Internal capsule

Internal capsule

Horizontal section of right cerebral hemisphere. (Capsula interna labeled at upper left.)

The motor tract.
Details
Identifiers
Latin capsula interna
NeuroNames hier-180
NeuroLex ID Internal capsule
Dorlands
/Elsevier		 c_07/12211417
TA A14.1.09.524
FMA 61950

Anatomical terms of neuroanatomy

The internal capsule is a white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain. It carries information past the basal ganglia, separating the caudate nucleus and the thalamus from the putamen and the globus pallidus. The internal capsule contains both ascending and descending axons. The internal capsule contains fibres going to and coming from the cerebral cortex.

The corticospinal tract constitutes a large part of the internal capsule, carrying motor information from the primary motor cortex to the lower motor neurons in the spinal cord. Above the basal ganglia the corticospinal tract is a part of the corona radiata, below the basal ganglia the tract is called crus cerebri (a part of the cerebral peduncle) and below the pons it is referred to as the corticospinal tract.

Structure

The internal capsule is V-shaped when cut horizontally, in a transverse plane.

When cut horizontally:

Genu

The genu is the flexure of the internal capsule. It is formed by fibers from the corticonuclear tracts. The fibers in this region are named the geniculate fibers; they originate in the motor part of the cerebral cortex and, after passing downward through the base of the cerebral peduncle with the cerebrospinal fibers, undergo decussation and end in the motor nuclei of the cranial nerves of the opposite side. It contains the Corticobulbar tract.

Anterior limb

The anterior limb of internal capsule (or frontal part) contains:

  1. fibers running from the thalamus to the frontal lobe;
  2. fibers connecting the lentiform and caudate nuclei;
  3. fibers connecting the cortex with the corpus striatum; and
  4. fibers passing from the frontal lobe through the medial fifth of the base of the cerebral peduncle to the nuclei pontis.

Posterior limb

The posterior limb of internal capsule (or occipital part) is the portion of the internal capsule posterior to the genu.

The anterior two-thirds of the occipital part of the internal capsule contains fibers of the corticospinal tract, which arise in the motor area of the cerebral cortex and, passing downward through the middle three-fifths of the base of the cerebral peduncle, are continued into the pyramids of the medulla oblongata.

The posterior third of the occipital part contains:

  1. sensory fibers, largely derived from the thalamus, though some may be continued upward from the medial lemniscus;
  2. the fibers of optic radiation, from the lower visual centers to the cortex of the occipital lobe;
  3. acoustic fibers, from the lateral lemniscus to the temporal lobe; and
  4. fibers which pass from the occipital and temporal lobes to the nuclei pontis.

Blood supply

The superior parts of both the anterior and posterior limbs and the genu of the internal capsule are supplied by the lenticulostriate arteries, which are branches of the M1 segment of the middle cerebral artery.

The inferior half of the anterior limb is supplied via the recurrent artery of Heubner, which is a branch of the anterior cerebral artery.

The inferior half of the posterior limb is supplied by the anterior choroidal artery, which is a branch of the internal carotid artery.

In summary, the blood supply of the internal capsule is

As in many parts of the body, some degree of variation in the blood supply exists. For example, thalamoperforator arteries, which are branches of the basilar artery, occasionally supply the inferior half of the posterior limb. Ellis clinical anatomy book says posterior cerebral artery also supply the posterior part of internal capsule

Function

Working anterior to posterior:

1) Frontopontine (corticofugal) fibers project from frontal cortex to pons;
2) Thalamocortical fibers (part of the thalamocortical radiations) connect the medial and anterior nuclei of the thalamus to the frontal lobes (these are severed during a prefrontal lobotomy).

Other fibers within the internal capsule

Clinical significance

The lenticulostriate arteries supply a substantial amount of the internal capsule. These small vessels are particularly vulnerable to narrowing in the setting of chronic hypertension and can result in small, punctate infarctions or intraparenchymal haemorrhage due to vessel rupture.

Lesions of the genu of the internal capsule affect fibers of the corticobulbar tract.

The primary motor cortex sends its axons through the posterior limb of the internal capsule. Lesions, therefore, result in a contralateral hemiparesis or hemiplegia. While symptoms of weakness due to an isolated lesion of the posterior limb can initially be severe, recovery of motor function is sometimes possible due to spinal projections of premotor cortical regions that are contained more rostrally in the internal capsule.

Additional images

References

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