Cortical Localisation & Subcortical White Matter Flashcards
What is meant by the “cortex”?
How can it be divided into different areas?
- the cortex is a thin layer of grey matter that surrounds the cerebral hemispheres
- it can be divided into 46 Brodmann’s areas based on cellular organisation
- different areas of the cortex are associated with different functions
What is meant by somatotopic organisation?
Why are some areas represented as larger than others?
- areas of the body are mapped differently to the cortex and are not proportional
- areas represented as large are areas that perform highly skilled motor functions or contain many more sensory receptors
How is sensory information coming from the face and the body relayed differently by the thalamus?
From the face:
- sensory information is relayed by the VPM nucleus and projected more laterally in the postcentral gyrus
From the body:
- sensory information is relayed by the VPL nucleus and projected more medially in the postcentral gyrus
How does motor information travel differently from the precentral gyrus depending on whether it is going to the face or body?
To the face:
- motor information travels to the face via corticobulbar (corticonuclear) fibres
To the body:
- motor information travels to the body via the corticospinal tract
- There is no involvement of the thalamus*
What acronym can be used to remember somatotopic organisation of the cortex?
FATL
- face is located most laterally, followed by the arms, trunk and legs
Where is the primary motor cortex located?
What Brodmann’s area is this?
- it is located in the precentral gyrus of the frontal lobe, anterior to the central sulcus
- it is BA 4
What fibres arise from the primary motor cortex?
What is the role of these fibres?
- the primary motor cortex gives rise to corticospinal and corticobulbar UMNs
- these descend via the posterior limb of the internal capsule
- the function of these tracts is to execute voluntary movement
What muscles are supplied by corticospinal fibres from the primary motor cortex?
How do they descend to reach these muscles?
- corticospinal fibres cross the midline at the decussation of the pyramids
- they enter the spinal cord to stimulate the muscles of the BODY** on the **CONTRALATERAL side
What muscles are stimulated by the fibres of the corticonuclear (corticobulbar) tract?
How do they travel to reach these muscles?
- corticonuclear fibres synapse with LMNs within cranial nerve nuclei in the brainstem
- the corticonuclear tract directly innervates nuclei for V, VII, IX and XII
- the corticonuclear tract innervates these nuclei bilaterally
- except for the lower facial nuclei that innervates the contralateral lower muscles of facial expression
- the fibres of the corticonuclear tract stimulate muscles of the FACE
What is the result of a lesion in the precentral gyrus?
contralateral paralysis / paresis
- as the precentral gyrus is somatotopically organised, a lesion in the most medial area leads to paralysis of the legs
- a lesion more laterally will lead to paralysis of the face / tongue
What are the association motor areas?
Where are they located?
- pre-motor and supplementary motor areas (BA 6)
- frontal eye field (BA 8)
- Broca’s area (BA 44, 45)
- these are all located immediately anterior to the precentral gyrus
- they do not initiate movement, but are responsible for other things related to movement
What is the function of the premotor and supplementary motor areas (BA 6)?
- they both receive inputs from the VA and VL nuclei of the thalamus
- together they generate and store programmes of motor routines for skilled voluntary actions
- e.g. tying shoe laces - this action needs to be learnt
- they relay instructions to the primary motor cortex about the pattern of muscle movement needed to perform a highly skilled task
What is the result of a lesion to BA 6 (premotor & supplementary motor areas)?
apraxia
- this is the inability to perform highly skilled, learned patterns of movement despite having the desire and physical ability to perform them
- there is no paralysis as the primary motor cortex is still intact
What is the role of the frontal eye field (BA 8)?
What happens if there is a lesion?
- it controls voluntary conjugate eye movements to the opposite side
- a lesion causes the eyes to deviate towards the ipsilateral side
What is the role of Broca’s area (44, 45)?
Where is it located?
- it is the expressive speech area that regulates the pattern of breathing and vocalisation needed for normal speech
- it is involved in the motor aspect of speech production
- it is located in the inferior frontal gyrus of the dominant hemisphere
- it consists of the opercular and triangular parts of the inferior frontal gyrus
What is the result of a lesion to Broca’s area?
Expressive aphasia
- the patient is aware of what they are trying to say, but is unable to articulate their speech properly or write language down
- there is hesitant and fragmented speech with little grammatical structure
Where is the prefrontal cortex located?
- it exists towards the frontal pole of the frontal lobe
- it is essentially the remainder of the frontal lobe once the primary motor cortex and association motor areas have been accounted for
What is the role of the prefrontal cortex?
- stimulation to this area does NOT elicit movement
- it has a role in monitoring behaviour and controlling higher mental processes, such as judgement and foresight
- it has extensive connections with the parietal, temporal and occipital lobes via fasciculi
- the lateral portion is involved in olfaction (which can evoke a wide range of emotions)
What is the result of a lesion to the prefrontal cortex?
- deficits in judgement and concentration
- patients often have a change in personality, act irrationally and demonstrate inappropriate social behaviours
Where is the primary somatosensory cortex located?
What information does it receive?
- located in the postcentral gyrus in the parietal lobe (BA 1, 2, 3)
- receives sensory pathways for touch, temperature, pain, vibration and proprioception
- the dorsal column pathway carries vibration / discriminative touch / proprioception information
- the spinothalamic tract carries pain and temperature information
- this sensory information is relayed via the thalamus
Which nuclei of the thalamus are involved in relaying sensory information to the somatosensory cortex?
Which regions do they project to?
- the VPL nucleus receives an input from the spinal cord, which is carrying sensory information from the body
- sensory information from the body is projected more medially within the somatosensory cortex (FATL)
- the VPM nucleus receives an input from the brainstem, which is carrying sensory information from the face
- sensory information from the face is projected more laterally within the cortex
- information from the VPM and VPL nuclei travels in the posterior limb of the internal capsule to reach the cortex
How is information related to proprioception and cutaneous sensation carried within the internal capsule?
What type of organisation is present here?
Cutaneous sensation:
- projected to the cortex via the anterior part of the posterior limb of the internal capsule
Proprioception:
- projected to the cortex via the posterior part of the posterior limb of the internal capsule
- this pathway is somatotopically organised so that information relating to the face is projected more laterally
What would be the result of a lesion to the postcentral gyrus and why?
hypesthesia (diminished sensation) or anaesthesia (lack of sensation) on the CONTRALATERAL side
- spinothalamic tract decussates 2-3 spinal nerve segments above the point of entry
- dorsal column pathway decussates the the level of the medulla and continues as the medial lemniscus on the contralateral side
Where are the somatosensory association areas located?
What is their role?
- they surround the primary somatosensory area (BA 5 & 7)
- they are located in the superior parietal lobule on the lateral and medial surfaces
- these areas do not perceive sensation, but are involved in interpretation, understanding and recognition that gives a particular sensation meaning
Which areas do the somatosensory association areas (BA 5, 7) receive fibres from?
- primary somatosensory area
- lateral posterior nucleus of thalamus
- pulvinar nucleus of thalamus
What is the result of a lesion to the somatosensory association areas?
tactile agnosia
- if someone was to close their eyes and hold a key, they could recognise the object just through touch
- someone with tactile agnosia would be unable to name the object without a visual stimulus
What is found within BA 3 and why is it significant?
sensorimotor strip
- this is a small strip of neurones within the primary somatosensory area that are capable of eliciting motor responses
Where is the primary visual area located?
How is it organised?
- it is the area located immediately superior and inferior to the calcarine sulcus that extends to the occipital pole
- it is BA 17
- it is retinotopically organised - retinal image is mapped onto the primary visual cortex
What is the result of a lesion to the primary visual cortex (BA 17)?
- results in visual field defects - hemianopias or quadrantopias
- these are contralateral if the lesion is in one hemisphere or bilateral if both are affected
Where are the association visual areas located?
What are the roles of the visual association areas?
- they surround the primary visual cortex (BA 18, 19) and are involved in the interpretation of visual information
- 3 roles of the visual association areas:
- relating past and present experiences
- recognition of what is being seen
- appreciation of the significance of what is being seen
What is meant by the “where?” and “what?” streams?
“Where?” dorsal stream:
- describes areas located more superiorly towards the parietal lobe
- this stream is involved in analysis of motion and spatial awareness
“What?” ventral stream:
- describes are towards the inferior aspect of the temporal lobe
- this stream is involved in identifying colours and familiar shapes / faces
What is the blood supply to the dorsal “where?” stream?
What happens if there is a lesion to this region?
- blood supply is via the middle cerebral artery
- there are usually bilateral lesions of the superior parietal lobe
- lesions to this area lead to visual disorientation as there is difficulty interpreting motion and spatial awareness
What is the blood supply to the ventral “what?” stream?
What happens if there is a lesion here?
- supplied by the P3 and P4 branches of the posterior cerebral artery
- there is usually a lesion to the inferior temporal lobe anterior to the occipital lobe
- this leads to acquired achromatopsia (loss of colour vision in the contralateral halves of the visual field)
- there is also prospagnosia (inability to recognise faces)
Where is the primary auditory cortex located?
What is its role?
- located within the transverse temporal gyri (Heschl’s convolutions)
- located in the ventral wall of the lateral fissure in the superior temporal gyrus (BA 41, 42)
- the primary auditory cortex is invovled in the conscious perception of sound
What nucleus of the thalamus is associated with the primary auditory cortex (BA 41, 42)?
- information about sound is projected to the brainstem via the cochlear nerve
- it is projected to the medial geniculate nucleus (MGN) of the thalamus, and then to the primary auditory cortex
How is the primary auditory cortex organised?
tonotopically organised
- lower frequency sounds are perceived by more anterolateral aspects of the primary auditory cortex
- higher frequency sounds are interpreted more posteromedially
Where is the association auditory cortex located?
What is its role?
- it is the planum temporale - located in the superior temporal gyrus and extends slightly into the parietal lobe
- it is involved in the interpretation of auditory information, which gives it meaning
- in the dominant hemisphere, BA 22 is Wernicke’s area
Where is the primary taste cortex located?
- it is located in the region of the insula (BA 43)
- it extends from the inferior margin of the postcentral gyrus and is adjacent to the general sensory area for the tongue
Where is the olfactory cortex located?
What is significant about this sensory pathway?
- olfactory fibres end in the area surrounding the uncus and its neighbouring area (BA 28, 34)
- the olfactory pathway is the only sensory pathway that has no thalamic involvement
- fibres travel directly from the olfactory tract to the olfactory cortex
What are the roles of the additional association cortex?
What are the 2 parts?
- involved in more complex aspects of behavioural and intellectual functioning
- not specific to giving meaning to one sensation, but integrate multiple modalities
- prefrontal cortex and parieto-temporal cortex make up the additional association cortex
What is the role of the prefrontal cortex?
- it regulates moods and feelings and is involved in higher cognitive functions
- it has a role in conceptualisation, planning and judgement
- flexible and complex behavioural patterns are constructed using experience
What is the role of the parieto-temporal cortex?
- it integrates analysed information from different sensory association areas to give a comprehensive understanding of the immediate environment
- it has a role in memory and allows us to retrieve specific information
- e.g. the memory of someone’s face, how they smell, what they sound like etc.
Which structure is larger in the left hemisphere compared to the right?
planum temporale
- it is larger in the left hemisphere as it is part of Wernicke’s (receptive language) area
What are the 3 different types of nerve fibres found in the white matter of the cerebral hemispheres?
- association fibres
- commissural fibres
- projection fibres
What structures are connected by the cingulum?
What is its purpose?
- it is the fasciculus of the limbic system that allows for communication between its different components
- it interconnects the cingulate gyrus, parahippocampal gyrus (of temporal lobe) and the septal area below the genu of the corpus callosum
What structures are connected by the superior longitudinal fasciculus?
What is its role?
- it runs above the insula to connect the parietal, temporal and occipital lobes with the frontal lobe
- it is a pathway for sensory information to influence appropriate motor function
What structures are connected by the inferior longitudinal fasciculus?
What is its function?
- it connects the temporal and occipital lobes
- thought to be involved in functions linked to the ventral “what?” stream, such as object recognition and face perception
What structures are connected by the superior & inferior occipitofrontal fasciculus?
Which part involves the temporal lobe?
- it connects the occipital lobes to the frontal lobes
- it passes backwards from the frontal lobe, along the lateral border of the caudate nucleus and onto the medial aspect of the corona radiata
- the uncinate fasciculus is part of this that sweeps around the lateral sulcus to the temporal lobe
- the uncinate fasciculus connects the orbital and anterior temporal cortex
What is the role of short association fibres?
short association fibres connect adjacent areas of cortex** within the **same hemisphere
What are the following commissural fibres?
What are the 4 parts of the corpus callosum?
What is it derived from?
- rostrum
- genu
- body
- splenium
- it is derived from the lamina terminalis
- the rostrum is continuous with the lamina terminalis (anterior wall of IIIrd ventricle)
What are the 2 major radiations from the corpus callosum that interconnect the 2 hemispheres?
Forceps minor / frontalis:
- this connects the lateral and medial surfaces of the frontal lobes
- it crosses the midline via the genu of the corpus callosum
Forceps major / occipitalis:
- this connects the occipital lobes
- it crosses the midline via the splenium of the corpus callosum
What structures are connected by the anterior commissure?
- it interconnects the middle and inferior temporal gyri and the olfactory bulbs
- derived from the lamina terminalis
What structures are connected by the fornix?
- also called the hippocampal commissure as it connects the hippocampi
What do projection fibres begin with?
What is this continuous with and what passage does it take to reach its target?
- projection fibres begin as the corona radiata, which are continuous with the internal capsule
- the internal capsule passes between the head of the caudate and the lentiform nuclei
- it connects to the cerebral peduncles, ventral pons and medullary pyramids
What are the 5 parts of the internal capsule?
- anterior limb
- genu
- posterior limb
- sublentiform part
- retrolentiform part
How is the internal capsule organised?
it is somatotopically organised
