The corticospinal tract is the largest pathway in the central nervous system. It produces movement and senses that are essential for daily life.
The pyramidal tract, better known as the Corticospinal tract, is a collection of neuron axons that transfer movement-related information from the brain's center (the cerebral cortex) to the spinal cord. The tract is part of the descending tract system that starts at the brainstem.
The corticospinal tract serves many functions. Some of the most important include controlling afferent inputs, motor neuron activity, reflexes of the spine, and limb movement.
Spinal Tracts
The spinal cord has various nerve fiber groups that move towards and from the brain.
These movement lanes are known as ascending and descending tracts of the spinal cord. The tracts are important in relating sensory and motor information.
Ascending And Descending Spinal Tracts
The ascending and descending spinal tracts are pathways that relay information up and down the spinal cord between the body and the brain.
The ascending tracts in the spinal cord carry sensory information from the body to the brain. An example of this would be something like pain.
When you have the misfortune of stepping on a piece of lego, a signal from the site, in this case, the foot, will travel to the brain to let it know that something is wrong.
In this case, your body will signal to your brain that the nerves in your foot hurt.
Descending tracts carry motor information – like instructions to move your foot off and away from the lego piece.
Once the pain has been signaled in the brain, it will respond with appropriate action to remove the cause of discomfort.
Both these tracts comprise neuronal axons that are grouped into columns known as fasciculi.
These fasciculi are located inside the spinal cord's ventral, lateral, and dorsal parts.
A Quick Run-Down On Ascending Tracts In The Spine
Ascending tracts are sensory pathways that start from the spinal cord and travel down to the cerebral cortex.
There are three tracts in this category and include:
- The dorsal column-medial lemniscus system
- The spinothalamic system
- The spinocerebellar system
- The cuneocerebellar tract
- The spinotectal, reticular, and olivary tract
These tracts comprise four connected neurons.
- First-order neurons – inside the dorsal root ganglions and gather sensory information.
- Second-order neurons – are found inside the spinal cord and receive the message from first-order neurons.
- Third-order neurons – are found in the thalamus and further convey second-order neuron signals.
- Fourth-order neurons – these neurons are found in the cerebral cortex and act as the final location for the signals to conduct to.
Any signals received travel up the spine in an interesting way.
The tracts cross over to the other side of the central nervous system. This means that the left side of the brain receives sensory information from the right side of the body and the same the other way around.
This process is called decussations and occurs across different levels of the central nervous system.
The corticospinal tract does not form part of the ascending spinal tracts, but if you are interested in the ascending pathways, you can read this post.
But now that you have the lowdown on ascending tracts, it's time to look at descending tracts, of which the corticospinal tract is one.
Descending Tracts
Descending tracts of motor pathways that are in charge of controlling muscles of the trunk and other extremities.
Descending tracts are also known as motor tracts due to their role in the coordination of movement.
The descending tracts can be classified by their structural arrangement, including lateral and medial tracts.
Like ascending tracts, they can be classified into two major groups.
These groups include pyramidal tracts and originate from the motor cortex. These tracts carry motor fibers to the spinal cord and brain stem.
The pyramidal tract is also important for the voluntary control of striated muscles in the body and face.
The other group includes the extrapyramidal tract and originates from the brainstem to carry motor fibers to the spinal cord.
These fibers are responsible for the automatic control of muscle balance, posture, and motor action.
Similar to ascending tracts descending tracts also have first-order and second-order neurons.
A singular descending tract is formed by two of these neurons.
First-order neurons (also known as upper motor neurons) arise from the cerebral cortex and travel down the spinal cord to synapses in the anterior grey horn.
Second-order neurons (Also known as lower motor neurons) travel from the spinal cord to skeletal muscles causing them to move (or become innervated).
Some of the descending tracts include:
- Corticobulbar (corticonuclear) tract
- Corticospinal tract
- Extrapyramidal tracts
- Rubrospinal tract
- Vestibulospinal tracts
- Lateral vestibulospinal tract (LVST)
- Medial vestibulospinal tract (MVST)
- Reticulospinal tracts
- Tectospinal tract
The one this post will be looking at is the corticospinal tract.
The Corticospinal Tract
The corticospinal tract consists of white matter and has a motor Parkway running from the cerebral cortex to the spinal cord.
This motor pathway is responsible for the movement of the limbs and the trunk.
The corticospinal past starts in the motor cortex, where the bodies of the first-order neurons lie.
These neurons are also known as the pyramidal cells of Betz. The neurons descend via the corona radiata.
These specialized neurons descend through the following structures in the following order:
- Starts at the corona radiata
- The internal capsule
- The cerebral peduncle of the midbrain
- The pons
- Ends in the medulla
When the signal reaches the lower medulla, the corticospinal fibers decussate (or cross over) to the opposite side – the process described earlier as pyramidal decussation.
When pyramidal decussation occurs, the corticospinal tract divides into two parts.
The first of these is the lateral corticospinal tract, and the second is the anterior tract.
The fibers that cross enter the lateral tract, while uncrossed fibers create the anterior tract.
Both these corticospinal tracts are found throughout the spinal cord and synapse with lower motor neurons in part known as the anterior horn.
Both these tracts travel through the spinal cord and synapse with lower motor neurons -- this process occurs in the anterior gray horn.
The lower motor neurons exit the spinal cord through the ventral root and form peripheral nerves that innervate the body's muscles.
Many of the fibers in the corticospinal tract terminate at some point.
Approximately 20% of them terminate at thoracic levels, 25% terminate at lumbosacral levels, and 55% terminate at cervical levels.
Many of the fibers that start at the motor cortex terminate in the ventral horn of the spinal cord.
Recent research has suggested that 30%-40% of corticospinal neurons originate from the primary motor cortex, while the rest arise from the supplementary motor area, parts of the somatosensory areas, parts of the posterior parietal cortex, and parts of the premotor cortex.
Because of these findings, many now consider that the corticospinal tract forms part of the motor system and carries a large sensory role too.
The Lateral Corticospinal Tract
The largest descending motor pathway is the lateral corticospinal tract.
The tract starts in the cerebral cortex receiving input from the primary motor Cortex and the premotor Cortex, and supplementary motor areas.
The lateral corticospinal tract also receives nerve fibers from the somatosensory area, which regulates ascending tracts.
Fibers from the upper motor neurons convert into the Corona radiator after exiting the motor cortex.
The fibers then pass inferiorly to form a white matter structure known as the internal capsule.
The internal capsule is a structure that is located between the basal ganglia and the thalamus.
After the fibers have passed through the posterior cerebral peduncle in the internal capsule, they continue to go down inferiorly through the center of the cerebral peduncle of the midbrain.
Once it has reached the midbrain, it continues entering the pons and the medulla.
The majority of the fibers decussate when they pass through the caudal medulla.
The fibers that are naturally crossed in the lateral corticospinal tract enter the anterior corticospinal tract and become uncrossed.
The lateral part of the corticospinal tract descends into the lateral funiculus and interacts with lower motor neurons in the ventral horn of each spinal cord level.
At these sites, impulses are generated in the second-order motor neurons, and they pass along the individual peripheral nerves to reach the skeletal muscles.
The neural impulses are then conveyed to the muscle cells through the motor endplates of the neuromuscular junction, resulting in muscle action or movement being generated.
The fibers involved in the pyramidal cells of Betz preserve the motor homunculus.
This somatotopic organization is preserved along the corticospinal tracts, whereby the more medial part of the lateral corticospinal tract is responsible for the cervical region, and the more lateral aspects supply the lower thoracic, lumbar and sacral regions, respectively.
The Motor Homonculus
The term used to describe motor and sensory distribution in the cerebral cortex is a Latin word translated as the little man – known to many as the homunculus.
Edwin Baldry discovered the concept of homunculus in the year 1937.
The idea of the homunculus was to create a map corresponding to body parts to touch sensitivity.
If you look at the homunculus will notice that the proportion of the sensory cortex to the size of the body region is not as you'd expect it to be.
One great example of this is that the trunk has a small level of sensation while large cortical areas are devoted to the face and the lips – parts of the body that are more sensitive to touch and sensation.
In total, there are three homunculi in the brain.
The first is the motor homunculus present in the primary motor cortex of the precentral gyrus.
The second is the somatosensory homunculus on the primary sensory cortex of the postcentral gyrus.
And the final one is the motor homunculus present in the internal capsule – the one that relates best to the lateral aspect of the corticospinal tract.
The Anterior Corticospinal Tract
The anterior corticospinal tract emerges from uncrossed fibers in the corticospinal tract itself at the level where the bulbomedullary junction is found.
The anterior tract travels to the anterior funiculus of the spinal cord.
The anterior corticospinal tract fibers then cross over at the spinal level they innervate.
At the site of innervation, they synapse with lower motor neurons in the anterior horn, and their axons form peripheral nerves that extend to target muscles.
At the target muscles, the anterior corticospinal tract synapses with the motor endplate at a site known as the neuromuscular junction.
The lateral corticospinal tract controls fine movements like writing, typing, or knitting, as well as the gross-motor movement of the arms and legs like walking and throwing.
The anterior corticospinal tract controls movements of the axial muscles, muscles located in the trunk, and can generate movement patterns like bending over or flexing your abdominal muscles.
Injury And Treatment For Corticospinal Tract Lesions
When any part of the brain or spine gets damaged, it's very likely that the person will experience injury related to that area.
When the upper part of motor neurons in the corticospinal tract or damaged, it can lead to deficits known as upper motor neuron syndrome.
Whenever a person experiences a lesion to the cranial part of the corticospinal tract, it is very likely that the decussation of the pyramids will result in deficits on the contralateral side.
When the lesion is in the caudal part of the corticospinal tract, decussation of the pyramids will result in ipsilateral (same side) deficits.
Essentially, if a person experiences damage to the left side of the brain, it will show on the right side of the body. This, in comparison to if the body experiences injury to the left side, it will show on the left side.
Traumatic Brain Injury And Stroke In The Corticospinal Tract
Returning to the idea of the homunculus – depending on what aspect of the brain areas are damaged, motor deficits will show on the other side of the body.
For example, if the left somatosensory area in the brain that controls the fingers is damaged, the person will experience symptoms, like numbness or tingling, in the right hand.
One of the most common causes of corticospinal lesions occurs as a result of a stroke.
Once the tract has a lesion, its function becomes impaired and results in contralateral motor deficits. Some people may recover some motor function, but a complete recovery is rarely achieved.
Spinal Cord Injuries In The Corticospinal Tract
After a traumatic spinal cord injury, both voluntary and involuntary movement and control can be limited.
The extent of a person's recovery depends on the severity of the lesion.
Because the corticospinal tract has already decussated, the deficits in motor control will be ipsilateral to the side of the lesion.
Tests to measure motor action and sensation provide an indication of the level of the spinal cord lesion. Generally, there will be two outcomes – complete spinal cord injury or incomplete spinal cord injury.
People with partial damage to the corticospinal tract are usually able to regain the ability to make gross motor movements after some time, but in fewer cases show improvement in fine motor function.
After the damage to the corticospinal tract, there are a series of events that occur on a cellular and neural level that results in movement reorganization – this means that the patient's brain will sometimes rewire itself to perform another function.
The rewiring process is known as neuroplasticity and can be achieved through rehabilitation.
Some of the rehabilitative techniques include:
- Retraining gait
- Mirror therapy
- Constraint-induced movement therapy
- Task-specific training
Some of these activities can result in axonal remodeling not only in the corticospinal tract but also in the corticorubral tract, the rubrospinal tract, and the reticulospinal tract.
Conclusion
The corticospinal tract is a vital part of the central nervous system. It is known to regulate movements, especially that of the limbs.
The corticospinal tract comprises two tracts, including the lateral and anterior tracts. Each of these is responsible for its specified tasks.
Damage to the tract can result in impaired movement, and the site of injury will determine the severity of impaired function.
References:
https://www.kenhub.com/en/library/anatomy/descending-tracts-of-the-spinal-cord
