The internal capsule is located in the human brain and consists of nerve fibers that connect deeper areas and the cerebral cortex. The numerous tracts that pass through the internal capsule include the fibrae frontopontinae, the tractus corticospinalis of the pyramidal tract, the fibrae temporopontinae, the tractus corticotectalis, and parts of the auditory and visual pathways. A wide variety of neurologic syndromes may develop in the setting of stroke and other damage, including hemiparesis.
What is the internal capsule?
Various nerve tracts that pass through the brain are grouped together by neurology to form the internal capsule. Two basic types of nervous tissue can be distinguished in the human brain: Gray matter contains many cell bodies (somata), while white matter consists mainly of nerve fibers. These fibers are extensions of the neurons through which electrical signals travel from one cell to the next. Outwardly, they are surrounded by an insulating layer known as the myelin sheath, which makes the tissue appear white. The myelin sheath is made up of specialized glial cells called Schwann cells. They grow in a spiral around the axon. Strictly speaking, axons are the mere extensions of nerve cells, while the term “nerve fiber” refers to the unit of axon and myelin layer. However, since most axons of the human nervous system are myelinated, this formal distinction plays only a minor role in practice. The capsula interna is also composed of white matter.
Anatomy and structure
The fibers of the internal capsule extend from the cortex at the surface of the brain to deeper areas such as the cerebral crus (crus cerebri). Their course is identical in both hemispheres. Towards the center of the brain, the thalamus and caudate nucleus are adjacent to the neural pathways of the internal capsule, while on the other side is the lentiform nucleus, itself composed of the putamen and pallidum. Anatomically, three areas can be distinguished within the internal capsule: the crus anterius, the genu capsulae internae, and the crus posterius. The crus anterius (“anterior limb”) consists of the nerve fibers located in the cephalic part of the cup-shaped accumulation. The fibrae frontopontinae, which transmit nerve signals from the frontal lobe to the crus cerebri, run here, as do nerve fibers connecting the frontal lobe to the thalamus, also known as the anterior thalamic peduncle. The genu capsulae internae contains only the corticonuclear pathway. Significantly more neural pathways are found in the crus posterior (“posterior limb”). These fibers include part of the pyramidal tract (tractus corticospinalis), fibrae temporopontinae, tractus corticotectalis, tractus corticorubralis, tractus corticoreticularis, fibers of the central and posterior parts of the thalamus (radiatio centralis thalami and radiatio posterior thalami), tracts of the auditory pathway (radiatio acustica), and nerve fibers of the optic pathway (radiatio optica).
Function and Tasks
No area of the brain has as many nerve tracts running through it as the internal capsule. The fibers belong to different tracts and, accordingly, perform different functions. The tractus corticospinalis carries motor information that originates in the praecentral gyrus in the frontal lobe and first traverses the capsula interna before proceeding to the cerebral peduncle and onward across the medulla oblongata (medulla oblongata), dividing at the pyramidal junction (decussatio pyramidum) into the pyramidal anterior cord pathway and the pyramidal lateral cord pathway; The latter switches sides of the body so that fibers from the right hemisphere supply the left side of the body and vice versa. In the human body, the pyramidal tracts are responsible for controlling voluntary movements. The fibrae temporopontinae have the function of connecting the temporal coil of the brain to the lateral posterior nuclei of the bridge (pons). In contrast, the tractus corticotectalis is involved in the control of the eyes, mediating both voluntary movements and reflexes.
Diseases
Lesions of the internal capsule usually result in various neurologic disorders because the density of nerve fibers is particularly high here. The impairments may affect several functional areas simultaneously. One possible consequence is hemiparesis.It is related to the internal capsule mainly due to lesions on the pyramidal tract and other motor fibers that pass through this area. In this case, the contralateral side of the body is affected. Complete paralysis of one side of the body, on the other hand, is what medicine calls hemiplegia or hemiparalysis. The extent of the paralysis depends on how many fibers of the motor pathways are destroyed. Damage to the auditory and visual pathways, whose nerve fibers also pass through the internal capsule, can cause impairments in the corresponding sensory modalities. Complex neurological disorders characterized by a variety of different symptoms are also possible. Stroke, for example, can be considered as a cause of damage to the internal capsule. An interruption of the blood supply leads to a lack of oxygen, energy and nutrients in the nerve cells located in the affected area. If the undersupply continues for too long, the cells die. In the case of a medial infarction, this process is due to occlusion of the middle cerebral artery. Another possible cause of lesions on the internal capsule is multiple sclerosis, which manifests itself in the destruction of the white matter. Inflammatory lesions in the brain lead to atrophy of the myelin sheaths, which electrically insulate the individual nerve fibers. This impairs the transmission of signals. In most cases, multiple sclerosis progresses in relapses; no causal treatment is currently available.
