Topography of White Matter and the Capsular System
The space between subcortical nuclei is filled with robust sheets of white matter anatomically termed capsules. Two main structures are distinguished:
- External capsule (capsula externa) — a layer of fibers topographically located between the claustrum and the putamen.
- Internal capsule (capsula interna) — a massive and functionally vital plate of white matter. It reliably separates the lentiform nucleus from two other structures: the caudate nucleus and the thalamus. The vast majority of pathways connecting the cortex to lower structures pass through it. Upon exiting the capsule, these projection fibers radiate in a fan-like manner, forming the corona radiata (corona radiata).
Structure of the Internal Capsule
Anatomically, the internal capsule (capsula interna) features a complex spatial configuration. Traditionally, it is divided into several parts, each containing strictly defined tracts.
- Anterior limb (crus anterius). This part contains the frontopontine tract (tractus frontopontinus) and anterior thalamic radiations (radiationes thalamicae anteriores).
- Genu (genu). The bend of the capsule, through which critical motor corticonuclear fibers (tractus corticonuclearis) pass in transit.
- Posterior limb (crus posterius). This is the most functionally dense part of the internal capsule. It concentrates corticospinal fibers (fibrae corticospinales), corticorubral (fibrae corticorubrales), corticoreticular (fibrae corticoreticulares), and corticothalamic pathways (fibrae corticothalamicae). Sensory thalamoparietal fibers (fibrae thalamoparietales), optic radiation (radiationes opticae), and acoustic radiation (radiationes acusticae) also run through the posterior limb.
Additionally, the retrolenticular part (pars retrolentiformis) is located posterior to the lentiform nucleus. It includes the posterior thalamic radiations (radiationes thalamicae posteriores) and the parieto-occipitopontine fasciculus (fasciculus parietooccipitopontinus).
Classification of Nerve Fibers
Functionally and morphologically, all cerebral white matter is subdivided into three major groups.
Projection fibers (neurofibrae projectoriae) They provide two-way communication between the cerebral cortex and subcortical structures, brainstem nuclei, and the spinal cord. The bulk of these tracts courses compactly through the internal capsule.
Association fibers (neurofibrae associationes) This group of tracts integrates the activity of centers within a single hemisphere. Association pathways are classified as short and long:
- Short fibers are represented by arcuate fibers (fibrae arcuatae), which loop over sulci to connect adjacent gyri.
- Long fibers connect distant lobes. These include the superior longitudinal fasciculus (fasciculus longitudinalis superior), inferior longitudinal fasciculus (fasciculus longitudinalis inferior), uncinate fasciculus (fasciculus uncinatus), and cingulum (cingulum).
Commissural fibers (neurofibrae commissurales) Their primary role is to connect symmetrical regions of both hemispheres, ensuring synchronized function. The corpus callosum is the largest commissure. This group also includes the anterior commissure (commissura anterior) and the hippocampal commissure (commissura fornicis).