Anatomy of the white matter tracts

Our central nervous system consists of three major parts, the cerebrum, the cerebellum, and the brainstem. The largest part is the cerebrum, which consists of two nearly symmetrical halves called the cerebral hemispheres.

If we were to cut through the cerebral hemispheres in the coronal plane, we would see that the outermost area is the cerebral cortex, which consists of gray matter that contains billions of neuronal cell bodies.

Deep to the gray matter is the subcortical white matter, which is made up of myelinated axons that are extensions of the neuronal cell bodies, allowing them to send and receive signals.

The term white matter is used because the myelination of the axon fibers gives this area a white appearance on gross inspection.

Axon fibers can be divided into three main groups - commissural fibers, association fibers, and projection fibers - depending on the target of the axons.

The cerebral commissural fibers connect the left and right cerebral hemispheres and consist of the corpus callosum, the anterior commissure, the posterior commissure, and the hippocampal commissure.

Association fibers connect different regions within the same hemisphere and they include the U-fibers, uncinate fasciculus, cingulum bundle, arcuate fasciculus, superior longitudinal and inferior longitudinal fasciculi, and the occipitofrontal fasciculus.

Finally, projection fibers connect the cerebral cortex with more caudal structures in the central nervous system, like the thalamus, brainstem, and spinal cord.

These ascending and descending projection fibers all travel in a structure called the internal capsule, which has five parts that we will explore later!

Okay, first, let’s cover the cerebral commissural fibers. Starting with the corpus callosum, which means tough body in latin, this is the largest white matter structure in the brain. It sends signals between the two cerebral hemispheres and connects them together at the same time.

If we make a mid sagittal section through the brain, we can see that the corpus callosum has four parts, the rostrum, genu, body, and splenium.

The body is the central and longest part of the corpus callosum and continues rostrally as the genu and caudally as the splenium.

The body of the corpus callosum occupies the bottom of the longitudinal fissure and arches between its rostral and caudal extents.

The genu, meaning knee, is the rostral bend of the corpus callosum that curves in front of the septum pellucidum, which is a thin membrane that separates the anterior horns of the lateral ventricles.

Just ventral and posterior to the genu is the rostrum, which gets its name because it is similar in shape to a bird’s beak. The rostrum connects with the lamina terminalis of the diencephalon.

Finally, most caudally, the corpus callosum ends as the splenium, which is an enlarged part of the commissure that sits above the midbrain.

Now let’s look at an axial, or transverse, section of the brain so that we can follow the fibers of the corpus callosum as they fan out towards the cerebral cortex.

The fibers from the genu spread rostrally, towards the frontal cortex, forming the forceps minor; while the fibers from the splenium spread caudally, towards the occipital cortex, forming the forceps major.

Lastly, fibers from the body of the corpus callosum spread laterally towards the temporal cortex. A part of this lateral radiation forms a roof over the central part of the lateral ventricle, and hence it is called the tapetum, which means covering or carpet.

On a coronal section of the brain, we can see the corpus callosum at the bottom of the longitudinal fissure and superior to the lateral ventricles, forming a bridge between the hemispheres.

Next, there is the anterior commissure, which is a much smaller white matter structure that mainly connects the temporal lobes of each hemisphere, but also has some decussating fibers from the olfactory tracts that will terminate in the primary olfactory cortex.

On a mid sagittal section of the brain, the anterior commissure can be observed just posterior to the lamina terminalis and rostral to the column of the fornix.

On an axial section of the brain, we can also see the anterior commissure just rostral to the fornix. Now, the fibers of the anterior commissure split laterally into two bundles: anterior and posterior.

The anterior bundle is smaller and turns ventrally towards the anterior perforated substance and olfactory tracts, while the posterior bundle turns caudally, towards the temporal lobe.

On a coronal section, we can see the posterior bundle of fibers cutting through the inferior part of the lentiform nuclei while heading towards the temporal lobe.

Another commissure to consider is the posterior commissure, which is located between the midbrain and the diencephalon just rostral to the pineal gland.

It is a small commissure and contains axons related to the oculomotor complex, for example, for the pupillary light reflex.

Note that the corpus callosum, anterior commissure and posterior commissure can all easily be seen in the mid sagittal plane upon gross inspection or MRI.

Lastly, there is the hippocampal commissure, also known as the commissure of the fornix, which connects the left and right hippocampi located in the medial temporal lobes.

The fibers from the hippocampi form the posterior columns of the fornix that travel between the thalamus, inferiorly, and the corpus callosum, superiorly.

Before merging to form the body of the fornix, the posterior columns are connected by the hippocampal commissure, which is a thin membrane located just inferior to the splenium of the corpus callosum.

Let’s take a short break and see if you can identify the main commissural fibers and the regions that they connect.

All right then! Let’s switch gears and talk about the association fibers. These fibers connect different cortical areas within the same cerebral hemisphere, enabling those regions to communicate with each other.

Depending on their length, the association fibers can be divided into two groups, the short and long fibers. The short association fibers connect adjacent gyri of the cerebral cortex.

They are also known as arcuate fibers, or U fibers, as they resemble a U shape while arching beneath the sulci separating the gyri.