Topographic anatomy and surroundings
The internal capsule (capsula interna) is a thick layer of white matter wedged between large accumulations of gray matter—the basal nuclei and diencephalic structures.
Medially (internally), the capsule is bounded by two anatomical structures: anteriorly by the head of the caudate nucleus (caput nuclei caudati), and posteriorly by the thalamus (thalamus).
Laterally (externally), the lentiform nucleus (nucleus lentiformis) lies closely adjacent to the capsule. The nucleus itself is heterogeneous, consisting of an outer part, the putamen (putamen), and an inner part, the globus pallidus (globus pallidus), which is divided into lateral and medial segments. Slightly further lateral to the putamen are the external capsule (capsula externa) and a thin sheet of gray matter, the claustrum (claustrum).
Anatomical segments
Due to its curved shape, the internal capsule is anatomically divided into three distinct parts. Each of them serves as a pathway for strictly defined nerve tracts:
- Anterior limb (crus anterius). This is the anterior portion of the "boomerang", located between the head of the caudate nucleus and the anterior part of the lentiform nucleus.
- Genu (genu). The point of maximum curvature of the capsule, where the anterior limb transitions into the posterior limb at an angle.
- Posterior limb (crus posterius). The longest and most massive part, running between the thalamus and the posterior part of the lentiform nucleus.
Pathways of the anterior limb and genu
The internal capsule is the main collection hub for projection fibers. The following tracts pass through its anterior parts:
- The anterior limb contains fibers of the anterior thalamic radiation (radiatio thalami anterior) as well as the frontopontine tract (tractus frontopontinus), which connects the frontal lobe cortex with pontine structures.
- The genu of the internal capsule contains a compact bundle of corticonuclear fibers (fibrae corticonucleares). This is a crucial part of the pyramidal system, which originates in the cerebral cortex and descends to the motor cranial nerve nuclei of the brainstem. These fibers provide voluntary movements.
Pathways of the posterior limb
The posterior limb (crus posterius) transmits vital descending and ascending pathways. The fibers here are arranged in a strict topographic sequence (from anterior to posterior):
- Corticospinal fibers (fibrae corticospinales). This is the basis of the pyramidal system, the main motor pathway for voluntary movements. It originates from the precentral gyrus and paracentral lobule, passes through the capsule and cerebral peduncles, and descends into the spinal cord.
- Central thalamic radiation (radiatio thalami centralis). These are sensory fibers carrying information from the ventrolateral nucleus of the thalamus to the postcentral gyrus cortex (the analyzer for temperature, pain, tactile, and proprioceptive sensitivity).
- Pontine tracts. Positioned more posteriorly are the parietopontine, occipitopontine, and temporopontine fibers (fibrae parietopontinae, occipitopontinae, temporopontinae), which participate in movement coordination.
- Auditory and optic radiations (radiatio acustica et radiatio optica). These occupy the most posterior parts of the limb, transmitting impulses to the cortical analyzers in the temporal and occipital lobes.
Connections with the cortex and brainstem
Fibers passing through the internal capsule fan out to various areas of the cerebral cortex, forming the corona radiata (corona radiata). After passing through the capsule, descending pathways gather into compact bundles and enter the base of the cerebral peduncles (crus cerebri), continuing their course through the midbrain, pons, and medulla oblongata. Thus, the capsule acts as a critical node where virtually all pathways controlling voluntary movements and collecting somatic sensation are concentrated within a small area.
To remember the order of pathways in the posterior limb of the internal capsule (from anterior to posterior), use the rule "From action to perception": motor pathways come first (corticospinal fibers), followed by general sensation (central thalamic radiation), and finally specific sensory organs (auditory and optic radiations).
Class and exam questions
What is the internal capsule of the brain?
This is a massive layer of white matter located between the thalamus and basal nuclei. It serves as the main highway for projection nerve fibers connecting the cortex to the brainstem and spinal cord.
Which structures form the boundaries of the internal capsule?
Medially, it is bounded by the head of the caudate nucleus (anteriorly) and the thalamus (posteriorly). Laterally, the lentiform nucleus lies closely adjacent to it.
Where do the pathways of the pyramidal system run?
Corticonuclear fibers pass through the genu of the internal capsule, while corticospinal fibers pass through the anterior part of its posterior limb.
Which sensory pathways pass through the capsule?
The central thalamic radiation (general sensation) passes through the posterior limb, along with the auditory and optic radiations heading to their respective cortical analyzers.
What else to study on this topic
- Basal nuclei: caudate nucleus, putamen, and globus pallidus
- Pyramidal system: corticospinal and corticonuclear pathways
- Extrapiramidal system and red nucleus
- Thalamic nuclei and their cortical projections
- Functional localization in the cerebral cortex (cortical analyzers)
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