stem.
Describe the internal capsule
The internal capsule is a white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain. It carries information past the basal ganglia, separating the caudate nucleus and the thalamus from the putamen and the globus pallidus
Allows communication with the brainstem.
What is the neocortex
A part of the cerebral cortex concerned with sight and hearing in mammals, regarded as the most recently evolved part of the cortex
How many layers of grey matter are there
3-6 (they are usually numbered by roman numerals) with letters for laminar subdivisions (layers IVa, b, and c in the visual cortex, for example).
Describe the different connections of the six layers of grey matter
Layers 1-3 = mainly cortico-cortical connections
Layer 4 = input from the thalamus
Layer 5-6 = connections with subcortical, brainstem and spinal cord (Betz cells- larger motor neurones with projections down to the brainstem).
What are Broadman’s areas
§ Brodmann split the cerebral cortex up into areas with distinct histological patterns and functions (respond to same stimuli).
42 different areas- variations in 6 layer structure
E.g motor cortex thicker than somatosensory cortex due to the presence of Betz cells in the motor cortex.
What different structures are found in each of the different layers of grey matter
The generally smaller pyramidal neurons in layers II and III (which are not as distinct as their Roman numeral assignments suggest) have primarily corticocortical connections, and layer I contains mainly neuropil.
For example, cortical layer IV is typically rich in stellate neurons with locally ramifying axons; in the primary sensory cortices, these neurons receive input from the thalamus, the major sensory relay from the periphery.
Layer V, and to a lesser degree layer VI, contain pyramidal neurons whose axons typically leave the cortex. Brodmann described about 50 distinct cortical regions, or cytoarchitectonic areas.
What does layer 1 mainly consist of
Neutropil – an area composed mostly of unmyelinated axons, dendrites and glial cell processes that forms a synaptically dense region containing a relatively low number of cell bodies
What do all layers of the cortex receive input from
In addition all layers of the cortex receive modulatory inputs from RAS and brainstem monoaminergic nuclei.
What are the two different types of cortex
Neocortex: largest, most complex
Archicortex & paleocortex: phylogenetically older, part of limbic system
Describe the vertical and horizontal organisation of cells in the grey matter
Neocortex is arranged in layers (lamina structure) and columns (vertical)
More dense vertical connections – basis for topographical organization
Neurons with similar properties are connected in the same column
Describe the importance of the vertical organisation of grey matter
Columnar organisation – inputs and outputs are matched for all cells in column – basis of topographical maps
Summarise the primary cortices of the different lobes of grey matter
function predictable organised topographically left-right symmetry
Summarise the occipital lobe
visual association cortex analyses different attributes of visual image in different places
form & colour analyzed along ventral pathway; spatial relationships & movement along dorsal pathway
lesions affect specific aspects of visual perception
Summarise the parietal lobe
posterior parietal association cortex creates spatial map of body in surroundings, from multi-modality information
injury may cause disorientation, inability to read map or understand spatial relationships, apraxia, hemispatial neglect
Summarise the temporal lobe
language, object recognition, memory, emotion
injury leads to agnosia, receptive aphasia
Summarise the frontal lobe
judgement, foresight, personality, appreciation of self in relation to world
injury leads to deficits in planning and inappropriate behaviour
Where is the auditory cortex found
In the superior temporal gyrus of the temporal lobe
Describe how the sulci divide the brain into four lobes
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The frontal lobe is separated from the parietal lobe by the central sulcus.
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The temporal lobe is separated from these two lobes by the lateral sulcus.
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Demarcation of the occipital lobe is difficult to appreciate from a lateral view but, on the medial (midsagittal) view (Fig. 1.5), the parieto-occipital sulcus can be seen. The leaf-like folia of the cerebellum (sitting behind the midbrain, pons and medulla) can also be seen.
What is important to remember about the gustatory and olfactory cortices
They are less well defined.
Gustatory (taste - temporal)
Olfactory (smell - temporal)
Summarise the association cortices
function less predictable not organised topographically left-right symmetry weak or absent
Ultimately, what is the role of association areas
Association areas where information from different modalities comes together for processing, forming a cohesive representation of the experienced world. For example, the sights, sounds and smells of a rugby match coming together so we can enjoy the ‘whole scene’.
Association fibres within the cortex and with different cortices
Describe the prefrontal association area in the frontal lobe
An extensive area of cortex lying anterior to the motor fields, known as the prefrontal cortex (PFC), is rich in connections from the parietal, temporal and occipital cortices. Association fibres run through the PFC primarily through the subcortical white matter. The PFC is also innervated by several brainstem nuclei
Coordinates information from other associaiton areas, controls some behaviours.
Describe the auditory association cortex in the temporal lobe
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An auditory association cortex which lies immediately posterior to the primary auditory cortex. In the dominant hemisphere this is known as Wernicke’s area, where language comprehension and responses occurs.
Summarise the visual association cortices
The processing circuits are formed by pathways through distinct areas of the visual cortex each of which contain a retinotopic map. This allows representation of different types of activity in the visual field:
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V2 has an unknown function, but possibly acts as a processing and relay station for higher areas.
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V3 may have a role in depth perception and visual acuity.
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V4 has a role in colour perception.
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V5 is concerned with motion detection.
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Inferotemporal areas have complex cells which respond to particular stimuli such as faces.
Describe the motor association cortex
The premotor and supplementary motor areas just anterior to M1 are involved in the cortical programming and preparation of movement and control of posture
Describe Broca’s area and the frotnal eye field
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Broca’s area is where the muscle movements that articulate speech are determined before being sent to the bulbar muscles via the cranial nerves. This is the ‘how’ of speech.
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A frontal eye field (Brodmann’s area 8) lying on the middle frontal gyrus controls voluntary conjugate deviation of the eyes while scanning the visual field.
Describe the sensory association area
Sensory information from skin, musculoskeletal system, viscera and taste buds.
Compare briefly the functional methods to measure brain function
The different methods used to measuring brain function have a trade off between invasiveness, spatial precision, and temporal precision (i.e. direct measures of activity, or delayed)
MEG, ERP, Optical, fMRI, SPECT and PET are all non-invasive and look at the macroscopic structure.
Subdural ECG and lesions- invasive- but look at macroscopic structure
extracellular single unit recordings, intracellular patches, light microscopy and microlesions- can look down to levels of neurone and are invasive.
What do SPECT and PET rely on
Ligands binding in the brain.
What are the two different pathways in the visual association cortex
Dorsal Pathway – responsible for interpretation of spatial relationships and movements
Ventral Pathway – responsible for form and colour and facial identification
SO the interpretation of images takes place in the visual association cortices.
What happens in the visual association cortex
Image attributes are processed separately
What (colour, form); Where (spatial relationships)
Describe the consequence of a lesion in the visual cortex
Lesions of the visual posterior association area (fusiform gyrus) can result in the inability to recognize familiar faces or learn new faces—a deficit called prosopagnosia (aka face blindness).
What is a key function of the frontal lobe
judgement, foresight, personality, appreciation of self in relation to world
injury leads to deficits in planning and inappropriate behaviour
What are lesions in the frontal cortex characterised by
Characterised by a lack of planning, behaviour becomes disorganised, attention span and concentration diminish, self-control is hugely impaired
Describe the consequences of lesions to the frontal lobe
§ Prefrontal Association Area:
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 crate speech but preserved comprehension.
§ Broca’s area is in the frontal lobe.
What are the consequences of prefrontal lobotomy
With a unilateral or bilateral prefrontal lobotomy there is a lack of ability remember and relate things over time. Delayed reward has a greater detrimental effect on learning.
Attention span and ability to concentrate are greatly diminished. Abstract reasoning largely disappears. The prefrontal cortex receives massive inputs from the sensory association
cortices (somatosensory, visual and auditory) and also from the dorsomedial nucleus of the thalamus. Lesions of the dorsomedial nucleus of the thalamus can produce many of
the same symptoms as from prefrontal lobotomy.
What is the role of the posterior parietal association cortex
posterior parietal association cortex creates a spatial map of the body in surroundings, from multi-modality information
What are the consequences of lesions to the posterior parietal association cortex
injury may cause disorientation, inability to read maps or understand spatial relationships, apraxia, hemispatial neglect
Describe the consequences of lesions to the parietal lobe in general
§ Primary somatosensory cortex:
o Sensory deficits – in perception of basic sensory information.
§ 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.
Describe how space is not equally represented by each parietal lobe
The parietal lobes in each hemisphere represent the contralateral half of space (the world around us). However, space is not equally represented in the hemispheres. The left hemisphere represents only the right side of space, whereas the right hemisphere represents all of the left side of space and some of the right. This means that the right side of space is ‘viewed’ by both hemispheres and is slightly over-represented. Clinically, this means that lesions to the left parietal cortex will not completely disrupt processing of the right side of space because the right hemisphere can compensate to some degree. Right-sided lesions will produce more severe effects on the processing of the left half of space, because this is not carried out elsewhere in the brain. This phenomenon is known as neglect
What is apraxia, and why may a parietal lobe lesion lead to apraxia
Inability to make skilled movements with accuracy
If you can’t see where you are in space- then this may lead to difficulty performing motor tasks- i.e apraxia.
What are the functions of the temporal lobe
Language, object recognition, memory, emotion. Injury leads to agnosia, receptive aphasia
What are the two main consequences of injury to the temporal lobe
AGNOSIA – inability for the brain to interpret sensory information although the nerves carrying sensory information to the brain are fine
E.g. visual agnosia – patients can see perfectly fine but they can’t interpret sympbols such as letters
RECEPTIVE APHASIA – unable to understand language in the spoken or written forms
Describe how injuries to the temporal lobe can lead to anterograde amnesia
o Anterograde amnesia – inability to form new memories
Patient HM. Bilateral resection of anterior medial temporal lobe structures to cure epilepsy. HM was left with dense anterograde amnesia.
Explain the lateralisation and specialisation of hemispheric function
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The left hemisphere is involved in intellectual reasoning and language.
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The right hemisphere is more concerned with spatial construction (including depth perception and the internal ‘map’ of our surroundings) and emotion.
Why should the left and right hemispheres carry out different functions at all? It is believed that to speed up the processing power of the brain, the two hemispheres operate largely independent of each other on their particular functions and hemispheric communication only ensues when more ‘processing power’ is needed.
Where does evidence for the two hemispheres carrying out different functions come from
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Patients with lesions localized to one hemisphere (e.g. patients who have aphasia following a left hemisphere stroke)
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Patients with severe grand mal epilepsy who have undergone sectioning of the corpus callosum (commissurotomy) to prevent spread of seizures
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‘Split-brain’ animal studies, in which a commissurotomy was carried out
callosotomy = commissurotomy
Describe the experiment that showed hemispheric specialisation
§ Left is more verbal and analytical.
§ Right is more non-verbal.
When image (saying FACE) is shown on the right hand side- the image is processed on the contralateral side (i.e the left side) and so as the left hemisphere is dominant for verbal processing- the subject is able to right the word face
However, when the image is shown on the left- the subject can only draw the image as information cannot be shared with the left (he has had a callostomy).
Where do Wernicke’s area and Broca’s area lie
For example, the planum temporale (the superior aspect of the temporal lobe lying within the lateral sulcus and forming the junction between the temporal, occipital and parietal lobes) tends to be larger in the left hemisphere of most people. Wernicke’s area lies in the posterior part of the planum temporale and is central to the comprehension of speech. Damage here (e.g. following a stroke) results in a lack of language comprehension. Furthermore, when one considers that the left-hand side of the brain controls the right side of the body (which tends to be the dominant hand in most people) it is unsurprising that the left hemisphere is usually described as the dominant hemisphere. However, a small but significant number of left-handed people have a dominant right hemisphere. To screen for this during the history ask which hand the patient writes with.
Describe how diffusion tensor imagine (tractography) can be used to look for lesions in white matter
Structural imaging can be used to measure the effect of lesions in white matter – or how these lesions might disconnect different brain areas, and lead to complex symptoms.
The movement of water molecules in the brain can be used to infer the underlying structure of white matter. This information be used to estimate the location and
Connections between different white matter pathways. This technique is called diffusion tensor imaging.
For patients with traumatic brain injury or concussion injuries in sports such as boxing, it is thought that white matter connections become disrupted.
Summarise transcranial magnetic stimulation
The magnetic field induces an electric current in the cortex, causing neurons to fire.
This can be used to test whether a specific brain area is responsible for a function, e.g. speech
Summarise the benefits of transcranial magnetic stimulation over lesion/patient studies
§ Measures the effects if interference with normal information processing due to electro-magnetic stimulation of neurones.
§ Benefits 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 transcranial direct current stimulation
· This changes the excitability of neurones but does NOT directly induce neuronal firing
· Anode = increases neuronal excitability
· Cathode = decreases neuronal excitability
· TDCS could be used to reduce motion sickness by suppressing the area of the cortex associated with perceiving vestibular information
Summarise PET
PET (Positron emission tomography) uses a radioactive tracer attached to a molecule to locate brain areas where that particular molecule, e.g. dopamine, is being absorbed in the brain. It is an expensive process but has good spatial resolution and specificity in terms of the underlying biology (i.e. it is the only way to identify brain regions absorbing particular substances).
Describe the use of PET scans in Parkinson’s
PET scans following administration of 18-fluoro-levodopa (18F-FDOPA) to label dopaminergic terminals in the striatum. The degree of uptake is shown here in a color scale with red indicating the highest presence of terminals. In the normal state (left), dopamine innervation is homogenous throughout the striatum. In PD (right), there is profound loss in the posterolateral putamen with relative preservation of the caudate. This example corresponds to a patient with mild PD, mainly affecting the left arm; thus reduced dopaminergic innervation is maximal on the right posterolateral putamen.
What is the key difference between EEG and MEG
MEG: measures magnetic fields
EEG: measures electric fields
Describe MEG
Magnetoencephalography(MEG) is afunctional neuroimagingtechnique for mapping brain activity by recordingmagnetic fieldsproduced by electrical currents occurring naturally in thebrain, using very sensitivemagnetometers.
Describe EEG
Electroencephalography (EEG) is an electrophysiological monitoring method to record electrical activity of the brain. It is typically noninvasive, with the electrodes placed along the scalp, although invasive electrodes are sometimes used in specific applications (intracortical EEG, for example to localise function during neurosurgery). EEG measures voltage fluctuations resulting from ionic current within the neurons of the brain
What are the disadvantages with EEG and MEG
It is quite noisy – there is a lot of background activity
This is resolved by doing a trial of a large number of participants so that an average can be found
Once the average has been found, it can be deducted from the captured signal to see the underlying activity
By averaging a large number of trials, the effect of random noise can be eliminated to reveal the underlying wavefunction – which represents the brain activity associated with a particular stimulus.
MEG/ EEG measures the surface activity of the brain, but cannot directly measure the activity of interior structures.
What is fMRI
Function MRI
It detects changes in blood flow in the brain
It relies on the fact that blood flow in the brain and neuronal activity are coupled – more active parts of the brain require increased blood flow
Good spatial localisation of the brain- but can also show the different regions of the brain involved in different tasks
The ‘blobs’ on fMRI results are areas which are slightly more active than the surrounding brain regions (e.g. 1-2%). The rest of the brain is still active, but in the same way
As an EEG/MEG average, for a particular task these regions show slightly different activity.
Which areas of the brain are active when we imagine positive events
When participants imagined positive events in the future or the past, the amygdala and
Rostral anterior cingulate cortex were more active than when they imagined negative events
Historiacally, what was the main method of observing brain lesions
Historically the main method was observing the effects of cerebral lesions on behaviour. Interpretation is limited by poor reproducibility, inter-subject variation, lack of premorbid measures, plasticity/redundancy.
How long can transcranial direct current stimulation last for
Effects can last 1-2 hours following a single session
Prolonged application is associated with longer term changes
Describe evoked potential
non-invasive
refined form of EEG recording – directly measure of neuronal activity
very good temporal resolution – measured in msec
poor spatial resolution – measured in cms
computer analysis reveals waveforms which are timelocked to particular events. Eg. response of visual cortex to light, activity related to movement, object recognition, cognition
Describe the pros and cons of MEG
non-invasive
measures changes in magnetic fields induced by electric currents in the cortex
also very good temporal resolution
magnetic fields not affected by artefacts in the same way as EEG
very expensive Functional magnetic resonance imaging
minimally invasive
measures neuronal activity indirectly
poor temporal resolution – measured in secs
better spatial resolution than EEG / MEG – measured in mms
Describe the pros and cons of PET
Invasive, patients are injected with a radioactive tracer
PET then measures radioactivity emitted by the tracer
Different types of receptor can be mapped by attaching the tracer to different molecules, e.g. glucose
Expensive, radionuclides must be produced using a cyclotron
Allows imaging of specific receptor populations
Sometimes combined with a computed tomography (CT) scan
Generally ,what are the consequences of injury to the temporal lobes
§ Primary auditory cortex:
o Deafness.
§ Auditory association area:
o Visual deficits – in interpreting auditory information.
§ Wernicke’s/receptive aphasia – impaired comprehension but preserved speech function.
Generally, what are the consequences of injuries to the occipital lobe
§ Primary visual cortex:
o Blindness.
§ Visual association area:
o Visual deficits – in interpreting visual information.
§ Prosopagnosia – inability to recognise familiar faces.
generally, what are the consequences of injury to the frontal lobe
§ Prefrontal Association Area:
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 crate speech but preserved comprehension.
§ Broca’s area is in the frontal lobe.
