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.
First, there is the superior longitudinal fasciculus, which connects the frontal, temporal and occipital lobes. Its fibers pass from the rostral portion of the frontal lobe and through the operculum, which are parts of the frontal, parietal, and temporal lobes that cover the insula.
These fibers then reach the caudal aspect of the lateral sulcus and spread to the temporal and occipital lobes. Then, there is the fronto-occipital fasciculus, which also connects the frontal, temporal, and occipital lobes.
However, its fibers are found in a deeper part of the white matter, passing just lateral to the caudate nucleus. Next is the uncinate fasciculus, which connects the inferior portion of the frontal lobe with the anterior part of the temporal lobe, traversing the bottom of the lateral sulcus.
The arcuate fasciculus connects the area of the brain responsible for speech production, or Broca’s area, found in the inferior frontal gyrus, with the region responsible for speech comprehension, or Wernicke’s area, located at the posterior end of the superior temporal gyrus and neighboring gyri of the parietal lobe.
In order to connect these two important speech centers, the arcuate fasciculus must pass around the insula and parallel to the superior longitudinal fasciculus.
The cingulum bundle, or cingulate fasciculus, connects the frontal and parietal lobes with the parahippocampal gyrus and temporal lobe, passing through the white matter of the cingulate gyrus, superior to the corpus callosum.
And lastly, there is the inferior longitudinal fasciculus, or the temporo-occipital bundle, that connects the temporal and occipital lobes with fibers running below the inferior horn of the lateral ventricle and lateral to the optic radiations.Good, now that we have covered these fibers, let’s make a coronal section through the brain and identify a few more white matter structures.
As you can see here, between the putamen and the claustrum, there’s a thin sheath of white matter called the external capsule.
Its dorsal part contains the claustrocortical fibers, which run between the claustrum and the frontal and parietal lobes, while the ventral part contains fibers of the uncinate fasciculus and fronto-occipital fasciculus that we previously discussed.
Also, more laterally, the claustrum and the cortex of the insula are connected by the fibers of the extreme capsule.Finally, we get to a structure called the internal capsule, which contains efferent and afferent projection fibers.
Think of it as a highway of axons that signals both to and from the cerebral cortex, connecting it with more caudal structures, like the thalamus, brainstem, and spinal cord.Let’s make an axial section through the brain to visualize it better.
As you can see, the internal capsule runs lateral to the thalamus and through the nuclei of the basal ganglia, separating them into the lentiform nucleus, laterally, and the caudate nucleus, medially.
The internal capsule has a V shape and consists of five parts: the anterior limb, genu, posterior limb, retrolentiform, and sublentiform parts.
If we look at the anterior limb of the internal capsule, it is located lateral to the head of the caudate nucleus, medial to the lentiform nucleus, and contains two types of fibers: the corticopontine fibers, or frontopontine fibers, that run from the frontal lobe to the nuclei of the pons; and the thalamocortical fibers that form the anterior thalamic radiation, originating from sensory relay nuclei of the thalamus and traveling to the frontal cortex.
The genu is a flexure just medial to the tip of the lentiform nucleus and contains the corticobulbar fibers, which run from cortex to the motor nuclei of the cranial nerves that control the movements of the head and neck.
The posterior limb of the internal capsule is located lateral to the thalamus and medial to the lentiform nucleus, and contains the corticospinal fibers, which run from cortex to lower motor nuclei in the spinal cord that control the voluntary muscles of the trunk and extremities.
The fibers of the corticospinal tract within the posterior limb are organized in a rostral to caudal fashion, with fibers controlling the upper extremities located rostrally and those controlling the lower extremities located more caudally.
The posterior limb also contains axons of the medial lemniscus, the spinothalamic tract, and also some thalamocortical fibers.
These thalamocortical fibers form the superior thalamic radiation that spreads from thalamic nuclei, processing sensory information, towards the cortex of the parietal lobe.
The retrolenticular part of the internal capsule is located posterior to the lentiform nucleus and contains the geniculocalcarine fibers, otherwise known as the optic radiation.
The optic radiation originates from the lateral geniculate nucleus of the thalamus and spreads posteriorly, carrying visual system information to the primary visual cortex of the occipital lobe.
Lastly, the sublenticular part of the internal capsule is located just inferior to the lentiform nucleus, and contains the fibers of the auditory radiation, which originate from the medial geniculate nucleus of the thalamus and spread towards the primary auditory cortex of the temporal lobe.All right, as a quick recap… The white matter of the brain contains axons that connect the cerebral cortex with other parts of the nervous system.
The connections include commissural, association and projection fibers. Commissural fibers connect the left and right cerebral hemispheres and form the corpus callosum, the anterior commissure, posterior commissure, and the hippocampal commissure.
The largest bundle of white matter is the corpus callosum, which sends signals between the two cerebral hemispheres and connects them together.
The anterior commissure is a much smaller white matter structure that mainly connects the olfactory bulbs and the temporal lobes of each hemisphere.
The posterior commissure is a small commissure located in the posterior diencephalon and contains crossing axons from the oculomotor complex.
And lastly, the hippocampal commissure connects the left and right hippocampi. On the other hand, association fibers connect different cortical regions within the same hemisphere.
These fibers include the short association fibers, which connect the cortices of the adjacent gyri, and the long association fibers that connect the cortices of distant gyri.
Long association fibers include the superior and inferior longitudinal fasciculi, the fronto-occipital fasciculus, and the uncinate, arcuate, and cingulate fasciculus.
Finally, the projection fibers are axons that send either ascending or descending signals and travel within the limbs of the internal capsule.
These fibers connect the cerebral cortex with other nervous system structures like the thalamus, brainstem, and the spinal cord.
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