Forebrain: Internal Capsule and Subcortical Tracts

Question 1

Outline key features of the 2 lateral ventricles.

Answer 1

• C-shaped cavities within each cerebral hemisphere
• location: surrounding the telencephalon
• parts: body (mostly in parietal lobe) + horns (anterior/frontal, posterior/occipital, inferior/temporal)

Question 1

What connects the 2 lateral ventricles to the 3rd ventricle?

Answer 1

Interventricular foramina

Question 2

Where do the body + posterior horn + inferior horns of the 2 lateral ventricles meet?

Answer 2

Collateral trigone/atrium

Question 3

What is the septum pellucidum?

Answer 3

Collection of white fibres separating the lateral ventricles

Question 4

Where is the 3rd ventricle?

Answer 4

Surrounds the diencephalon, between the 2 thalami

Question 5

What connects the 3rd and 4th ventricles?

Answer 5

Cerebral aqueduct

Question 6

What connects the thalamus and 3rd ventricle?

Answer 6

Interthalamic adhesions

Question 7

Where is the fornix found?

Answer 7

Just above the 3rd ventricle

Question 8

What shape is the 4th ventricle?

Answer 8

Pyramidal

Question 9

Where is the 4th ventricle?

Answer 9

Surrounds the hindbrain
- leads to the central canal + 2 lateral apertures + 1 medial aperture
- aperture leads to the subarachnoid space

Question 10

Identify the parts of the basal ganglia.

Answer 10

Corpus striatum, claustrum, amygdala and hippocampus

Question 11

What are the two parts of the corpus striatum?

Answer 11

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

Question 12

What is the claustrum?

Answer 12

• thin layer of grey matter (condensation of cell body)
• location: covering the external capsule, between the lentiform nucleus and insula

Question 13

What is the amygdala?

Answer 13

• lies deep to the uncus (primary olfactory area) in the temporal lobe
• anteriorly in the inferior horn of the LV

Question 14

What is the hippocampus?

Answer 14

• region of cortex
• located along the medial edge of the temporal lobe
• forms the medial wall of the inferior horn of the LV

Question 15

What are the three white matter fibres?

Answer 15

Projection, commisural and association fibres

Question 16

What is the role of projection fibres?

Answer 16

• 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)

Question 17

What do commisural fibres do?

Answer 17

Connect corresponding parts of 2 hemisphere by crossing the midline

Question 18

What are the three distinct bundles of commissural fibres?

Answer 18

Corpus callosum, anterior commissure and hippocampal commissure + fornix

Question 19

Describe the corpus callosum bundle of commissural fibres.

Answer 19

• contain 200-300 million fibres
• divided into: rostrum, genu, body, splenium, tapetum
• is intersected in ‘split brain’ surgery

Question 20

Outline key features of anterior commissure fibres (commissural).

Answer 20

• 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

Question 21

What parts make up the hippocampal commissure + fornix?

Answer 21

Mamillary bodies, columns, body, crura and hippocampal commissure

Question 22

What is the function of the hippocampal commissure + fornix?

Answer 22

Connect axons from the hippocampus together in a bundle

Question 23

What is the path of the hippocampal commissure + fornix?

Answer 23

• 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

Question 24

What do association fibres do?

Answer 24

- 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

Question 25

What are the 3 capsules?

Answer 25

Internal, external and extreme

Question 26

Where is the internal capsule located?

Answer 26

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

Question 27

Describe the 5 parts of the internal capsule.

Answer 27

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

Question 28

Why is the internal capsule an important region clinically?

Answer 28

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

Question 29

Key features of the external capsule?

Answer 29

- thin layer of white matter on the outer surface of the lentiform nucleus (putamen) - formed mainly by fibres from the cortex --> putament

Question 30

Key features of the extreme capsule?

Answer 30

Thin layer of white matter lying between the claustrum and insula