White Matter and Internal Capsule of the Cerebral Hemisphere

Organization and Structure of the Cerebral White Matter

The cerebral hemisphere is organized into several distinct layers and structures, primarily the cerebral cortex (gray matter), the underlying white matter, the basal ganglia, and the lateral ventricle. The white matter itself is composed of tracts of myelinated axons supported by neuroglia. These nerve fibers are categorized into three primary functional groups based on the regions they connect:

Association Fibers: These fibers connect various cortical regions within the same cerebral hemisphere.
Commissural Fibers: These fibers facilitate communication between corresponding regions of the two cerebral hemispheres.
Projection Fibers: These fibers establish connections between the cerebral cortex and subcortical regions, such as the basal ganglia, thalamus, brainstem, and spinal cord.

Detailed Classification of Association Fibers

Association fibers are subdivided based on the distance they travel between gyri within the same hemisphere:

Short Association Fibers: Often referred to as "U-fibers," these connect adjacent gyri. They are entirely intracortical in their distribution.
Long Association Fibers: These bundles connect more separated gyri within the same hemisphere. Major bundles include: * Uncinate Fasciculus: This tract connects the motor speech area and the orbital gyri of the frontal lobe with the temporal lobe. * Cingulum: Located deep to the cingulate gyrus, this tract connects the frontal and parietal lobes with the parahippocampal and temporal regions. * Superior Longitudinal Fasciculus: This is the largest association bundle. It connects the anterior part of the frontal lobe to the occipital and parietal lobes. * Inferior Longitudinal Fasciculus: This tract connects the occipital lobe directly to the temporal lobe. * Fronto-occipital Fasciculus: This bundle connects the frontal lobe to both the temporal and occipital lobes. * Arcuate Fasciculus: A specific tract noted for its role in connecting language areas.

Anatomy and Function of Commissural Fibers

Commissural fibers connect the corresponding regions of the left and right cerebral hemispheres to allow for interhemispheric communication. The primary commissural structures include:

Corpus Callosum: This is the largest and main commissure of the brain, situated at the bottom of the longitudinal cerebral fissure. It consists of four distinct parts: * Rostrum (Head): Connects the orbital surfaces of the frontal lobes. * Genu: The anterior curve of the corpus callosum. It forms the forceps minor to connect the frontal lobes of both hemispheres. * Body: Fibers from the body pass laterally and intersect the corona radiata to reach the parietal lobes. * Splenium: The thickened posterior end of the corpus callosum. It forms the forceps major, connecting the two occipital lobes.
Anterior Commissure: This bundle crosses the midline in front of the anterior column of the fornix. It primarily connects the temporal lobes and is associated with functions such as smell (Aroma), emotion (Amygdala), and memory.
Posterior Commissure: This crosses the midline above the opening of the cerebral aqueduct. It is functionally linked to the pupillary light reflex.
Fornix: These C-shaped fibers serve as the primary connecting pathway of the limbic system. It acts as the output tract of the hippocampus, passing signals to the mamillary bodies of the hypothalamus.
Habenular Commissure: This small bundle crosses the midline superior to the pineal stalk, connecting the left and right habenular nuclei.

Projection Fibers: Corona Radiata and Internal Capsule

Projection fibers are responsible for transmitting information between the cerebral cortex and the rest of the Central Nervous System (CNS). These include:

Corticofugal Fibers: Axons carrying impulses away from the cortex.
Corticopetal Fibers: Axons carrying impulses towards the cortex.

Corona Radiata: This is a dense mass of subcortical white matter that carries motor and sensory fibers between the cortex and deeper brain structures. It is continuous with the internal capsule.

Internal Capsule: This is a tightly packed collection of ascending and descending projection fibers (the continuation of the corona radiata) situated between the thalamus and the basal ganglia. Physically, it is a band of white matter with a lateral concavity.

Anatomical Relations and Divisions of the Internal Capsule

Anatomical Relations

Medial: Bordered by the thalamus and the head of the caudate nucleus.
Lateral: Bordered by the lentiform nucleus.
Superior: Continues as the corona radiata.
Inferior: Becomes the crus cerebri of the midbrain.

Parts of the Internal Capsule

Anterior Limb: Situated between the head of the caudate nucleus (medially) and the lentiform nucleus (laterally).
Genu: The "bend" of the capsule, located at the apex of the globus pallidus.
Posterior Limb: Situated between the thalamus (medially) and the lentiform nucleus (laterally).
Retrolentiform Part: Located behind the posterior end of the lentiform nucleus.
Sublentiform Part: Located below the lentiform nucleus.

Functional Contents of the Internal Capsule

Anterior Limb

Thalamocortical Fibers: Part of the anterior thalamic radiation traveling to the frontal lobe.
Frontopontine Fibers: Traveling from the frontal lobe to the pontine nuclei.

Genu

Corticonuclear Fibers: These fibers control the head and neck, traveling from the motor cortex to the motor cranial nuclei in the brainstem.

Posterior Limb

Thalamocortical Fibers: Part of the superior thalamic radiation traveling to the parietal lobe.
Corticospinal Fibers: Mediating motor control for the upper limb, lower limb, and trunk.
Corticorubral Fibers: Connecting the cerebral cortex to the red nucleus.
Frontopontine Fibers: Traveling from the frontal lobe to the pontine nuclei.

Retrolentiform Part

Optic Radiation (Posterior Thalamic Radiation): Connects the lateral geniculate body to the primary visual area in the occipital lobe.
Parietopontine Fibers: From the parietal lobe to the pons.
Occipitopontine Fibers: From the occipital lobe to the pons.

Sublentiform Part

Auditory Radiation (Inferior Thalamic Radiation): Connects the medial geniculate body to the primary auditory area in the temporal lobe.
Temporopontine Fibers: From the temporal lobe to the pons.

Vascular Supply and Clinical Significance

Arterial Supply

The internal capsule receives its blood supply from central branches of the circle of Willis, specifically the anterior and middle cerebral arteries:

Medial Striate Artery (branch of the Anterior Cerebral Artery): Supplies the anterior limb and the genu.
Lateral Striate Arteries (Lenticulostriate arteries of the Middle Cerebral Artery): These supply the anterior limb, the genu, and the majority of the posterior limb.
Anterior Choroidal Artery (branch of the Internal Carotid Artery): Supplies part of the posterior limb and the retrolentiform part.

Clinical Applications: Ischemic Stroke and Lesions

An internal capsule infarct is a common site for ischemic stroke, which results from reduced blood supply due to thrombosis or vessel occlusion. The clinical presentation depends on the specific part of the capsule affected:

General Stroke: Occlusion of the lateral striate (lenticulostriate) artery affecting the posterior limb lead to contralateral hemiparesis.
Anterior Limb Lesions: May manifest as confusion, impaired attention, agitation, and dysarthria.
Genu Lesions: Affecting corticonuclear tract fibers, these can cause face and tongue weakness as well as dysarthria.
Posterior Limb Lesions: * Damage to the anterior half of the posterior limb causes pure motor hemiparesis contralateral to the lesion. * Damage to the posterior third causes contralateral hemisensory deficits.
Retrolenticular Segment Lesions: May impair vision due to the involvement of optic radiations.
Sublenticular Segment Lesions: May result in auditory deficits due to the involvement of auditory radiations.