corona radiata

**1. Neuroanatomy (most high-yield meaning)** - Location: White matter lying superior to the internal capsule, deep to the cerebral cortex. Fibers fan out (radiate) from the internal capsule to all cortical regions, and below it they funnel/converge into the internal capsule (anterior limb, genu, posterior limb), then the cerebral peduncles of the midbrain. - Composition: Projection fibers, both ascending and descending. - Descending: corticospinal, corticobulbar (corticonuclear), corticopontine, and corticothalamic fibers. - Ascending: thalamocortical fibers (including somatosensory radiations from VPL/VPM to S1). - Continuity: Corona radiata → internal capsule → cerebral peduncle (crus cerebri) → basis pontis → medullary pyramids (corticospinal tract decussation at the caudomedial medulla). - Somatotopy: The motor fibers are organized somatotopically. They are more compact as they converge at the internal capsule, so small lesions there produce larger deficits than in the broad corona radiata. - Blood supply: Largely the deep penetrating branches of the MCA (lenticulostriate arteries) and, for the superficial/periventricular white matter, the medullary branches of the MCA and ACA. The anterior choroidal artery (from the ICA) supplies the posterior limb of the internal capsule.

**Clinical significance** - Lacunar infarcts (small-vessel disease, lipohyalinosis from chronic hypertension; also diabetes) in the corona radiata or internal capsule produce classic lacunar syndromes: - Pure motor hemiparesis: the most common. Contralateral face, arm, and leg weakness without cortical signs (no aphasia, neglect, or visual field deficit). - Pure sensory stroke (thalamic VPL/VPM, or thalamocortical fibers). - Ataxic hemiparesis, dysarthria–clumsy hand syndrome. - Because the lesion is above the decussation, deficits are contralateral. UMN signs follow: spasticity, hyperreflexia, Babinski sign. Initial flaccidity may occur acutely. - Cortical signs (aphasia, neglect, agnosia, visual field cut) are absent because the cortex is spared. This helps distinguish a subcortical lacune from a cortical MCA stroke. - Imaging: MRI DWI is the most sensitive for acute small infarcts. CT is often negative early. Chronic small-vessel disease shows as T2/FLAIR white matter hyperintensities. - Other pathology: Multiple sclerosis (periventricular demyelination), leukoaraiosis/vascular dementia (Binswanger disease: subcortical white matter ischemia in chronic hypertension), and diffuse axonal injury (shearing at the gray–white junction, corpus callosum, brainstem).

**2. Embryology/Reproductive biology** - The corona radiata is the layer of granulosa (cumulus) cells surrounding the secondary oocyte, outside the zona pellucida. It is ovulated with the oocyte. - Sperm must penetrate it during fertilization: 1. Sperm pass through the corona radiata, aided by hyaluronidase from the acrosome. 2. Sperm bind ZP3 (a zona pellucida glycoprotein) → acrosomal reaction releases acrosin and other enzymes → penetration of the zona pellucida. 3. Sperm–oocyte membrane fusion → cortical reaction (Ca²⁺-mediated release of cortical granules) → zona reaction (ZP2 cleavage) → block to polyspermy → completion of meiosis II. - Order from outside to inside: corona radiata → zona pellucida → oocyte plasma membrane. - Capacitation in the female genital tract is required before sperm can undergo the acrosomal reaction.

**Board pearls** - "Pure motor stroke" = posterior limb of the internal capsule, corona radiata, or basis pontis. All involve lacunar infarction. - The corona radiata is a continuation of the internal capsule. Don't confuse it with the corpus callosum (commissural fibers) or arcuate/uncinate fasciculi (association fibers). - Three fiber types: projection (corona radiata/internal capsule), commissural (corpus callosum, anterior commissure), association (arcuate, cingulum, uncinate).