14. Organisation of the Cerebral Cortex Flashcards
1
Q
What are the three types of fibres in the cerebral white matter?
A
- Association Fibres = connect with areas in the SAME hemisphere
- Commissural Fibres = connect the two hemispheres (lateralised fibres, biggest=corpus callosum)
- Projection Fibres = connect the cortex with lower brain structures (e.g. thalamus), brainstem and spinal cord
2
Q
How is the grey matter arranged? Describe the NEOCORTEX.
A
- The grey matter is organised into layers (laminar structure) with 3-6 cortical layers which are usually designated by roman numerals, with letters for laminar subdivisions
- Each of the cortical laminae in the neocortex (which covers the bulk of the cerebral hemispheres and is defined by six layers) has characteristic functional and anatomical features
- NEOCORTEX = a part of the cerebral cortex concerned with sight and hearing in mammals, regarded as the most recently evolved part of the cortex - 6 layers
- Layer 1 - contains mainly fibres/neuropil (area composed mostly of unmyelinated axons, dendrites and glial cells processes that form a synaptically dense region containing a relatively low number of cell bodies)
- Layers 2 and 3 - you begin to see neurones
- Layer 4 - input layer - is typically rich in stellate neurones with locally ramifying axons, in the primary sensory cortices - these neurones receive an input from the thalamus
- Layers 5 and 6 contain pyramidal neurones (layer 4 have a few as well)
- The neurones are also locally organised into columns
- Neocortex is arranged in layers (lamina structure) and columns
- Along the column of neurones, there are dense vertical connections
- These neurones are all talking to each other and they each column has a particular function
- This is the basis for topographical organisation
3
Q
How is the cortex arranged
A
- Cortex is divided into lobes
-
Occipital
- Visual association cortex analyses different attributes of visual image in different places
- Form and Colour are analysed along the ventral pathway
- Spatial relationships and movements are analysed along the dorsal pathway
-
Parietal
- Posterior parietal association cortex creates a spatial map of the body in its surroundings from multi-modality information
- Injury may cause disorientation, inability to read a map or understand spatial relationships, apraxia, hemispatial neglect
- DEFINITION OF APRAXIA = inability to make skilled movements with accuracy
-
Temporal
- Language
- Object recognition
- Memory
- Emotion
- Injury leads to:
-
Agnosia = disorder of the brain whereby the patient cannot interpret sensations correctly although the sense organs and nerves conducting sensation to the brain are functioning normally
- E.g. auditory agnosia = patient can hear but can’t interpret sounds
- Tactile agnosia = retains normal sensation in the hands but cannot recognise three dimensional objects by touch alone
- Visual agnosia = patient can see but cannot interpret symbols, including letters
- Receptive Aphasia = the patient is unable to understand language in its spoken or written form
-
Agnosia = disorder of the brain whereby the patient cannot interpret sensations correctly although the sense organs and nerves conducting sensation to the brain are functioning normally
-
Frontal
- Judgement
- Foresight
- Personality
- Appreciation of self in relation to the world
- Injury leads to deficits in planning and inappropriate behaviour
4
Q
Describe the primary and association cortices.
A
-
Primary Cortices:
- Primary somatic sensory cortex
- Visual cortex
- Taste: Gustatory cortex
- Smell: Olfactory cortex
- Primary motor cortex
- Auditory cortex
- Their function is predictable
- Organised topographically
- Left-right symmetry
-
Association Cortices:
- Sensory association area
- Visual association area
- Motor association area
- Auditory association area
- Prefrontal association area
- Function is less predictable
- It is NOT organised topographically
- Involved in many functions
- Left-right symmetry is weak or absent
5
Q
Explain how the visual association cortex works.
A
- Vision is not a single modality input - requires lots of processing but can largely be broken down into 2 processes: the ventral stream (what pathway) and the dorsal stream (where pathway)
- Image attributes are processed separately:
- __What (colour, form)
- Where (spatial relationship)
6
Q
Describe visual association cortex lesions.
A
- Lesions of the visual posterior association area (fusiform gyrus) can result in the inability to recognise faces or learn new faces
- Other parts of visual recognition will be intact
- This is called PROSOPAGNOSIA aka face blindness
- NOTE: Oliver Sacks has prosopagnosia
7
Q
Describe frontal cortex lesions.
A
- Phineas Gage - the rod went through his frontal lobe destroying many parts of his prefrontal cortex but he survived
- His personality changed - he became unreliable and impulsive with little regard for consequences, he became an alcoholic
- With unilateral or bilateral prefrontal lobotomy there is a lack of ability to remember and relate things over time
- Attention span and ability to concentrate are diminished
- Abstract reasoning largely disappears
8
Q
Describe parietal cortex lesions.
A
- Right parietal lobe strokes are MORE COMMON than left parietal lobe strokes
- These patients have no problem with vision
- But if you give them something to draw then they will draw half of it (the right side) and then stop half way
- This is a deficit of attention
- If you remind them to draw the left side they will end up drifting back to the right
- DYSPRAXIA/APRAXIA – Difficult in activities requiring coordination and movement - inability to read maps, special neglect, inability to understand 3D images
9
Q
Describe temporal cortex lesions.
A
- The temporal lobe connects emotions, memory and language – very important in decision making
- Lesions of the temporal lobe will impair short-term memory
- They are effectively trapped in a 30 second window of memory
- Patient H.M. had his anterior medial temporal lobes resected to cure him of his epilepsy but this resulted in terrible anterograde amnesia
10
Q
Describe hemispheric specialisation
A
- There is some degree of specialisation within hemispheres
- Right Hemisphere = artistic + creative
- Left Hemisphere = logical + scientific + dominant for verbal processing
- Callosotomy (corpus callosum is split)= palliative surgical procedure for the treatment of seizures because the corpus callosum is key for the interhemispheric spread of epileptic activity.
- If you perform a callosotomy, then you can show things to one eye or another and you’ll know that that image is only being processed by one hemisphere
- E.g. if you show the word face to the left hemisphere you will be able to read it but if you show it to the right hemisphere you will be able to draw it but not read it
11
Q
How does diffusion tensor imaging (DTI) work
A
- The movement of water molecules in the brain can be used to infer the underlying structure of the white matter
- This information is used to estimate the location and connections between different white matter pathways
- For patients with traumatic brain injury or concussion injuries in sports such as boxing, it is though that the white matter connections becomes disrupted
12
Q
Describe transcranial magnetic stimulation.
A
- This is a method of focally stimulating different areas of the cortex
- Putting a current through the wire coil induces a current through your brain
- This means that you can briefly activate a brain area and hence test which specific brain area is responsible for a certain function
13
Q
Describe Transcranial Direct Current Stimulation.
A
- Changes the local excitability of neurones, increasing or decreasing the firing rate
- This does NOT directly induce neuronal firing
- The anode will INCREASE excitability
- The cathode will DECREASE excitability
- TDCS could be used to reduce motion sickness by suppressing the area of the cortex associated with processing vestibular information
14
Q
Describe how Positron emission tomography works.
A
- PET uses a radioactive tracer attached to a molecule to locate brain areas where that particular molecule (e.g. dopamine) is being absorbed
- It is expensive but has good spatial resolution and specificity in terms of underlying biology (it is the only way to identify brain regions absorbing particular substances)
- Image on the right shows a PET scan following administration of 18F-FDOPA to label dopaminergic terminals in the striatum
- Left scan shows a normal state where dopamine innervation is homogenous throughout the striatum
- In Parkinson’s Disease, there is profound loss in the posterolateral putamen with relative preservation of the caudate
15
Q
Explain how Magnetoencephalography and electroencephalography works.
A
- MEG = measures magnetic fields
- EEG = measures electric fields
- MEG maps brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain
- EEG records the electrical activity in the brain
- It is typically non-invasive with the electrodes placed along the scalp
- EEG measures voltage fluctuations resulting from ionic current within the neurons of the brain
- EEG is a bit less elaborate than the MEG
- EEG and MEG signals are noisy
- So participants perform a large number of trials so that an average can be found
- Once the average is known, you can take that away from the captured signal to identify the underlying activity
