Internal capsule, corpus callosum and fornix

Show numbers

1. Globus pallidus

Shown in Images 4 and 5, the globus pallidus is one of the five main components of the basal ganglia. Together with the caudate nucleus and putamen, the globus pallidus is one of the three basal ganglia components located in the forebrain and has two segments:

External segment (GPe): receives afferents from basal ganglia nuclei, especially the caudate nucleus, putamen and subthalamic nucleus, and sends efferent projections primarily to other basal ganglia nuclei such as the subthalamic nucleus.

Internal segment (GPi): receives the same afferents as the external segment but, together with the pars reticulata of the substantia nigra, is one of the two main output nuclei of the basal ganglia. Efferent projections from the internal segment of the globus pallidus go to the thalamus.

Note that in addition to the GPe and GPi, the pallidal complex also contains the ventral pallidum, which is located inferior to the anterior commissure and extending into the ventral striatum.

Show list

1.
Globus pallidus
2.
Putamen
3.
Nucleus accumbens
4.
Internal Capsule
5.
Lateral ventricle
6.
Body of corpus callosum
7.
Caudate nucleus
8.
Fornix (plural fornices)
9.
Corona radiata
10.
Claustrum
11.
Insula

In this view we see:

A coronal section of the brain cutting vertically through the lateral ventricles revealing the anterior part of the globus pallidus.
In addition to the globus pallidus, two other components of the basal ganglia: the caudate nucleus and putamen.
Between the darker brown areas (indicating grey matter nuclei) are light brown areas indicating white matter tracts, which include the internal capsule, corpus callosum and fornix.

Location

Shown in Images 4, 5, 6 and 7, the internal capsule is a continuation of the corona radiata (seen in Image 3) with the putamen and globus pallidus on its lateral side, and the caudate nucleus and thalamus on its medial side.

Structure

The internal capsule has five parts, named for their relationship to the putamen and globus pallidus:

The anterior limb is largely between the putamen and the head of the caudate nucleus.
The posterior limb has the putamen and globus pallidus laterally and thalamus medially.
The genu is the junction between the anterior and posterior limbs.
The retrolenticular part is behind the putamen and globus pallidus.
The sublenticular part is beneath the putamen and globus pallidus.

Function

The internal capsule consists of fibres conducting electrical impulses (information) in both directions between the superficially located cortex and the deeper thalamus and brainstem. This function varies for each of the five parts of the internal capsule:

The anterior limb conveys information from the thalamus to the cingulate gyrus (seen in Image 6) and prefrontal cortex (seen in Image 1).
The posterior limb importantly contains corticospinal tract fibres as well as fibres interconnecting the thalamus and motor cortex, and sensory fibres from the thalamus to the somatosensory cortex.
The genu contains fibres connecting the thalamus to the prefrontal cortex.
The retrolenticular part of the internal capsule interconnects the thalamus with the posterior parts of the cerebral hemispheres, including some of the optic radiation.
The sublenticular part of the internal capsule contains the auditory radiation and the remainder of the optic radiation.

Strokes affecting the internal capsule can have devastating effects because important connections are funnelled through a small, confined area between the putamen and globus pallidus of the basal ganglia on one side, and the thalamus on the other side.

When a stroke impacts on the corticospinal tract in the posterior limb of the internal capsule, there is likely to be loss of movement in the arm and leg on the opposite side of the body (the left and right corticospinal tracts swap to opposite sides at the pyramidal decussation in the inferior medulla).