Corticospinal Tract

The corticospinal tract, also known as the pyramidal tract, is one of the descending spinal tracts necessary for the passing of information from the central nervous system to the peripheral nervous system, particularly to musculature of the axial region of the body (the trunk) and distal regions (limbs and fingers/toes).

This tract is said to represent the highest order of motor function in humans and is most directly in control of fine, digital movements. Many of this tract’s fibers that terminate on interneurons of the spinal cord, though, are responsible for reflexes. Of all the corticospinal fibers…

  • 20% terminate at the thoracic levels
  • 25% terminate at the lumbosacral levels
  • 55% terminate at the cervical levels

Just like many other major nerve tracts, the corticospinal tract can be divided into two sub-tracts: the lateral corticospinal tract and the ventral (anterior) corticospinal tract.

The lateral corticospinal tract is responsible for controlling the distal musculature whereas the ventral corticospinal tract controls the axial musculature. Generally, you can expect the nerve fibers of the corticospinal tract to innervate skeletal muscle more than cardiac or smooth muscles, if at all.

The Neurons of the Corticospinal Tract

The neurons which feed information into the corticospinal tract are known as “upper motor neurons.” These neurons send fibers that move down through the cerebral cortex, through the brainstem, to the spinal cord, and synapse in the ventral, or anterior, gray horn. The fibers of the upper motor neuron then leave the ventral gray horn to innervate a given muscle or muscle group.

As the nerve fiber travels out of the central nervous system, they transition into what is known as the “lower motor neuron.” The lower motor neurons are the extension of the nerve fibers that travel out to the peripheral nervous system in order to innervate the skeletal muscles.

Innervation of the distal musculoskeletal system is not the only thing the corticospinal tract is good for though. In fact, this tract also innervates nerves not only through the lower spinal cord, but also supply muscles via cranial nerves from the cervical spinal levels.

As the fibers descend through the spinal cord, they give off small extensions that supply information to cranial nerves and innervate muscles in the trunk of the body. These nerves are not called corticospinal nerves, however, they are instead called “corticonuclear fibers” since they innervate cranial nerve nuclei.

How Does the Corticospinal Tract Communicate With the Rest of the Nervous System?

The corticospinal tract maintains connections with multiple regions of the cerebrum, primarily the motor cortex. The motor cortex is recognized to have three main components, the primary motor cortex, premotor cortex, and the supplementary motor area – each of these maintain their own unique connections and methods of communication with the corticospinal tract.

The Corticospinal Tract and the Motor Cortex

One of the structures that is in direct communication with the corticospinal tract (located just anterior to the central sulcus) is known as the “precentral gyrus,” or the “primary motor cortex.”  Most of the axons that originate in this cortex control the voluntary movement of skeletal muscles.

Just anterior to the primary motor cortex is the “premotor cortex.” This cortex is associated with learned, repetitive, or planned motor actions. Last, but not least, is the “supplemental motor area,” positioned superiorly to the premotor cortex. Of course, this structure is also associated with the control of movement.

Additional Cerebral Cortices Connected to the Corticospinal Tract

Believe it or not, there are even more cortices in the brain, yet the remaining cortices are not considered to be a part of the motor cortex. Posterior to the primary motor cortex is the “postcentral gyrus,” a structure that is home to the primary somatosensory cortex. Now, what the heck is this the primary somatosensory cortex? It is a part of the brain that is incredibly important to voluntary movement since a large portion (30-40%) of the nerves that control movement originate here.

The frontal lobe of the cerebrum holds a few cortices of its own: The prefrontal cortex can be said to be where the process of voluntary movement truly begins. It is where the thought of a movement will form (i.e., thinking of flexing your arm or scratching an itch).

When you notice an itch on your eyebrow, the thought of you scratching that itch forms and the prefrontal cortex then sends signals to the appropriate cortex/cortices to begin composing the motor plan.

From the cortices, the thought is sent to the basal nuclei which further perfects the motor plan before sending it to the cerebral cortex, the last stop before the execution of the action impulse in the peripheral nervous system.

It is important to note that when the action impulse is sent from the cerebral cortex to the cerebellum, the motor plan is sent along with proprioceptive information. Why is this important and what part does the proprioceptive information play in the formation of the motor plan?

With proprioceptive information, your brain is able to develop a more accurate motor plan since it will be aware of the original positioning of your body. For instance, if you wanted to scratch the itch on your eyebrow, but your arm was buried under a blanket, your motor plan would be to free your arm from the blanket, lift your arm, and scratch.

If your arm was already near your head, the motor plan would only entail the movement of your hand to the left or the right to reach the itchy spot. In consideration of the proprioceptive information, the final motor plan is sent back up to the cerebrum from the basal ganglia to be distributed to the peripheral nervous system through the spinal cord.

Central and Peripheral Nervous System Communication Through the Corticospinal Tract

If it isn’t already clear, the corticospinal tract has an abundance of connections throughout the nervous system to ensure not only the execution of the motor plans, but also to control the intensity of the movement (you want to scratch the itch not rip your face off) and the implementation of the necessary proprioceptive information.

Some of the structures outside of the cortices that maintain connections with the corticospinal tract are the nuclei of the parietal lobe, cerebral peduncles, pons, and the medullary pyramids.

In the parietal lobe are a bunch of different types of motor neurons, the cell bodies of which are located in separate laminae – not the Rexed laminae of the spinal cord, but of the cerebral cortex. In the 5th lamina of the cerebral cortex are the pyramidal cells (some of which are known as the “Cells of Betz,” particularly the larger cells). What part do these pyramidal cells play in the transmission of neuronal information through the corticospinal tract?

The significance of these cells comes from the nerve extensions they send outward into the cerebrum for the passing of motor-related information. The pyramidal cells send off their axons in a fan shape known as the “corona radiata,” the fibers of which squeeze together and become compacted before proceeding into a white matter structure called the “internal capsule.” In the posterior region of the internal capsule, these newly-compacted fibers funnel down and condense even more as they travel to the midbrain.

Next, these nerve fibers continue their descent and progress into the the stalks of white matter found at the front of the midbrain called the “cerebral peduncles.” The specific point at which the fibers move through the midbrain are known as the “crus cerebri,” the three subdivisions of which contain fibers specific to three major descending tracts – but that’s another story for another time. From the crus cerebri, the nerve fibers funnel lower into the midbrain, into the pons.

Within the pons are even more clusters of nuclei integral to the transmission of nervous system information through the corticospinal tract. These clusters of nuclei are known as the “pontine nuclei.”

They are continuously receiving information from the cerebral cortex, and subsequently distributing this information contralaterally within the cerebellum, through the middle cerebellar peduncles. (Note that some fibers extending from the corona radiata scatter across the pontine nuclei – not all of them follow a route that is so neat and easily traced.

Ultimately, though, they do all return to the main bundle and condense once again before exiting the pontine nuclei to travel to the medulla oblongata.)

As far as where the groups of fibers go:

  • The majority of the fibers (80%) of the fibers decussate and travel through the dorsal portion of the spinal cord (the point of decussation, on the distal end of the medullary pyramid, is known as the pyramidal decussation)
  • A small percentage (15-20%) of the fibers remain ipsilateral and descend into the ventral portion of the spinal cord.

Why does this matter? The significance of this is that the decussation or ipsilateral movement of the nerve tracts is a direct influence on the columns of the spinal cord into which the tract flows. Let’s take a closer look at this in the next section.

The Corticospinal Tract: Through the Spinal Cord and Peripheral Nervous System

The columns of the spinal cord are composed of white matter and the individual sections are as follows:

  • Lateral white columns (also known as the “lateral funiculi”)
  • Dorsal white columns (also known as the “dorsal funiculi”)
  • Anterior white columns (also known as the “ventral funiculi”)

The Lateral Corticospinal Tract

The fibers that travel through the lateral funiculi are what make up the lateral corticospinal tract. The nerves of the lateral corticospinal tract synapse onto the motor neurons in the ventral gray horn, specifically on both alpha and gamma motor neurons, and then go out to innervate the skeletal muscles.

When nerves innervate muscles, they have to maintain contact with specific types of fibers, each of which are stimulated by varying fibers. The types of fibers necessary for movement are:

  • Extrafusal: These are contractile fibers, controlling the contraction, or the shortening and extending, of muscles. These fibers are stimulated by alpha motor neurons.
  • Intrafusal: These are muscle spindles and provide information regarding proprioception. These fibers are stimulated by gamma motor neurons.

What is the difference between alpha and gamma motor neurons? Gamma motor neurons maintain muscle tone by causing muscle spindles to become taught. In order to do this, the gamma motor neuron must work with the alpha motor neuron to accomplish this tension or other type of flex in the muscle. This collaboration between the two motor neurons is called “co-activation.”

The musculature controlled by the lateral corticospinal is  the distal limb musculature, particularly the arms, hands, and fingers, meant for very precise movements.

The Ventral Corticospinal Tract

The ventral corticospinal tract, on the other hand, is more closely associated with control of the axial musculature for “gross” or larger movements. Nerves from the ventral corticospinal tract decussate as they pass through the ventral funiculus before going on to stimulate both the alpha and the gamma motor neurons.

The fibers of this tract terminate in the ventral gray horn of the spinal cord, which is what enables the decussation in the ventral funiculus and subsequent activation of the alpha and gamma motor neurons.

What about the dorsal white column? Well, that’s known as the medial lemniscus pathway, and that tract gets its own time to shine.

Summary of the Role of the Corticospinal Tract

Between thirty and forty percent of the nerves in the corticospinal tract arise from the primary motor cortex, whereas the rest come from the supplementary motor area, premotor cortex, somatosensory areas, and parts of the posterior parietal cortex.

Because of the numerous points of origin for only this singular tract, it is believed that the corticospinal tract not only participates in the control of movement, but that it is also a part of the processing of sensory information as well.

It is also thought that the corticospinal tract may act as a “gate” by controlling information that is deemed useful or irrelevant for the body’s reaction to a given stimuli. The information passed through the corticospinal tract is essential for distal and axial movements in response to external stimuli, particularly motor actions or motor-related memory.

References

  1. The Editors of Physiopedia. (n.d.). Corticospinal tract. Retrieved November 29, 2019, from https://www.physio-pedia.com/Corticospinal_Tract
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  3. Neafsey, E. J. (1995). Neuroscience lab | Slide 18: Midbrain – Superior colliculus. Retrieved from http://www.meddean.luc.edu/lumen/MedEd/Neuro/frames/nlBSsL/NL18SF.HTM
  4. Ninja Nerd Science. (2018, January 9). Descending tracts | Corticospinal tract [Video file]. Retrieved from https://www.youtube.com/watch?v=zKA8iaxfEww
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