CorpusHuman Anatomy
NervousBrain region

Internal capsule

Capsula interna

A compact V-shaped sheet of projection fibres between the thalamus and caudate nucleus medially and the lentiform nucleus laterally, carrying almost all traffic between the cortex and the brainstem and spinal cord; a small lesion here causes dense contralateral hemiplegia.

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Overview

Corticospinal fibres per side≈1 million (counted in the medullary pyramid)

In axial section the internal capsule forms a "V" (or boomerang) pointing medially. It has five parts:
• the anterior limb, between the head of the caudate and the lentiform nucleus;
• the genu, at the apex beside the interventricular foramen;
• the posterior limb, between the thalamus and the lentiform nucleus;
• a retrolenticular part, behind the lentiform nucleus;
• a sublenticular part, beneath it.
Above, its fibres fan out into the corona radiata. Below, they condense into the cerebral peduncle of the midbrain.

Because so many pathways converge in such a small space, even a small lacunar infarct or haemorrhage here can paralyse the whole contralateral side. Its contents by part are:
• anterior limb: frontopontine fibres and the anterior thalamic radiation (mediodorsal and anterior nuclei ↔ prefrontal and cingulate cortex);
• genu: corticobulbar fibres to the cranial motor nuclei;
• posterior limb: corticospinal fibres, somatotopically arranged with the arm anterior to the leg, and the superior thalamic radiation (VPL/VPM → somatosensory cortex) carrying all somatic sensation;
• retrolenticular part: the optic radiation;
• sublenticular part: the auditory radiation and part of the optic radiation (Meyer's loop).

Location

Deep cerebral white matter: medial to the lentiform nucleus (putamen + globus pallidus), lateral to the caudate head (anterior limb) and thalamus (posterior limb).

Parts & subdivisions

  1. Anterior limb
    Frontopontine fibres and the anterior thalamic radiation. Supplied by the recurrent artery of Heubner (ACA) and lenticulostriate arteries.
  2. Genu
    Corticobulbar fibres (face, tongue, pharynx). Lenticulostriate (MCA) and direct ICA branches.
  3. Posterior limb
    Corticospinal (arm → trunk → leg, anterior to posterior), superior thalamic (sensory) radiation, corticorubral and corticopontine fibres. Upper part: lenticulostriate arteries; lower part: anterior choroidal artery.
  4. Retrolenticular part
    Optic radiation (LGN → V1) and parieto-occipital corticopontine fibres.
  5. Sublenticular part
    Auditory radiation (MGN → Heschl gyri) and Meyer's loop of the optic radiation.

Anatomy

Arterial supply

Lateral lenticulostriate arteries (from the M1 segment of the MCA), the anterior choroidal artery (from the ICA), and the recurrent artery of Heubner (from the ACA).

Function

  • Conducts descending motor fibres (corticospinal, corticobulbar, corticopontine)
  • Conducts ascending thalamocortical sensory radiations
  • Carries the optic and auditory radiations
  • Connects the prefrontal cortex with the thalamus (anterior limb)

Clinical significance

Lacunar stroke: pure motor hemiparesis

Occlusion of a small penetrating lenticulostriate artery (lipohyalinosis from hypertension or diabetes) produces a lacune in the posterior limb. The result is equal weakness of the contralateral face, arm and leg with no cortical signs: no aphasia, neglect or field defect. A thalamic lacune causes pure sensory stroke.

Hypertensive haemorrhage

The lenticulostriate arteries ("arteries of cerebral haemorrhage", Charcot) are the commonest source of hypertensive intracerebral haemorrhage, typically in the putamen with extension into the internal capsule. It causes dense contralateral hemiplegia, hemisensory loss and conjugate gaze deviation towards the lesion.

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.