PYRAMIDAL PATHWAY: TRACTS, STRUCTURE AND LESIONS - ANATOMY AND PHYSIOLOGY - 2024

The pyramidal pathway or pyramidal tract is a group of nerve fibers that originate in the cerebral cortex and end in the spinal cord. They direct the voluntary control of the musculature of the whole body. This pathway includes two tracts: the corticospinal and the corticobulbar. The first ends in the brainstem, and the second in the spinal cord.

The pyramidal pathway is a descending pathway, that is, it sends impulses from the brain to the body's motor neurons. The latter directly innervate the muscles so that we can move them.

It differs from the extrapyramidal pathway in that it directs involuntary and automatic muscle control such as coordination, balance, muscle tone, posture, etc.

There are no synapses (neuronal connections) within the pyramidal pathway. The bodies of the cells are in the cerebral cortex, or in the brainstem.

The neurons in this pathway are called upper motor neurons, since once completed, they connect with lower motor neurons that directly control the muscles.

The pyramidal pathway is so named because its fibers pass through the pyramids of the medulla oblongata. In this area, the fibers converge in many directions, taking the appearance of an inverted pyramid.

Tracts of the pyramidal tract

Tracts of the pyramidal pathway. Source: Polarlys and Mikael Häggström via Wikimedia Commons

The pyramidal pathway can be functionally subdivided into two parts: the corticobulbar tract and the corticospinal tract. Next, I'll explain what each of them consists of.

Corticobulbar tract

This tract directs the muscles of the head and neck. Thanks to this structure we can control facial expression, chew, produce sounds and swallow.

It arises in the lateral part of the primary motor cortex. The fibers then converge on the internal capsule of the brain stem.

From there, they travel to the motor nuclei of the cranial nerves. In these nerves they connect with lower motor neurons to innervate the muscles of the face and neck.

Generally, fibers from the left primary motor cortex control neurons bilaterally. That is, they direct the right and left trochlear nerves. However, there are exceptions. An example is the motor neurons of the hypoglossal cranial nerve, which innervate contralaterally (on the opposite side).

Corticospinal tract

Source: slideshare.net

The corticospinal tract controls the voluntary movement of the body. They begin in the cerebral cortex, specifically, from the pyramidal cells of the V layer.

Fibers arise from several structures: the primary motor cortex, the premotor cortex, and the supplemental motor area. It also receives nerve impulses from the somatosensory area, the parietal lobe, and the cingulate gyrus; although to a lesser extent.

The nerve fibers converge in the internal capsule, which is located between the thalamus and the basal ganglia.

From there, they pass through the cerebral peduncle, the pons, and the medulla oblongata. In the lower part of the bulb, the corticospinal tract divides into two: the lateral and anterior corticospinal tract.

The fibers of the former cross to the other side of the central nervous system and descend to the ventral horn of the spinal cord. Once there, they connect with the lower motor neurons that direct the muscles directly.

On the other hand, the anterior corticospinal tract is ipsilateral. That is, the right side activates the right part of the body (as with the left). It runs down the spinal cord, ending at the ventral horn of the cervical and thoracic segments. In that place, it connects with the lower motor neurons present there.

The corticospinal tract has a special type of cell that does not exist anywhere else in the body. They are called Betz cells, and they are the largest pyramidal cells in the entire cortex.

Large diameter axons arise from them, which mainly control the legs. Its characteristics allow nerve impulses to travel very fast.

This tract has more than one million axons, most of which are covered in myelin.

Development of the pyramidal pathway

When we are born, the pyramidal pathway is not completely myelinated. Little by little it is myelinated from below (trunk or medulla) upwards (cortex). As it becomes covered with myelin, each time we make more perfected and precise movements.

This pathway ends myelination at two years of age, although it continues to progress gradually in the opposite direction until age 12.

Structure

The pyramidal pathway is composed of upper motor neurons that originate in the cerebral cortex and end in the brain stem (corticobulbar tract) or in the spinal cord (corticospinal tract). The pathway itself is mainly made up of axons.

The axons that run through the tracts are called efferent nerve fibers, because they send information from the cerebral cortex to the muscles (if it received the information instead of sending it, it would be called afferent).

They can intersect in the medulla oblongata and travel through the spinal cord. There, they usually connect with interneurons in the middle of the cord, called the gray matter.

Interneurons are normally small and have a short axon. They serve to connect two different neurons. They usually link sensory and motor neurons.

These interneurons connect with lower motor neurons, which control muscles. Although, in some cases, axons travel through the white matter of the spinal cord until they reach the vertebral level of the muscle they are going to direct.

Once there, the axons connect to the lower motor neurons.

Lesions of the pyramidal tract

The pyramidal pathway can be damaged as it runs through most of the central nervous system. One especially vulnerable area is the internal capsule. It is common for strokes to occur in this area.

Damage to the pyramidal tract can be due to both stroke and hemorrhage, abscesses, tumors, inflammation, multiple sclerosis… As well as trauma to the spinal cord or herniated discs.

The lesions can give different symptoms if they affect the corticospinal tract or the corticobulbar.

Damage to the corticospinal tract produces upper motor neuron syndrome. If only one side of the corticospinal tract is damaged, symptoms will be seen on the side of the body opposite the injury. Some of them are:

- Increased muscle tone (hypertonia).

- Muscular weakness.

- Increased muscle reflexes (hyperreflexia).

- Babinski sign.

- Clonus, which refers to rhythmic and involuntary muscle contractions.

- Problems to make fine movements.

On the other hand, an injury to the corticobulbar tract if it is unilateral would produce mild muscle weakness in the face or neck. Although this changes according to the affected nerves:

- Hypoglossal nerve: is responsible for directing the movements of the tongue. If damaged, a spastic paralysis would occur to one side of it, causing it to drift to one side.

- Facial nerve: its injury would lead to spastic paralysis of the muscles of the lower quadrant of the face, on the side opposite the injury.

If the corticobulbar tract injury is complete, pseudobulbar paralysis may occur. It consists of difficulties in pronouncing, chewing and swallowing. In addition to suffering sudden mood swings.

References

1. Pyramidal pathway. (sf). Retrieved on April 6, 2017, from Quizlet: quizlet.com.
2. Pyramidal tracts. (sf). Retrieved on April 6, 2017, from Wikipedia: en.wikipedia.org.
3. Pyramidal tracts. (sf). Retrieved on April 6, 2017, from Science direct: sciencedirect.com.
4. Swenson, R. (nd). Chapter 8A - Pyramidal System. Retrieved on April 6, 2017, from Review of clinical and fuctional neuroscience: dartmouth.edu.
5. THE DESCENDING TRACTS. (sf). Retrieved on April 6, 2017, from Teach me anatomy: teachmeanatomy.info.