Organisation of Cerebral Cortex Flashcards
What are the three types of fibre that make up white matter?
- Association Fibres – connect with areas in the same hemisphere
- Commissural Fibres – connect the two hemispheres
- Projection Fibres – connect the cortex with lower brain structures (e.g. thalamus)
In humans, how many layers of grey matter are there?
Three to six cortical layers, which are usually designated by Roman numerals, with letters for laminar subdivisions
Distinguish between the terms ‘cortex’ and ‘neocortex’. Summarise the main function s of the neocortex
The neocortex has 6 horizontal layers of different neuronal structure, while the rest of cortex, called “allocortex” has 3 layers; in humans, almost all cerebral cortex is neocortex.
The neocortex is involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning, conscious thought, and language
Describe the different connections of the 6 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
Note that the neocortex is also arranged in columns so that neurons with similar properties are connected in the same column; more dense vertical connections (basis for topographical organisation)
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 type of neurone is found in layer 4?
Stellate neurones;
in the primary sensory cortices, receive input from the thalamus
What type of neurone is found in layer 5, and to a lesser degree layer 6?
Pyramidal neurones whose axons typically leave the cortex
What are the two parts of the visual association cortex and what are they responsible for?
Dorsal Pathway – responsible for interpretation of spatial relationships and movements
Ventral Pathway – responsible for form and colour
What is the role of the posterior parietal association cortex?
It creates a SPATIAL MAP of the body in its surroundings from multi-modality information
What could injury of this part of the association cortex lead to?
- Disorientation
- Inability to read a map or understand spatial relationships
- Apraxia
- Hemispatial Neglect
Define apraxia.
Inability to make skilled movements with accuracy
What is the temporal association cortex responsible for?
- Language
- Object Recognition
- Memory
- Emotions
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 symbols such as letters
RECEPTIVE APHASIA –unable to understand language in the spoken or written forms
Name a consequence of a lesion of the visual association cortex (in fusiform gyrus)
Prosopagnosia – inability to recognise familiar faces or learn new faces
What is the role of the frontal lobe?
Executive functions e.g. planning, judgement, foresight, personality
What are the consequences of a prefrontal lobotomy?
- Change in personality
- Inappropriate behaviour
- Lack of ability to remember and relate things over time
- Attention span and ability to concentrate are diminished
Why would a lesion of the dorsomedial nucleus of the thalamus have similar consequences to prefrontal lobotomy?
Prefrontal cortex receives massive inputs from the dorsomedial nucleus of the thalamus
In the case of Hemispatial neglect (as a consequence of a lesion of the posterior parietal association cortex), if give the patient something to draw, what will you expect him or her to do?
Draw only half of it
deficit in attention to and awareness of one side of the field of vision is observed
What effect do temporal cortex lesions have on memory?
Impaired short-term memory
=> They are effectively trapped in a 30 second window of memory amnesia
Define hemispheric specialisation
The idea that one hemisphere has specialised functions, or that it exerts greater control over a particular function
e.g. the left hemisphere is language dominant, the right hemisphere is largely spatial processing
What is a callosotomy?
A palliative procedure used for the treatment of seizures where the corpus callosum is cut through.
The corpus callosum is key for the interhemispheric spread of epileptic activity
State a type of imaging that uses the movement of water molecules in the brain to infer the underlying structure of white matter.
Diffusion Tensor Imaging – Tractography
State two types of brain stimulation testing and what it can be used for.
Transcranial Magnetic Stimulation (TMS):
- Magnetic field induces a current in the cortex
- This is method of focally stimulating different areas of the cortex and testing what each area is responsible for
Transcranial Direct Current Stimulation (TDCS):
- 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
Describe and explain how PET scans work. What can it be used for?
A radioactive tracer is attached to a molecule to locate areas of the brain where that molecule is being absorbed
The tracer emits positrons, which are then detected by the receptors
It can be used in Parkinson’s disease to see the uptake of dopamine precursors by dopaminergic neurones
What is the difference between MEGs and EEGs?
MEGs = magnetoencephalography – measures magnetic fields EEGs = electroencephalography – measures electric fields
What is a major problem with MEGs and EEGs and how is this resolved?
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
What is fMRI?
Functional 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
Which areas of the brain become more active when participants imagine positive events?
Amygdala
Rostral anterior cingulate cortex
