White matter topography and capsule system
The space between subcortical nuclei is filled with thick layers of white matter anatomically termed capsules. Two main structures are distinguished:
- External capsule (capsula externa) is a layer of fibers topographically located between the claustrum and the putamen.
- Internal capsule (capsula interna) is a massive and functionally crucial 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 with lower structures pass through it. Upon exiting the capsule, these projection fibers diverge in a fan-like manner, forming the so-called corona radiata (corona radiata).
Structure of the internal capsule
Anatomically, the internal capsule (capsula interna) has a complex spatial configuration. Traditionally, several parts are distinguished within it, each containing strictly defined pathways.
- 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. It serves as a transit route for the critical motor corticonuclear fibers (tractus corticonuclearis).
- Posterior limb (crus posterius). This is the most functionally dense part of the internal capsule. It concentrates the corticospinal fibers (fibrae corticospinales), corticorubral fibers (fibrae corticorubrales), corticoreticular fibers (fibrae corticoreticulares), and corticothalamic pathways (fibrae corticothalamicae). Also passing through the posterior limb are sensory thalamoparietal fibers (fibrae thalamoparietales), the optic radiation (radiationes opticae), and the auditory radiation (radiationes acusticae).
A separate division is the retrolentiform part (pars retrolentiformis), which 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 the white matter of the hemispheres is divided into three major groups.
Projection fibers (neurofibrae projectoriae) They provide two-way communication between the cerebral cortex and subcortical structures, brainstem structures, and the spinal cord. The bulk of these tracts pass 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 association fibers are represented by arcuate fibers (fibrae arcuatae), which arch over sulci and connect adjacent gyri.
- Long association 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 main task is to connect symmetrical areas of both hemispheres, ensuring their coordinated function. The largest commissure is the corpus callosum. This group also includes the anterior commissure (commissura anterior) and the commissure of the fornix (commissura fornicis).
Briefly: answers to topical questions
Where is the extreme capsule (capsula extrema) located?
The extreme capsule (extreme capsule) is located lateral to the claustrum (claustrum).
The claustrum is a thin layer of gray matter situated between the lentiform nucleus and the insular cortex (insula cortex). It divides the white matter into two structures:
- External capsule (external capsule) is located medial to the claustrum.
- Extreme capsule (extreme capsule) is located lateral to the claustrum.
Class and exam questions
What forms the corona radiata?
The corona radiata is formed by projection fibers of the hemispheres that radiate fan-like from the internal capsule to the cerebral cortex.
Which fibers connect the cortex within a single hemisphere?
Communication between cortical areas of the same hemisphere is provided by association fibers. Adjacent gyri are connected by short arcuate fibers, while distant lobes are linked by long bundles (e.g., the superior and inferior longitudinal fasciculi).
What is the difference between the internal and external capsules?
Topographically, the internal capsule separates the lentiform nucleus from the caudate nucleus and thalamus, whereas the external capsule lies between the putamen and the claustrum. The internal capsule serves as the primary pathway for projection tracts.
What passes through the genu of the internal capsule?
The corticonuclear tract (tractus corticonuclearis) passes through the genu of the internal capsule.
What else to study on this topic
- Topography and functions of the long association tracts of the hemispheres
- Detailed composition and course of the optic and auditory radiations
- Anatomy of the corpus callosum and other commissural pathways
- Course of motor pathways through the posterior limb of the internal capsule
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