Forebrain: Internal Capsule and Subcortical Tracts Flashcards
Outline key features of the 2 lateral ventricles.
- C-shaped cavities within each cerebral hemisphere
- location: surrounding the telencephalon
- parts: body (mostly in parietal lobe) + horns (anterior/frontal, posterior/occipital, inferior/temporal)
What connects the 2 lateral ventricles to the 3rd ventricle?
Interventricular foramina
Where do the body + posterior horn + inferior horns of the 2 lateral ventricles meet?
Collateral trigone/atrium
What is the septum pellucidum?
Collection of white fibres separating the lateral ventricles
Where is the 3rd ventricle?
Surrounds the diencephalon, between the 2 thalami
What connects the 3rd and 4th ventricles?
Cerebral aqueduct
What connects the thalamus and 3rd ventricle?
Interthalamic adhesions
Where is the fornix found?
Just above the 3rd ventricle
What shape is the 4th ventricle?
Pyramidal
Where is the 4th ventricle?
Surrounds the hindbrain
- leads to the central canal + 2 lateral apertures + 1 medial aperture
- aperture leads to the subarachnoid space
Identify the parts of the basal ganglia.
Corpus striatum, claustrum, amygdala and hippocampus
What are the two parts of the corpus striatum?
1) Lentiform nucleus
- wedge-shaped nucleus embedded within the cerebral white matter
- 2 parts:
–> globus pallidus medially and putamen laterally
2) Caudate nucleus
- located in the lateral walls of the lateral ventricles (LV)
- parts: head, body and tail
- head is continuous inferiorly with the putamen of the lentiform nucleus
What is the claustrum?
- thin layer of grey matter (condensation of cell body)
- location: covering the external capsule, between the lentiform nucleus and insula
What is the amygdala?
- lies deep to the uncus (primary olfactory area) in the temporal lobe
- anteriorly in the inferior horn of the LV
What is the hippocampus?
- region of cortex
- located along the medial edge of the temporal lobe
- forms the medial wall of the inferior horn of the LV
What are the three white matter fibres?
Projection, commisural and association fibres
What is the role of projection fibres?
- connect the cortex with subcortical structures
- cortex –> corona radiata –> internal capsule –> midbrain –> pons –> pyramids of medulla
- from the cerebral cortex, these tracts form the corona radiata (sheet of white matter) and capsules (internal, external, extreme)
What do commisural fibres do?
Connect corresponding parts of 2 hemisphere by crossing the midline
What are the three distinct bundles of commissural fibres?
Corpus callosum, anterior commissure and hippocampal commissure + fornix
Describe the corpus callosum bundle of commissural fibres.
- contain 200-300 million fibres
- divided into: rostrum, genu, body, splenium, tapetum
- is intersected in ‘split brain’ surgery
Outline key features of anterior commissure fibres (commissural).
- located inferior to the rostrum of the corpus callosum, anterior to the fornix
- contains +1/-1 million fibres
- connects the inferior parts of the temporal lobe including the olfactory areas + frontal lobe
What parts make up the hippocampal commissure + fornix?
Mamillary bodies, columns, body, crura and hippocampal commissure
What is the function of the hippocampal commissure + fornix?
Connect axons from the hippocampus together in a bundle
What is the path of the hippocampal commissure + fornix?
- Arises from posterior end of hippocampus –> passes up over the thalamus
- beneath the splenium, some fibres cross the midline to enter the opposite fornix, forming the hippocampal commissure
- body of the fornix passes forward until it reaches the front of the thalamus, where it diverges from the midline as it turns ventrally
- fornix forms anterior boundary of the interventricular foramen before it enters the hypothalamus, where it terminates
What do association fibres do?
- connect associated areas of the cortex on the same side
- short association fibres connect the cortex of adjacent gyri
- long association fibres connect important functional areas and form distinct bundles
What are the 3 capsules?
Internal, external and extreme
Where is the internal capsule located?
Between the lentiform nucleus (laterally) and the caudate nucleus + thalamus (medially)
- continues downwards into the brainstem as the crus cerebri and upwards as the corona radiata
- in horizontal sections, appears L-shaped and has 5 parts
Describe the 5 parts of the internal capsule.
1) Anterior limb
- location: between the head of the caudate nucleus (medially) and the lentiform nucleus (laterally)
- contains the anterior thalamic radiation and frontopontine fibres
2) Posterior limb
- location: between the thalamus (medially) and the lentiform nucleus (laterally)
- contains the middle (superior) thalamic radiation + corticospinal + corticobulbar + corticopontine tracts
3) Genu
- Junction of anterior and posterior limb
4) Retrolenticular part
- location: behind the lentiform nucleus
- contains the optic radiation + corticopontine fibres
5) Sublenticular part
- location: pass under the lentiform nucleus
- Contains the auditory radiation + some fibres of the optic radiation
Why is the internal capsule an important region clinically?
It is the most common site of cerebral haemorrhage.
i. Arterial supply: central (perforating) branches of the anterior and middle cerebral arteries
1. Anterior cerebral branches supply the anterior limb
2. Middle cerebral branches (anterior choroidal and lateral striate arteries) supply the remainder of the internal capsule
i
i. These arteries are end arteries and their small calibre make them susceptible to
1. Rupture if there is a sudden increase in blood pressure
2. Occlusion by emboli
iii. Posterior limb is most commonly affected, causing contralateral spastic hemiplegia + hemianesthesia. If the infarct extends into the retrolenticular part, it can cause contralateral hemianopsia
Key features of the external capsule?
- thin layer of white matter on the outer surface of the lentiform nucleus (putamen)
- formed mainly by fibres from the cortex –> putament
Key features of the extreme capsule?
Thin layer of white matter lying between the claustrum and insula
