The internal capsule is a structure of the brain composed of fibers with myelin , through which pass the nerve projections that go from the cortex to the medulla and subcortical structures, and vice versa.
In this article we explain in more detail what the internal capsule is, what its composition and anatomical structure is, what functions the most important nerve pathways perform and what the consequences of injuries in this region of the brain are.
What is the internal brain capsule?
The internal capsule is a region of the brain formed by a small band of myelinated fibres, also called the white substance , which separates the lenticular nucleus from the caudate nucleus and the thalamus. This substance is made up of projections that range from the cerebral cortex to different subcortical structures.
This structure is composed of an anterior and a posterior arm, as well as ascending and descending projections . Precisely, when these run between the lenticular nucleus and the area of the thalamus and the caudate nucleus, the matter is compacted giving rise to the internal capsule.
This area of the brain contains vitally important pathways, such as the corticobulbar and corticospinal tracts. In fact, specific lesions in the internal capsule (which can occur, for example, from strokes such as heart attacks or strokes) can lead to devastating clinical deficiencies for the affected person.
The internal capsule does not form one of the basal ganglia, but a bundle of fibers that crosses them . The shape of the internal capsule varies depending on how we analyse the brain. If we make a frontal cut, this brain structure takes the form of an externally open angle with a lower, horizontal side below the lenticular nucleus.
If we make a horizontal cut, it presents a V-shaped appearance, where the knee (its end) points medially, and we can divide it into three fundamental components:
The anterior arm
The anterior arm or lenticule-caudate portion of the internal capsule separates the lenticular nucleus from the caudate nucleus .
It contains fibres from the thalamus to the cortex (thalamocortical) and, conversely, from the cortex to the thalamus (corticothalamic), which link the lateral thalamic core to the frontal lobe cortex; the frontopontine tracts of the frontal lobe to the pontine core; and fibres running transversely from the caudate core to the putamen.
The rear arm
The posterior arm, or lenticuo-thalamic portion of the internal capsule, located between the thalamus and the lenticular nucleus , contains essential ascending and descending pathways.
The corticobulbar and corticospinal tracts pass through the anterior half of the posterior arm, with the fibers to the face and arm, in front of the fibers of the leg. The corticorubal fibers, which come from the frontal lobe cortex to the red core, accompany the corticospinal tract.
The posterior third of the posterior arm is composed of third order sensory fibers that go from the posterolateral nucleus of the thalamus to the postcentral gyrus (where the somatosensory cortex, area of sensory reception, such as touch, is located).
As in the case of corticospinal and corticobulbar fibres, there is a somatotropic organisation of the sensory fibres of the posterior arm, with the face and arm rising in front of the fibres of the leg.
Composition of your nerve fibers
The internal capsule is composed of white substance, i.e. nerve fibers covered with myelin, a substance that envelops and protects the axons, allowing a greater speed in the transmission of the nerve impulse.
Depending on the length of the path that each of the fibre bundles takes , we can divide these into short and long fibres.
This type of fibre links the thalamus with various areas of the cerebral cortex (thalamocortical fibres). They emerge along the entire ovoid mass of grey substance that makes up the thalamus and, seen on a macroscopic level, they seem to radiate in all directions (hence the name thalamic radiation).
The anterior thalamic stalk is composed of fibers that emerge from the anterior part of the thalamus and are directed to the frontal lobe, occupying the anterior arm of the internal capsule. The upper peduncle, on the other hand, emerges from the upper and rear part of the thalamus to go to the parietal lobe, passing through the rear arm of the internal capsule.
On the other hand, the fibres that join the posterior zone of the thalamus with the occipital lobe form the posterior peduncle, which also runs along the posterior arm of the internal capsule . And finally, the fibres that emerge from the anterior part of the thalamus and project to the temporal lobe occupy the sublenticular region of the internal capsule and form the lower peduncle.
This other type of fibre is responsible for linking the cerebral cortex with nerve areas further away from the brain , passing through the internal capsule and occupying the anterior arm, the knee and the posterior arm.
These fibres constitute: the frontopontine fascicle; the geniculate fascicle, responsible for voluntary movements of the head; the pyramidal pathway, responsible for voluntary movements of the trunk and extremities; the sensory pathway; the parietropontinal and occipito-pontine fascicles; and the retrolenticular or Wernicke’s corridor, an area of great relevance both at the motor and sensory levels, and where optical and acoustic radiations cross.
The internal capsule is a brain region in which two very important nerve fiber tracts converge: the corticospinal tract and the corticobulbar tract . Below, we will see what functions each of them performs.
The corticospinal tract
The tract or corticospinal tract is a group of nerve fibers that are part of the internal capsule and are responsible for controlling the voluntary movement of the body . It is essential for managing the execution of fine movements (with the fingers, for example). Its function is to ensure that movements of this type have the appropriate skill and precision.
On the other hand, it also intervenes in the regulation of the sensory relays and when selecting the sensory mode that finally reaches the cerebral cortex . The corticospinal tract stimulates the neurons responsible for flexion and inhibits those responsible for extension.
In the corticospinal tract there is a somatopic representation of the different parts of the body in the primary motor cortex, with each area of the lower limb located in the medial cortex and the area of the cephalic limb located in the lateral cortex, in the convexity of the cerebral hemisphere (the motor homunculus).
The motor area of the arm and hand occupies the most space (occupying the pre-central gyrus, located between the area of the lower limb and the face).
The corticobulbar tract
The tract or corticobulbar pathway is a bundle of nerve fibers that is responsible for directing the muscles of the head and neck . This nerve tract is responsible for the fact that we can control, for example, our facial expression, or that we can chew or swallow.
The tract arises in the lateral part of the primary motor cortex and its fibers end up converging in the internal capsule of the brain stem. From there, they are directed to the motor nuclei of the cranial nerves and these connect to the lower motor neurons to innervate the facial and neck muscles.
Consequences of damage to this region of the brain
Lesions in a region of the brain such as the internal capsule can selectively compromise motor and sensory function . For example, lacunar infarctions, strokes less than 15 mm in diameter and caused by occlusion of the perforating arteries of the brain, can selectively compromise the anterior part of the posterior arm of the internal capsule, producing pure motor hemiparesis.
Lacunar infarctions or strokes can cause a number of other symptoms and syndromes , depending on the area affected. Pure sensory syndrome is another condition that occurs with infarcts located in the internal capsule and/or the posterior nucleus of the thalamus. The affected person, in this case, suffers from facio-brachial hemi-hypesthesia (reduction of sensitivity in practically half of the body).
Another consequence associated with damage to the cerebral blood vessels, leading to infarction or thrombosis in the posterior arm of the internal capsule, is contralateral hemiplegia (on the opposite side of the body), due to an interruption of the corticospinal fibers that run from the cortex to the spinal cord and the muscles that perform the motor functions.
Finally, there are two more disorders that are associated with specific damage to the internal capsule and related structures. On the one hand, , the clumsy hand-dysarthria syndrome , due to damage to the knee of the internal capsule and which causes symptoms such as facial weakness, manual clumsiness, dysphagia and dysarthria; and on the other hand, paresis with hemiataxia, when the cortico-pontocerebellar duct and the posterior arm of the internal capsule are affected, and which causes symptoms such as paralysis and lack of coordination of the leg or arm.
- Pendlebury, S. T., Blamire, A. M., Lee, M. A., Styles, P., & Matthews, P. M. (1999). Axonal injury in the internal capsule correlates with motor impairment after stroke. Stroke, 30(5), 956-962.
- Snell, R. S. (2007). Clinical neuroanatomy. Panamerican Medical Ed.