White Matter of Cerebrum : Projection, Commissural and Association fibers


The white matter of the cerebral hemisphere basically contains two components
  • myelinated nerve fibers of many sizes*
  •  neuroglia (mostly oligodendrocytes)
*This note only describes different types of myelinated nerve fibers.

The white matter of the cerebral hemisphere are of three types:
  1. commissural fibers
  2. association fibers
  3. projection fibers

1. Commissural Fibers

The commissural fibers interconnect the corresponding regions of the two cerebral hemispheres. They are as follows:
-          Corpus callosum
-          Anterior commissure
-          Posterior commissure
-          Fornix
-          Habenular commissure

The corpus callosum comprises the largest bundle of fibers; most of these arise from parts of the neocortex of one cerebral hemisphere and terminate in the corresponding parts of the opposite cerebral hemisphere. It lies at the bottom of the longitudinal fissure. It is divided into from before backwards into
  • The rostrum, a thin lamina of nerve fibers that connects the genu to the upper end of the lamina terminalis
  • The genu, the curved anterior end
  • The body or trunk which arches posteriorly and ends as the thickened enlargement called
  • The splenium.
Traced laterally, the fibers of the genu curve forward into the frontal lobes and form the forceps minor. The fibers of the body extend laterally as the radiation of the corpus callosum. The fibers from the splenium runs backward into the occipital lobe and form the forceps major.

Anterior commissure
The anterior commissure is situated in the anterior wall of the third ventricle at the upper end of the lamina terminalis. The fibers passing through the commissure interconnect the olfactory bulbs of the two cerebral hemisphere. Other fibers interconnect the parahippocampal gyri and other parts of the temporal lobe.

Posterior commissure
It lies in the inferior lamina of the stalk of the pineal gland immediately above the opening of the cerebral aqueduct into the third ventricle.

It is a prominent bundle of fibers seen on the medial aspect of the cerebral hemisphere. It is made up of fibers arising from the hippocampus. The body of the fornix is suspended from the corpus callosum by septum pellucidum. The nerve fibers arise from the alveus, a thin layer of white matter covering the surface of the hippocampus and then converge to form the fimbria. The fimbriae form the posterior column or crus of the fornix arching below the corpus callosum. The two crura then come together in the midline to form the body of the fornix. The two crura are interconnected by fibers passing from one to the other – hippocampal commissure or commissure of the fornix which actually joins the two hippocampus. The anterior end of the body of fornix also divides into anterior columns or pillars. Each anterior column of the fornix turns downward just in front of the interventricular foramen and passess through the hypothalamus to reach the mamillary body.

Habenular commissure
It is a small bundle of nerve fibers that cross the midline in the superior stalk of the pineal gland. The commissure is associated with the habenular nucleus.

2. Association Fibers

These fibers connect the various cortical regions of a cerebral hemisphere. These fibers permit the cortex to function as a coordinated whole. 

Types of Association fibers

Short association fibers, or U fibers, connect adjacent gyri

  • intracortical fibers - located in the deeper portions of the white matter and
  • subcortical fibers - located just beneath the cortex

Long association fibers connect more widely separated areas. 

  • The uncinate fasciculus connects the inferior frontal lobe gyri with the anterior temporal lobe. 
  • The cingulum, a white band within the cingulate gyrus, connects the frontal and parietal lobe with the parahippocampal gyrus.
  • The arcuate fasciculus sweeps around the insula and connects the superior and middle frontal convolutions (which contain the speech motor area) with the temporal lobe (which contains the speech comprehension area).   
  • The superior longitudinal fasciculus connects portions of the frontal lobe with occipital and temporal areas. 
  • The inferior longitudinal fasciculus, connects the temporal and occipital lobes. 
  • The occipitofrontal fasciculus extends backward from the frontal lobe, radiating into the temporal and occipital lobes. 

3. Projection Fibers 

These fibers connect the cerebral cortex with lower part of the brain or brainstem and the spinal cord, in both directions. 

The corticopetal (afferent) fibers include 
  • the geniculocalcarine radiation from the lateral geniculate body to the calcarine cortex, 
  • the auditory radiation from the medial geniculate body to the auditory cortex, and 
  • the thalamic radiations from the thalamic nuclei to specific cerebrocortical areas

The corticofugal (efferent) fibers proceed from the cerebral cortex to the thalamus, brain stem, or spinal cord.

Internal Capsule
Most of the nerve fibers interconnecting the cerebral cortex with centers in the brainstem and spinal cord, and with the thalamus pass through the interval between the thalamus and caudate nucleus medially and lentiform nucleus laterally. This region at the upper end of the brainstem form a compact band and is called the internal capsule. Above, the internal capsule is continuous with corona radiate and  below, with the crus cerebri of midbrain.

As seen on the horizontal section, the internal capsule consists of
  • The anterior limb lies between caudate nucleus medially and the anterior part of the lentiform nucleus laterally
  • The posterior limb lies between the thalamus medially and the posterior part of the lentiform nucleus laterally
  • And the genu where both limb meets.
In addition, some fibers of the internal capsule lie behind the posterior end of the lentiform nucleus constituting its retrolentiform part whereas some other fibers pass below the lentiform nucleus (and not medial to it), constitute the sublentiform part.

Arrangement of fibers within the internal capsule:
i. The fibers to and from the anterior part of the frontal lobe pass through the anterior limb of the internal capsule.

ii. The fibers to and from the posterior part of frontal lobe and greater part of parietal lobe occupy genu and posterior limb of the internal capsule.

iii. Fibers to and from the temporal lobe  (and lowest part of parietal lobe) occupy the sublentiform part.

Whereas those to and from the occipital lobe occupy the retrolentiform part of the internal capsule.

Ascending fibers in the internal capsule:
These are predominantly thalamocortical fibers - from thalamus to all parts of the cerebral cortex and forming thalamic radiations or peduncles, as given below:

Anterior thalamic radiation (or frontal thalamic peduncle) 
  • Pass through the anterior limb
  • Fibers to the frontal lobe
  • From anterior and medial thalamic nuclei
  • Also carries fibers from hypothalamus and limbic structures
Superior thalamic radiation (superior or dorsal thalamic peduncle)
  • Pass through the genu and posterior limb of the capsule
  • To the somatosensory area
  • From ventral posterior nuclei of the thalamus
Posterior thalamic radiation (posterior or caudal thalamic peduncle)
  • From thalamus to the occipital lobe
  • Includes the optic radiation from lateral geniculate body to the visual cortex
  • Lie in the retrolentiform part of the internal capsule
Inferior thalamic radiation (ventral thalamic radiation)
  • From thalamus to temporal lobe
  • Includes acoustic radiation from medial geniculate body to the acoustic area
  • Pass through the sublentiform part of the internal capsule

Descending fibers in the internal capsule
These are as follow:

Corticopontine fibers

  • Frontopontine fibers are the most numerous - pass mainly through the anterior limb
  • Parietopontine fibers mainly pass through the retrolentiform part
  • Temporopontine fibers from the sublentiform part
  • Occipitoponitne fibers from the retrolentifrom part

Corticospinal and corticonuclear fibers
Corticonuclear fibers
(for cranial nerve nuclei to head and neck) pass through the genu of the capsule.

Corticospinal fibers
form several discrete bundles in the posterior limb and are arranged from before backward – upper limb, trunk and lower limb.

Corticothalamic fibers
are from cortex to thalamus and form part of the thalamic radiation.


      Gray’s Anatomy, 39th Edition
      Snell’s Clinical Neuroanatomy 7th Edition 

      Lange Clinical Neuroanatomy 25th Edition