WHITE MATTER

 

White matter is formed by association fibers, projection fibers and commissural fibers. The association fibers unite cortical areas within one cerebral hemisphere; the projection fibers unite the cerebral cortex with the brain stem and the spinal medulla; and the commissural fibers communicate both the cerebral hemispheres.

 

The association fibers can be short or long. The short ones are arched because they pass ventrally to the sulci. The long ones form the superior longitudinal fasciculus[1], the inferior longitudinal fasciculus[2], the uncinate fasciculus[3], the subcallosal fasciculus (fronto-occipital fasciculus), and the cingulum[4] (see images).

 

The most extensive band of projection fibers is the internal capsule.  It is limited medially by the caudate nucleus and the thalamus, and laterally by the lenticular nucleus. Anatomically, it is divided into a rostral crus, a genu and a caudal crus. The rostral crus is situated between the caudate nucleus and the lenticular nucleus. It is formed by corticopontine fibers[5], thalamocortical fibers, corticothalamic fibers and caudadoputaminal fibers. The caudal crus is located between the thalamus and the lenticular nucleus. It is formed by corticopontine[6] fibers, corticospinal fibers, corticorubral fibers, corticothalamic fibers and thalamocortical fibers. The rostral and caudal crus of the internal capsule are continuous with each other at the level of the genu[7] of the internal capsule. It is formed of corticonuclear fibers and corticoreticular fibers. The acoustic radiation[8] and the optic radiation[9] are fibers that, from the medial geniculate body and the lateral geniculate body respectively, join the caudal crus of the internal capsule and terminate in the cortex (the auditory fibers terminate in the temporal cortex while the visual ones terminate in the occipital cortex).

 

In relation to the lenticular nucleus, the internal capsule can be divided into a subventicular portion, which occupies the most ventral position, and a retrolenticular portion, which is located just caudally to the lentiform nucleus.

 

The fibers of the internal capsule are situated dorsal to the caudate nucleus and they extend to the cerebral cortex in a fan-like fashion, forming the corona radiata.

 

During development, the rostral wall of the diencephalic vesicle, the lamina terminalis[10], connects the two telencephalic vesicles.  Since the growth of the telencephalic vesicles occurs in the shape of a “C”, part of the lamina terminalis (the lamina terminalis alba) is dragged upwards and caudally, giving rise to the anterior commissure, the corpus callosum and the commissure of the fornix. The rest of the lamina terminalis forms the lamina terminalis grisea and is considered part of the hypothalamus.

 

Three large commissures connect both cerebral hemispheres: the rostral commissure (connects the paleocortex), the fornix (connects the archicortex), and the corpus callosum (connects the neocortex).

 

The rostral commissure, is situated in the rostral wall of the third ventricle, connects the rhinencephalon from both sides. The rostral part, is made by fibers from the intermediate olfactory tract, connects both olfactory bulbs. The caudal part, connects both piriform lobes.

 

The corpus callosum is the largest of the commissures of the brain. It serves an important function in the transfer of information between neocortical areas of the two cerebral hemispheres.  From rostral to caudal, it is divided into: rostrum, genu, body and splenium[11]. The rostrum connects the premotor and the supplementary motor cortices; the genu connects the prefrontal cortices; the body connects the primary motor cortices, the somesthetic and the parietal cortices; the splenium connects the temporal and occipital cortices. The area where the white mater of the internal capsule mixes with the corpus callossum is the centrum semiovale.

 

The fornix[12] is formed by association and commissural fibers. The association fibers constitute two arched bands, one on each side, which connect the hippocampus with the mamillary bodies of the hypothalamus and the telencephalic septum. The fibers that exit the hippocampus do so at the level of the alveous hippocampi to form the fimbria of fornix. The free border of the fimbria is the taenia[13] of fornix where the choroid plexus of the lateral ventricle is attached to. The acumulation of fibers of the fimbria form the crus of fornix that is directed dorsally and rostrally to fuse with the contralateral crus to form the unpaired body of the fornix. The continuation of the fibers forms the columns of the fornix (one on each side) that follow the wall of the third ventricle. The commissural fibers of the fornix are transverse fibers that cross the median plane before the formation of the body of the fornix. Phylogenetically, these commissural fibers are older than the ones of corpus callosum. For this reason, the most primitive mammals like the opossum, have commissure of the fornix but not corpus callosum (Page 163 of “Anatomia Humana Sistema nervioso central y órganos de los sentidos. Sistema neurovegetativo”. by Orts Llorca Ed. Científico-médica. 4ª ed. 1972).

 

[1] Connects the frontal cortex with the parietal cortex and the occipital cortex.

[2] Connects the occipital cortex with the temporal cortex.

[3] Connects the frontal cortex with the temporal cortex.

[4] This is a band of fibers that are located in the cingulated gyrus and consists of the main afferents to the parahippocampal gyrus.

[5] Frontopontine fibers.

[6]  Parietopontine, temporopontine and occipitopontine fibers.

[7] Due to the angle that forms the lenticular nucleus, the internal capsule folds in on itself and forms a rostral crus and caudal crus.

[8]  The acoustic radiation connects the medial geniculate body of the thalamus with the temporal cortex.

[9] The optic radiation connects the lateral geniculate body of the thalamus with the occipital cortex.

[10] From the Latin, “that forms or belongs to the extreme”.

[11] From the Greek, splenion, “similar to a bandage.”

[12] From the Latin, fornix, “vault, arch.”

[13] From the Latin, taenia, and the Greek, tainia, “flat band, ribbon.”

01 CEREBRAL CORTEX[1] Connects the frontal cortex with the parietal cortex and the occipital cortex.

[2] Connects the occipital cortex with the temporal cortex.

[3] Connects the frontal cortex with the temporal cortex.

[4] This is a band of fibers that are located in the cingulated gyrus and consists of the main afferents to the parahippocampal gyrus.

[5] Frontopontine fibers.

[6]  Parietopontine, temporopontine and occipitopontine fibers.

[7] Due to the angle that forms the lenticular nucleus, the internal capsule folds in on itself and forms a rostral crus and caudal crus.

[8]  The acoustic radiation connects the medial geniculate body of the thalamus with the temporal cortex.

[9] The optic radiation connects the lateral geniculate body of the thalamus with the occipital cortex.

[10] From the Latin, “that forms or belongs to the extreme”.

[11] From the Greek, splenion, “similar to a bandage.”

[12] From the Latin, fornix, “vault, arch.”

[13] From the Latin, taenia, and the Greek, tainia, “flat band, ribbon.”

[14] Forms the roof of the third ventricle and supports the choroid plexus of the third ventricle.