A summary of the parts and functions of this bundle of nerve fibers in the brain.
The internal capsule is a structure of the brain composed of fibers with myelin, through which 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 are, what functions the most important nerve pathways perform, and the consequences of injuries in this region of the brain.
What is the internal capsule of the brain?
The internal capsule is a region of the brain made up of a small band of myelinated fibers, also called the white matter, that separates the lenticular nucleus from the caudate nucleus and the thalamus. This substance is made up of projections that go from the cerebral cortex to different subcortical structures.
This structure is composed of an anterior arm 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 compacts giving rise to the internal capsule.
This area of the brain contains vital pathways such as the corticobulbar and corticospinal tracts. In fact, specific injuries to the internal capsule (which can occur, for example, from strokes such as heart attacks or strokes) can generate devastating clinical deficiencies for the affected person.
The internal capsule does not make up one of the basal ganglia, but rather a bundle of fibers that passes through them. The shape of the internal capsule varies depending on how we analyze 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 lenticulo-caudate portion of the internal capsule separates the lenticular nucleus from the caudate nucleus.
It contains fibers that go from the thalamus to the cortex (thalamocorticales) and, vice versa, from the cortex to the thalamus (corticothalamic), which join the lateral thalamic nucleus with the cortex of the frontal lobe; to the frontopontine tracts of the frontal lobe with the pontine nucleus; and to fibers that run transversely from the caudate nucleus to the putamen.
The posterior arm or lenticulo-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 travel in the anterior half of the posterior arm, with the fibers to the face and arm, in front of the fibers of the leg. Corticorubral fibers, which come from the cortex of the frontal lobe to the red nucleus, 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 is located, an area of sensory reception, such as touch).
As in the case of corticospinal and corticobulbar fibers, there is a somatotopic organization of the sensory fibers of the posterior arm, with the face and arm ascending in front of the leg fibers.
Composition of your nerve fibers
The internal capsule is composed of white matter, that is, nerve fibers covered with myelin, a substance that envelops and protects the axons, allowing there to be a greater speed in the transmission of the nerve impulse.
Depending on the length of the path that each of the fiber bundles travels, we can divide these into short and long fibers.
These types of fibers join the thalamus with various areas of the cerebral cortex (thalamocortical fibers). They emerge along the entire ovoid mass of gray matter that makes up the thalamus and, viewed at the macroscopic level, they appear to radiate in all directions (hence the name thalamic radiation).
The anterior thalamic peduncle is composed of fibers that emerge from the anterior part of the thalamus and go to the frontal lobe, occupying the anterior arm of the internal capsule. The superior peduncle, for its part, emerges from the superior and posterior part of the thalamus to go to the parietal lobe, passing through the posterior arm of the internal capsule.
On the other hand, the fibers that join the posterior area 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 fibers 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 inferior peduncle.
This other type of fiber is responsible for joining 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 fibers constitute: the frontopontine fascicle; the geniculate fascicle, responsible for the voluntary movements of the head; the pyramidal pathway, responsible for voluntary movements of the trunk and limbs; the sensitive pathway; the parietal-pontine and occipito-pontine fascicles; and the retrolental portion or corridor of Wernicke, an area of great relevance both at motor and sensory level, and in which optical and acoustic radiation intersect.
The internal capsule is a brain region in which two very important nerve fiber tracts converge: the corticospinal tract and the corticobulbar tract. Next, we will see what functions each of them fulfill.
The corticospinal tract
The corticospinal tract or pathway is a set of nerve fibers that are part of the internal capsule and are responsible for controlling the voluntary movement of the body. It is essential to manage the execution of fine movements (with the fingers of the hand, for example). Their role is to ensure that movements of this type have adequate skill and precision.
On the other hand, it also intervenes in the regulation of sensory relays and in selecting the sensory modality that finally reaches the cerebral cortex. The corticospinal tract stimulates the neurons responsible for flexion and inhibits those responsible for extension.
In the corticospinal pathway, there is a somatotopic 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 extremity located in the lateral cortex, in the convexity of the cerebral hemisphere. (the motor homunculus).
The motor area of the arm and hand is the one that occupies the most space (occupying the precentral gyrus, located between the area of the lower limb and the face).
The corticobulbar tract
The corticobulbar tract or tract is a bundle of nerve fibers that is responsible for directing the muscles of the head and neck. This nervous tract is responsible for the fact that we can control, for example, facial expression, or that we can chew or swallow.
The tract arises in the lateral part of the primary motor cortex and its fibers eventually converge in the internal capsule of the brainstem. From there, they go to the motor nuclei of the cranial nerves and these connect with 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 infarcts, cerebrovascular accidents of less than 15 mm in diameter and caused by the 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 heart attacks or strokes can cause another series of symptoms and syndromes, depending on the affected area. Pure sensory syndrome is another of the conditions that occur when localized infarcts occur in the internal capsule and / or the posterior nucleus of the thalamus. The affected person, in this case, suffers from facio-brachy-crural hemi-hypoesthesia (reduced sensitivity in practically half of the body).
Another consequence associated with damage to the cerebral blood vessels, which produces 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 go from the cortex to the spinal cord and the muscles that perform 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 dysarthria-clumsy hand 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-ponto-cerebellar pathway and the posterior arm of the internal capsule are affected, and which causes symptoms such as paralysis and incoordination of the leg or arm.
- Pendlebury, ST, Blamire, AM, Lee, MA, Styles, P., & Matthews, PM (1999). Axonal injury in the internal capsule correlates with motor impairment after stroke. Stroke, 30 (5), 956-962.
- Snell, RS (2007). Clinical neuroanatomy. Panamerican Medical Ed.