Cortex (from cortex, thalamus & hypothalamus) Flashcards
Raised fold in the cortex
Gyrus
Fissure in the cortex
Sulcus
Discuss the cerebral microstructure
- cell bodies of cortical neurones are always arranged in layers, that usually lie parallel to the surface of the brain
- the layer of neurons closest to the surface (the most superficial layer) is separated from the pia mater by a zone that lacks neurons → the molecular layer
- at least one cell layer contains pyramidal cells that emit large dendrite (apical dendrites) that extend up to layer I where they form multiple branches → pyramidal cells in the prefrontal cortex have up to 23 times more dendritic spines than those in the primary visual area (may allow for more extensive sampling of inputs, as required by higher-order processing of visual information)
What two divisions may the cortex consist of?
- the neocortex (differentiated into 6 horizontal layers)
2. the allocortex, which contains the hippocampus (which has 3 cortical layers) and olfactory cortex
How many layers does the hippocampus have?
3 at most
Discuss cortical columns
Neurons in various layers connect vertically to form small microcircuit → these are cortical columns
also known as hypercolumns
they consist of groups of neurons that have nearly identical receptive fields
What are Broadmann’s areas distinguished by?
Differences in histological structure and functional roles in:
- sensation, cognition and behaviour
What neurons are the most common in the cortex?
Pyramidal
What neurotransmitter do pyramidal neurons utilise?
Glutamate
Discuss the 3 different fibres types in the cortex
Commissural (transverse fibres) → connect the two hemispheres
Association → connect regions in the same hemisphere
Projection → leave the cortex and project to different regions of the CNS - e.g. thalamus & spinal cord
How many layers does the cortex have?
6
Mnemonic for layers of the cortex
From the outside to the inside:
Molecular (plexiform), external granular, external pyramidal, internal granular, internal pyramidal, multiform (fusiform) layers (‘Miles Per Gallon Is Gallon Per Miles’)
Descibre the molecular layer (layer I)
- Contains few scattered neurons and consists mainly of extensions of apical dendritictufts of
pyramidal neurons and horizontally oriented axons, as well asglial cells - Also, some spinystellate cellscan be found here.
- Inputs to the apical tufts are thought to be crucial for the ‘‘feedback’’ interactions in the
cerebral cortex involved in associative learning and attention - While it was once thought that the input to layer I came from the cortex itself, it is now
realized that layer I across the cerebral cortex mantle receives substantial input from ‘‘matrix’’
or M-type thalamus cells (in contrast to ‘‘core’’ or C-type that go to layer IV)
Describe the external granular layer (layer II)
Contains smallpyramidal neuronsand numerous stellate neurons
Can be long enough to be association fibres
Describe external pyramidal layer (layer III)
Contains predominantly small and medium-sizepyramidal neurons
- The apical dendrites of these cells extend superficially and reach the molecular layer
- the basal processes join the subcortical white matter and then project again to the cortex, so they serve as both association and commissural corticocortical fibers.
Describe the interal granular layer (layer IV)
- Contains different types ofstellateandpyramidal neurons,
- is the main target of
thalamocortical afferents from thalamus type C neurons as well as intra-hemispheric
corticocortical afferents - main input to cortical station (most stimuli from the periphery arrive here)
- therefore contains sensory areas:
- Stellate cells of the primary sensory cortex receive fibers from the ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei of the thalamus
- Primary visual cortex receives fibers from the lateral geniculate nucleus
- Stellate cells from primary auditory cortex receive projections from the medial geniculate nucleus
Describe the internal pyramidal layer (layer V)
Contains LARGEpyramidal neuronswhich give rise to axons leaving the cortex and running
down to subcortical structures (such as thebasal ganglia).
- In the primary motor cortex of the frontal lobe, layer V containsBetz cells, whose axons travel
through theinternal capsule, thebrain stemand the spinal cord forming the corticospinal
tract, which is the main pathway for voluntary motor control.
Describe the multiform layer (layer VI)
- Contains few largepyramidal neuronsand many small spindle-like pyramidal and multiform
neurons - Layer VI sends efferent fibers to the thalamus, establishing a very precise reciprocal
interconnection between the cortex and the thalamus – layer VI neurons from one cortical column connect with thalamus neurons that provide input to the same cortical column; these
connections are both excitatory and inhibitory - Neurons sendexcitatoryfibers to neurons in the thalamus and also send collaterals to
thethalamic reticular nucleusthatinhibitthese same thalamus neurons or ones adjacent to
them – one theory is that because the inhibitory output is reduced by cholinergicinput to the
cerebral cortex, this provides thebrainstemwith adjustable “gain control for the relay
oflemniscalinputs - Cortical areas that lack a layer IV are called agranular; cortical areas that have only a
rudimentary layer IV are
called dysgranular
Which layers are the output pyramidal cells located in?
3, 5 and 6
What do the columns represent?
The functional units of the cortex
Describe the anatomy of the corpus callosum
- Theposterior(back) portion of the
corpus callosum is called thesplenium - theanterior(front) is called thegenu(or knee)
- between the two is the truncus, or body
- The part between the body and the splenium is often markedly narrowed – isthmus
Describe the area of the brain connecting the prefrontal cortex between the two halves
Thinner axons in the genu
These fibres arise from a fork-like bundle of fibres in the tapetum (main body of fibres), called the forceps anterior
Describe the connection of the motor cortex between the two hemispheres
- Thicker axons in the mid-body or trunk, innerconnect areas of the motor cortex
- proportionately more of the corpus callosum is dedicated to supplementary motor regions, including Broca’s area
Describe the part of the the allows exchange of somatosensory information between the two hemispheres?
The posterior body of the corpus, known as the splenium, communicates somatosensory information between the two halves of the parietal lobe and the visual cortex at the occipital lobe
These are the fibres of the forceps posterior
What is the internal capsule?
A white matter structure situated in the inferomedial part of each cerebral hemisphere (boomerang shape)
What does the internal capsule separate?
The caudate nucleus and the thalamus from the putamen and globus pallidus
What does the internal capsule consist of?
Both ascending and descending axons
What forms a large part of the internal capsule? What does this carry?
Corticospinal tract
Motor info from the primary motor cortex to the lower motor neurons of the spinal cord
What does the anterior limb of the internal capsule contain?
a) Frontopontine(corticofugal) fibers project fromfrontal cortextopons
b) Thalamocortical fibers (part of thethalamocortical radiations) connect
themedialandanteriornuclei of the thalamus to thefrontal lobes(these are severed during
aprefrontal lobotomy).
What can the internal capsule be divided into?
Anterior limb
Genu
Posterior limb
What does the genu contain?
containscorticobulbar fibers, which run between thecortexand thebrainstem.
What does the posterior limb contain?
containscorticospinal fibers, sensory fibers (including
themedial lemniscusand theanterolateral system) from the body and a few corticobulbar fibers
What is the fornix?
The fornix is a C-shaped bundle of nerve fibers in the brain that carries signals from
the hippocampus to the mammillary bodies and then to
the anterior nuclei of thalamus
it is part of the limbic system
What connects the fornix to the corpus callosum?
Septum pellucidum
What are the posterior bifurcations of the fornix called?
Columns of the fornix
What does the fornix pass through to reach the mamillary body?
Hypothalamus
Describe the precommisurral fornix
composed of the fibers that descend in front of the anterior commissure. They relay onto the septal nuclei (medial olfactory area), the ventral striatum, and the cingulate gyrus.
Describe the postcommisurral fornix
the postcommissural fornix are the fibers that pass behind the anterior commissure and relay onto the mammillary bodies and the anterior nuclei of the thalamus. A small amount of fornical fibers pass above the splenium of corpus callosum to reach the structures above the corpus callosum. Those fibers constitute the dorsal fornix.
Main function of the fornix
the primary role of the fornix is to transmit the information from the hippocampus to the mammillary bodies and to the anterior nuclei of thalamus
thought to play a key role in memory
Functional anatomy areas of the cortex → draw as well
Somatosensory - Motor and pre-motor - Visual - Auditory - Posterior parietal - Orbitofrontal - Olfactory - Limbic Look on OneNote
Describe thalamocortical projections
Thalamocortical fibers have a bush or tree-like appearance as they extend into theinternal capsuleand
project to the layers of the cortex.
- The main thalamocortical fibers extend from different nuclei of thethalamusand project to the visual
cortex, somatosensory (and associated sensori-motor) cortex, and the auditory cortex in the brain.
- Visual input from the optic tract is processed by thelateral geniculate nucleusof thethalamus
- auditory input processed in themedial geniculate nucleus
- Somatosensory input processed in theventral posterior nucleus
- Thalamocortical radiations also innervate the gustatory pathways, as well as pre-frontal motor areas
Describe corticothalamic projections. Which nucleus recieves corticothalamic feedback? From which layer of the cortex do these mainly come?
Thalamic nuclei project to cortical areas of distinct architectural organization and relay the processed
information back to the area of original activity in the thalamus viacorticothalamic(CT) fibres
- Thethalamic reticular nucleus(TRN) receives incoming signals via corticothalamic pathways and
regulates activity within the thalamus accordingly
- Cortico-thalamic feedback neurons are mostly found in layer VI of the cortex.
- Reciprocal CT projections to the thalamus are of a higher order than, and synapse with the TRN in
much greater number than do thalamocortical projections to cortex – this suggests that the cortex has
a much bigger role in top down processing and regulation of thalamic activity than do the processes
originating in thalamic interneurons.
What s the arterial cerebral circulation normally divided into?
Anterior cerebral circulation and posterior cerebral circulation
What are the two main pairs of arteries that supply the cerebral arteries and the cerebrum?
- internal carotid
- vertebral
What are the anterior and posterior cerebral circulations interconnected via?
Bilateral posterior communicating arteries - part of the circle of Willis - this provides backup circulation
Describe the anterior cerebral circulation - what is it supplied by?
- Internal carotid arteries – these large arteries are
the left and right branches of thecommon carotid
arteriesin the neck which enter the skull, as
opposed to theexternal carotidbranches which
supply the facial tissues. The internal carotid artery
branches into theanterior cerebral arteryand
continues to form themiddle cerebral artery - Anterior cerebral artery(ACA) – left and right anterior
cerebral arteries connected by anterior communicating
artery, within and along the floor of the cerebral vault. - Middle cerebral artery(MCA)
What arteries form the posterior cerebral circulation?
- Vertebral arteries: These smaller arteries branch from
thesubclavian arteries; within thecranium the two vertebral arteries fuse into the basilar artery
- Posterior inferior cerebellar artery(PICA) is largest branch off of vertebral arteries - Basilar artery: Suppliesmidbrain,cerebellum, and usually branches into posterior cerebral artery
- Anterior inferior cerebellar artery(AICA)
- Pontine branches
- Superior cerebellar artery(SCA) - Posterior cerebral artery(PCA)
- Posterior communicating artery
What is the structure of the venous drainage of the brain?
Drainage by veins which empty into dural venous sinuses
Where do the dural venous sinuses lie?
Between the periosteal and meningeal layers of the dura mater
What do all the dural sinuses ultimately drain into?
The internal jugular vein
How many venous sinuses are there in total?
11
Describe the venous sinuses
The straight, superior, and inferior sagittal sinuses are found in the falx cerebri of the dura mater. They converge at the confluence of sinuses (overlying the internal occipital protuberance). The straight sinus is a continuation of the great cerebral vein and the inferior sagittal sinus.
From the confluence, the transverse sinus continues bi-laterally and curves into the sigmoid sinus to meet the opening of the internal jugular vein.
The cavernous sinus drains the ophthalmic veins and can be found on either side of the sella turcica. From here, the blood returns to the internal jugular vein via the superior or inferior petrosal sinuses
Draw them! OneNote
What may the veins of the brain be divided into?
Superficial and deep gorups
Discuss the superficial venous system
- Superior cerebral veins: Drain the superior surface, carrying blood to the superior sagittal sinus.
- Superficial middle cerebral vein: Drains the lateral surface of each hemisphere, carrying blood to the cavernous or sphenopalatine sinuses.
- Inferior cerebral veins: Drain the inferior aspect of each cerebral hemisphere, depositing blood into cavernous and transverse sinuses.
- Superior anastamotic vein (Trolard): Connects the superficial middle cerebral vein to the superior sagittal sinus.
- Inferior anastamotic vein (Labbé): Connects the superficial middle cerebral vein to the transverse sinus
Discuss the deep venous system
- Subependymal veins – There are numerous subependymal veins, which will not be described here in detail. These receive blood from the medullary veins and carry it to the dural venous sinuses. The great cerebral vein (vein of Galen) is worthy of a mention; it is formed by the union of two of the deep veins, and drains into the
straight sinus. - Medullary veins: Originate 1-2cm below the cortical grey matter, and drain into subependymal veins. These drain the deep areas of the brain.
Consequences of lesion to visual cortex
Scotoma (hole in visual field)
Consequences of lesion to motor cortex
Lesions of the precentral gyrus (primary motor cortex) result inparalysisof the
contralateral side of the body
Consequences of lesion to somatosensory cortex
Lesions in S1 impair somatosensation (dysfunction in texture, size and
shape discrimination)
Consequences of lesion to posterior parietal cortex
the two most striking consequences of PPC lesions are apraxia and hemospatial neglect (neglect syndromes and acquired dyslexias)
Left hemisphere lesions cause Broca’s aphasia
Right hemisphere lesions cause hemispatial neglect (failure to perceive and process stimuli
from one side of the body, most commonly contralateral side) – hemispatial neglect occurs
more commonly in right hemisphere, so often left-sided neglect
