White matter Tracts

Goals

• To introduce the concept of axonal fiber bundles, or white matter tracts, and projection neurons.
• To provide an annotated list of some of the major white matter fiber tracts in the brain and spinal cord.

Axons

The principal cell of the brain is the neuron. Neurons give rise to protoplasmic processes called dendrites and axons. In general, dendrites receive information from other neurons, while axons carry information from a neuron to other neurons. A neuron may have many dendrites and these may extensively branch. However, a neuron gives rise to only a single axon (although that axon may branch and contact the dendrites of many neurons). It is this communication among neurons that is the basis for information processing in the brain.

While dendrites are located near their neuron, the neuron’s axon can travel a great distance. Indeed, the longest axon in the human body travels about 1 meter (from lower spinal cord to the feet). Anatomists often distinguish between local circuit neurons that participate in local processing of information (e.g., within cortical layers) and of projection neurons that carry information from one brain region to a distant brain region. The axons from the projection neurons of a given brain region can travel together in distinct bundles. These bundles are called tracts, fiber tracts, or pathways.

When viewed through a microscope, the areas of cortex rich with neuronal cell bodies appear gray, and so we refer colloquially to these areas as 'gray matter'. Similarly, the axonal tracts appear lighter under the microscope, and so are referred to as 'white matter'.

Because the axonal ‘wiring’ paths between different parts of the brain, and between the brain and the spinal cord, are consistent across individuals and even across species, they have been given names. In fact, there are many named fiber tracts and pathways, and reviewing them all is beyond the scope of this outline. I have therefore listed only some of the major tracts, with an emphasis on those which we will encounter this semester.

I have grouped the major tracts in the following way:

• Tracts that ascend/descend in the spinal cord, and thus transmit cutaneous, nociceptive, proprioceptive and motor command information between the brain and neurons in the spinal cord and dorsal root ganglia.
• Intrahemispheric tracts that connect different thalamic and cortical regions within the cerebrum of a hemisphere.
• Interhemispheric or commissural tracts that connect different regions of the cerebrum between the two hemispheres.
• Cerebellar tracts, or peduncles, that convey information between the cerebellum and the cerebrum.

Spinal cord

The spinal cord provides a conduit for many tracts involved in motor control, cutaneous sensation, nociception (pain and thermal sensation), and proprioception.

The figure below illustrates the location of the various tracts in a cross-section of the spinal cord.

Descending tracts from brain to spinal cord

There are numerous tracts that connect cortex and subcortical regions of the brain to the spinal cord, primarily in the service of movement and posture. Here are some of the major tacts:

Corticospinal Tract

The CST connects primary motor cortex and other sensory and motor regions (i.e., premotor cortex, supplementary motor cortex, somatosensory cortex) to the motor neurons and interneurons in the ventral horn of the spinal cord.

Rubrospinal Tract

The Rubrospinal tract connects the red nucleus of the midbrain tegmentum to motor neurons and interneurons in the ventral horn of the spinal cord.

Reticulospinal Tract

The Retriculospinal tract connects neurons in the Pons and Medulla to motor neurons and interneurons in the ventral horn of the spinal cord. The Reticulospinal tract also plays a role in breathing and regulation of the heart.

Vestibulospinal Tract

The Vestibulospinal tract is composed of two tracts: the Lateral Vestibulospinal tract and the Medial Vestibulospinal tract. Both tracts have their origin from the vestibular nuclei in the pons and medulla.

Lateral Vestibulospinal Tract

The LVS tract controls muscle tone and posture.

Medial Vestibulospinal Tract

The MVS tract is confined to the upper spinal cord (cervical and upper thoracic) and is involved with stabilizing head position.

Ascending tracts from spinal cord to brain

There are many ascending tracts from peripheral cutaneous receptors, nociceptors, and proprioceptors that ascend in the spinal cord to targets in the medulla, pons, cerebellum, and thalamus. Here are the major tracts.

Dorsal Column-Medial Lemniscus System

The dorsal or posterior columns convey cutaneous and proprioceptive information from the spinal cord to the thalamus.

Anterolateral System

The anterolateral tracts carry information from the skin to the thalamus. The tracts differ in the nature of the information carried.

Lateral spinothalamic tract

The lateral spinothalamic tract conveys pain and temperature information.

Ventral spinothalamic tract

The ventral spinothalamic tract conveys pressure and touch.

Spinocerebellar tracts

As the name suggests, the spinocerebellar tracts connect the spinal cord to the cerebellum. These tracts primarily carry proprioceptive information from muscle spindles and Golgi tendon organs. There are several named tracts, which are mainly differentiated by what part of the body contributes to the tract.

Posterior spinocerebellar tract

Carries proprioceptive information from the lower limbs to the ipsilateral cerebellum.

Cuneocerebellar tract

Carries proprioceptive information from the upper limbs to the ipsilateral cerebellum.

Anterior spinocerebellar tract

Carries proprioceptive information from the lower limbs. The fibres decussate twice – and so terminate in the ipsilateral cerebellum.

Rostral spinocerebellar tract

Carries proprioceptive information from the upper limbs to the ipsilateral cerebellum.

Intra-hemispheric cortical tracts

Corona Radiata

The Corona Radiata sweep from the thalamic nuclei to cerebral cortex. With the thalamus at the center of the brain, the radiations of the axons appeared to early anatomists as the rays of the sun. I showed an example slide in class in which cortex has been digested away, leaving ‘ropes’ of white matter clearly visible.

Superior Longitudinal Fasciculus

The SLF connects the parietal, occipital and temporal lobes with frontal cortex. The SLF is bidirectional and is itself composed of different sub-pathways. The SLF plays an important role in language and attention.

Inferior Longitudinal Fasciculus

The ILF connects extrastriate cortex of the occipital lobe to the anterior temporal lobe.

Uncinate Fasciculus

The uncinate fasciculus is a bidirectional pathway that connects the anterior temporal lobe to the lateral orbitofrontal cortex. The UF is usually considered a component of the limbic system, although there is controversy as to whether the UF extends into the anterior amygdala and hippocampus, thus providing a direct connection from the amygdala and hippocampus to ventral frontal cortex.

Cingulum

The cingulum connects the cingulate cortex to the entorhinal cortex, which is the major source of input to the hippocampus.

The figure below illustrates several of the tracts discussed above.

Fornix

The fimbria-fornix tract provides output from the hippocampus to the hypothalamus.

Stria Terminalis

The stria terminalis provides output from the corticomedial nucleus of the amygdala to the bed nucleus of the stria terminalis and targets within the hypothalamus.

Ventral amygdalofugal pathway

The ventral amygdalofugal pathway connects the basal and accessory nuclei of the amygdala to numerous structures, including the hippocampus, orbitofrontal and anterior cingulate cortex, nucleus accumbens, dorsomedial thalamus, and hypothalamus.

Inter-hemispheric cortical tracts

Corpus Callosum

The Corpus Callosum (CC) refers to the large fiber tract that connect both homotopic (corresponding) and heterotopic (not corresponding) regions of the cerebral hemispheres. The CC is sometimes surgically disconnected in severe cases of epilepsy to keep seizures from spreading from one side of the brain to the other. It will be important later in the semester when we discuss split-brain patients. The fibers of the CC in the human can be seen in the image below as the string-like processes crossing the midline. In this view, the overlying cortical regions has been removed to expose the CC.

Anterior commissure

The Anterior Commissure (AC) connects the temporal lobe and amygdalae of each hemisphere. The AC is much smaller than the Corpus Callosum.

The figure below shows the relative position of the AC and CC in a coronal view of a human brain drawing.

Posterior commissure

Compared to the CC and the AC, the Posterior Commissure is a very small fiber bundle that connect pre-tectal nuclei and plays a role in the bilateral pupillary light reflect.

Hippocampal commissure

Unlike the commissures described above, the Hippocampal Commissure is a part of the fiber tract known as the fornix (see above). The HC is that part of the fornix that crosses the brain’s midline and connects the hippocampus in one hemisphere to the hippocampus in the other hemisphere.

Cerebellar tracts or peduncles

The cerebellum is connected to the cerebrum through thick fiber bundles known as peduncles (latin for ‘foot’). There are three major peduncles (per hemisphere): the superior, middle, and inferior peduncles. The peduncles can be seen in this cut-away image of the brainstem. On the left of the image, the cerebellum has been cut away and the peduncles cut and labeled. On the right of the image, the peduncles and their connections to the cerebellum can be seen.

Superior cerebellar peduncle

The superior cerebellar peduncle is a major efferent pathway from the cerebellum to the thalamus and to the red nucleus.

Middle cerebellar peduncle

The middle cerebellar peduncle is a major afferent pathway from the pons to the cerebellum.

Inferior cerebellar peduncle

The inferior cerebellar peduncle is the confluence of the spinocerebellar tracts discussed above. Thus, it provides the major source of proprioceptive information to the cerebellum from the spinal cord.

Diffusion imaging

Interest in white matter tracts has increased greatly over the past two decades as a form of magnetic resonance imaging (MRI), called diffusion-weighted imaging, can visual white matter tracts and the direction in which they travel. This has allowed for noninvasive quantitative studies of the white matter tracts in a large numbers of individuals.

Below is a 3D diffusion image of a living human brain with color coding indicating the primary direction of the tracts.