Definition: a massive layer (8–10 mm thick) of white matter separating the caudate nucleus and thalamus (medial) from the more laterally situated lentiform nucleus (globus pallidus and putamen). It consists of two gernal fiber populations: fibers ascending from the thalamus to the cerebral cortex that comprise, among others, the visual, auditory, and somatic sensory radiations, and fibers descending from the cerebral cortex to the thalamus, subthalamic region, midbrain, hindbrain, and spinal cord. The internal capsule is the major route by which the cerebral cortex is connected with the brainstem and spinal cord. Laterally and superiorly it is continuous with the corona radiata, which forms a major part of the cerebral hemisphere's white matter; caudally and medially it continues, much reduced in size, as the crus cerebri, which contains, among others, corticospinal fibers. In axial (horizontal) section it appears in the form of a V opening out laterally; the V's obtuse angle is called genu (knee); its anterior and posterior limbs, respectively, the crus anterior and crus posterior. The internal capsule consists of an anterior limb [TA], genu of internal capsule [TA], posterior limb [TA], retrolentiform (or retrolenticular) limb [TA], and sublentiform (or sublenticular) limb [TA].
Synonym(s): capsula internaTA