White matter of hemispheres - association and commissural fibers, internal capsule (draw scheme of tracts in internal capsule)

White matter in the central nervous system contains tracts of myelinated axons with supporting cells including astrocytes and microglia, as well as blood vessels and very few neuronal cell bodies. The presence of lipid-rich myelin sheaths encompassing the myelinated axons give these areas their unique white appearance.

It consists of 3 kind of fibers:

Association tracts

Association fibers

Association fibers are nerve fibers that connect different regions within the same hemisphere of the brain, specifically the cerebral cortex. They facilitate communication and integration of information between various brain areas, enabling higher-level cognitive functions.

Types of Association Fibers:

• Short Association Fibers (Arcuate Fibers): These connect adjacent gyri (ridges) on the cortical surface within the same lobe.
• Long Association Fibers: These connect different lobes of the same hemisphere, often organized into specific bundles or fasciculi.

Examples of Long Association Fibers:

• Uncinate Fasciculus: Connects the frontal and temporal lobes. Uncinate fasciculus is considered to belong to the limbic system and could be involved in emotion processing and memory
• Superior Longitudinal Fasciculus: Connects the frontal and occipital, parietal and temporal lobes. Its function includes integrating sensory information, motor planning, visuospatial attention, and complex motor behavior.
• Inferior Longitudinal Fasciculus: Connects the temporal and occipital lobes. Functions include visual processing, language, memory.
• Cingulum: Connects the cingulate gyrus to the entorhinal cortex and hippocampus.
• Articulate fasciculus: connect Wernicke's area and Broca's area

Projection fibers: Fibers that connect a cortical region to lower parts of the brain or the spinal cord.

Commissural fibers

Commissural fibers are a type of neural pathway that connects the two hemispheres of the brain. These fibers, also known as commissural tracts or commissures, are white matter tracts that cross the midline, facilitating communication and coordination between the left and right sides of the brain. Key examples include the corpus callosum, anterior commissure, and posterior commissure.

Corpus callosum

The corpus callosum is the main and largest of the commissural pathways that connects the right and left cerebral hemispheres. During embryonic development, this commissural pathway may fail to develop, leading to a congenital condition called split brain effect as the two cerebral hemispheres are not connected.

Anterior Commissure

The anterior commissure is located in the anterior wall of the third ventricle at the upper end of the lamina terminalis. It runs across the midline in front of the anterior columns of the fornix, above the basal forebrain and beneath the medial and ventral aspect of the anterior limb of the internal capsule.

The anterior commissure is involved in interhemispheric communication, particularly for sensory and cognitive information. It helps transfer information between the two hemispheres, including auditory, olfactory, and visual information.

Posterior Commissure

The posterior commissural pathway is major part of the epithalamus and forms one of the stalks that attach the pineal body (pineal gland) to the posterior wall of the third ventricle.

The posterior commissure is the inferior lamina or stalk of the pineal gland and is important in language processing and connects the language processing centers of both cerebral hemispheres. It is closely related in function to the splenium of the corpus callosum, and injury to it may lead to disorders such as alexia.

Habenular commissure

The superior lamina or stalk of the pineal body is called the habenular commissure and connects the habenular nuclei of the two cerebral hemispheres. Thus it is also a vital part of the epithalamus.

Commissure of fornix

The hippocampal commissure, or commissure of fornix, is a bundle of fibers interconnecting the hippocampi of the two cerebral hemispheres. More specifically, this commissure connects a part of the body of the fornix called the crus on both sides of the midline.

Hippocampal Commissure

Connects the hippocampus of the two hemispheres.

While the exact function is still being investigated, it is believed to play a role in memory and cognitive processes, particularly those involving the hippocampus. Damage to the fornixes, which the hippocampal commissure is part of, can lead to memory loss.

Internal capsule

Tracts though the internal capsule

The internal capsule (IC) is a subcortical white matter structure situated in the inferomedial portion of each cerebral hemisphere. It is composed of myelinated ascending and descending fiber tracts that course past the basal ganglia to connect the cerebral hemispheres with subcortical structures, the brainstem, and the spinal cord.

• Anterior Limb: Contains fibers from the frontal cortex to the pons and the thalamocortical fibers, which project from the thalamus to the frontal lobe.
• Genu (Knee): The genu contains corticobulbar fibers, which project from the cortex to the brainstem.
• Posterior Limb: This is the largest section and contains:
  • Corticospinal tract: The primary motor pathway, carrying signals from the motor cortex to the spinal cord.
  • Corticopontine tract: Projects from the cortex to the pons.
  • Sensory fibers: Including those from the thalamic radiation, carrying sensory information from the thalamus to the cortex.

Citations:

https://pubmed.ncbi.nlm.nih.gov/31194338/#:~:text=Excerpt,significant%20motor%20and%20sensory%20deficits.

https://www.kenhub.com/en/library/anatomy/commissural-pathways

https://academic.oup.com/brain/article/130/3/630/278415