# Internal Capsule > Internal capsule — limbs, fibre tracts, somatotopy, lenticulostriate blood supply and dense hemiplegia lesions for MBBS Anatomy. - Canonical URL: https://prepelephant.com/topics/mbbs/anatomy/internal-capsule - Exam / course: MBBS · Subject: Anatomy - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Internal Capsule", PrepElephant, https://prepelephant.com/topics/mbbs/anatomy/internal-capsule ## Direct answer Almost every fibre connecting the cerebral cortex with the brainstem, cerebellum and spinal cord is compressed into a V-shaped band of white matter between the thalamus and caudate nucleus medially and the lentiform nucleus laterally — the internal capsule. Its anterior limb carries frontopontine and thalamic fibres, the genu carries corticobulbar fibres for the face, and the posterior limb carries the corticospinal tract in an orderly face–arm–leg sequence from front to back, together with the thalamocortical sensory radiation. Supplied chiefly by the lenticulostriate branches of the middle cerebral artery, it is the commonest site of hypertensive intracerebral haemorrhage and of small lacunar infarcts, both producing the dense contralateral hemiplegia that out of all proportion to a lesion smaller than a peanut. ## What you must remember - **Shape and boundaries:** a V (or bent horn) with the genu pointing medially, between the caudate nucleus and thalamus medially and the lentiform nucleus laterally; seen best in a horizontal section. - **Anterior limb:** frontopontine fibres and the anterior thalamic radiation from the medial and anterior thalamic nuclei. - **Genu:** corticobulbar fibres to the cranial nerve motor nuclei, chiefly the lower facial nucleus — genu lesions spare the forehead. - **Posterior limb (thalamolentiform part):** corticospinal fibres occupy its anterior two-thirds, face most anterior then upper limb then trunk then lower limb posteriorly, with the thalamocortical sensory fibres behind them. - **Retrolentiform part:** visual radiation fibres from the lateral geniculate body sweeping to the occipital cortex; **sublentiform part:** auditory radiation from the medial geniculate body. - **Blood supply:** lateral lenticulostriate branches of the middle cerebral artery (the striate arteries that rupture in hypertension), medial striate including the recurrent artery of Heubner from the anterior cerebral artery, and the anterior choroidal artery to the inferior posterior limb. - **Haemorrhage:** Charcot–Bouchard microaneurysms on lenticulostriate arterioles rupture into the posterior limb — sudden contralateral dense hemiplegia, often with hemianaesthesia and homonymous field defects. - **Lacunar infarct:** a small deep infarct of the lenticulostriate territory producing a pure motor stroke — face, arm and leg all affected, cortex spared, hence no aphasia or neglect. ## Why a small lesion here devastates the body A diabetic hypertensive man collapses with a right face, arm and leg weakness of equal severity, yet speaks normally, neglects nothing and has no visual field cut — the clinical anatomy is worth reasoning through. In the cortex, motor fibres for face, arm and leg are spread over a long stretch of precentral gyrus, so a cortical infarct weakens one part more than others (a monoplegia pattern) and usually adds cortical features. In the internal capsule all these fibres, having converged, run in a bundle a few millimetres across the posterior limb in strict somatotopic order — corticobulbar in the genu, then arm, then leg — so a single lenticulostriate occlusion strikes them all at once, producing equal, dense weakness with no cortical sign. Sensory thalamocortical fibres just behind may share the damage, adding hemianaesthesia. Recovery is characteristically poor because the perforating arteries are end vessels with no collateral reach. The same packing explains why a larger hypertensive haemorrhage originating in the putamen extends medially into the capsule: consciousness is lost as the haematoma compresses the thalamus and midbrain. ## Where students slip Naming the parts is where marks leak: anterior limb, genu, posterior limb, retrolentiform and sublentiform — students invent a "central limb" or forget that the visual and auditory radiations occupy the retro- and sublentiform parts respectively. The second slip is somatotopy direction: the face lies anteriorly at the genu and the leg most posteriorly in the posterior limb, a reversal many drawings get wrong. In viva, "Why does an internal capsule lesion cause dense hemiplegia while a cortical lesion causes monoplegia?" must be answered with the convergence argument above; naming the lenticulostriate supply and Charcot–Bouchard aneurysms completes it. ## Frequently asked questions ### What are the parts of the internal capsule and their contents? The anterior limb (frontopontine and anterior thalamic fibres), genu (corticobulbar), posterior limb (corticospinal and thalamocortical sensory), retrolentiform part (visual radiation) and sublentiform part (auditory radiation). ### What is the somatotopy of the corticospinal tract in the posterior limb? Face fibres lie most anterior, near the genu, followed by upper limb, trunk and lower limb from before backwards in the anterior two-thirds of the posterior limb. Sensory thalamocortical fibres lie posterior to the motor fibres. ### Which arteries supply the internal capsule? Lateral lenticulostriate branches of the middle cerebral artery chiefly, the recurrent artery of Heubner from the anterior cerebral artery to the anterior limb and genu, and the anterior choroidal artery to the posterior part of the posterior limb. ### Why is the hemiplegia of internal capsule lesions dense and equal in face, arm and leg? Because all corticospinal and corticobulbar fibres converge into this narrow bundle, a small lesion destroys them all, unlike a cortical lesion that affects only part of the body representation. The cortex is spared, so aphasia, neglect and visual field defects are absent. ### What are Charcot–Bouchard microaneurysms? Small miliary aneurysms on lenticulostriate arterioles in chronic hypertension. Their rupture into the adjacent internal capsule and lentiform nucleus is the classical cause of spontaneous intracerebral haemorrhage.