Cerebral White Matter Overview

Cerebral white matter Overview

Association fibers

Cord fibers

Either directly connect areas on opposite sides of the neuroaxis or provide an important step in that cross-axis connection

Striatal fibers

Association fibers

Short association fibers (aka U-fi ber or arcuate bundle) travel between gyri just underneath the innermost cerebral cortical gray matter layer (layer 6).

Certain white matter diseases, such as subtypes of multiple sclerosis, spare the short association fibers.

Mid-range association fibers (aka neighborhood association fiber) extend into the deep white matter to connect areas a mid-distance away from one another.
Long-distance association fibers (long association fibers) extend deep into the brain and connect distant ipsihemispheric regions (they're trajectory is best seen in sagittal view).

They include:

The arcuate fasciculus (which is classically (although probably erroneously assigned as the pathway that passes language between the motor and sensory centers)
The cingulum, which passes through the limbic lobe and plays a key role in emotional processing.

Striatal fiber

Projections from throughout the cortex synapse in the basal ganglia and are evidentiary of the widespread role of the basal ganglia in behavioral as well as motor functions.
The two types of striatal fiber bundles are the: Muratoff bundle (aka subcallosal fasciculus) and the external capsule.

cord fibers

Connect opposite sides of the nervous system: they subdivide into commissural and subcortical fiber bundles and they form the dense aggregate of deep white matter underneath the cortical gray matter.

Commissural fibers

The key commissural fibers are:

Subcortical projection fibers

An ascending one from the thalamus to the cortex: the thalamic bundle.
A descending one from the cortex to the opposite side of the brainstem: the pontine bundle.

The cord fibers form the deep white matter region between the cortex and the diencephalon.
The centrum semiovale refers to the white matter above the level of the lateral ventricles.
The corona radiata lies at the level of the lateral ventricles (it's so-named for "radiating crown," which is appreciable only in detailed anatomic dissection).

The internal capsule

Brain Atlas: Internal Capsule

An important location of restricted cerebrovascular infarct.

Key landmarks: the frontal horn and body of the lateral ventricles, thalamus, and the basal ganglia (putamen, globus pallidus, and caudate (head)).
The internal capsule forms a V-shaped wedge in between key basal ganglia structures: it comprises fiber bundles that originate from widespread brain regions and it carries fibers of many disparate functional modalities, including motor, sensory, cognitive, and emotional fiber pathways.

internal capsule

The anterior limb lies between the head of the caudate and the lentiform nucleus.
The genu is the middle (bend) of the internal capsule, which occurs along the medial–lateral plane of the anterior thalamus.

Infarction of the genu and posterior limb of the internal capsule (Stroke: Internal Capsule) produces a classic and common pure motor stroke (see somatotopy, below).

The posterior limb lies between the thalamus and the lentiform nucleus (the combined putamen/globus pallidus).
The retrolenticular limb lies posterior to the posterior limb — its fibers run posterior to the lentiform nucleus.
The sublenticular limb fibers run underneath the lentiform nucleus (and lie inferior to the plane of this diagram).

Somatotopy of the Internal Capsule

As a simplification: we commonly associate the facial fibers purely with the genu.
The arm fibers descend through the anterior portion of the posterior limb.
The leg fibers originate posterior to the arm fibers and pass anteriorly during their descent through the posterior limb.

Clinical Correlation

Interestingly, the motor fibers crowd together inferiorly within the internal capsule: thus, the more inferior the infarct, the broader the motor deficit relative to the size of the infarct.

CNS Demyelinating Disorders

Select Key Central Nervous System Demyelinating Disorders