Corticospinal Tract (CST): Defination, Structure and Function

The Corticospinal tract (CST), also known as the pyramidal tract, is a collection of axons that carry movement-related information from the cerebral cortex to the spinal cord. It forms part of the descending spinal tract system that originate from the cortex or brainstem.

Corticospinal Tract (CST)

• The neurons that travel in the corticospinal tract are referred to as upper motor neurons; they synapse on neurons in the spinal cord called lower motor neurons, which make contact with skeletal muscle to cause muscle contraction.

The CST:

• Is one of the major pathways for carrying movement-related information from the brain to the spinal cord and has approximately 1 million nerve fibres (average conduction velocity of approximately 60m/s using glutamate as their transmitter substance).
• Signaling along the corticospinal tract involved in a variety of movements, including behaviors like walking and reaching, but it is especially important for fine finger movements e.g. writing, typing, or buttoning clothes.
• Represents the highest order of motor function in humans and is most directly in control of fine, digital movements.
• After selective damage to the corticospinal tract, patients are usually able to regain the ability to make crude movements (e.g. reaching) after a period of time, but they may be unable to fully recover the ability to make individual finger movements. 

Course

The CST

• Originates in several cortical areas, about half of these axons extend from neurons in the primary motor cortex, but others originate in the nonprimary motor areas of the brain as well as in regions of the parietal lobe like the somatosensory cortex. 
• The axons that travel in the CST descend into the brainstem as part of large fiber bundles called the cerebral peduncles.
• The tract continues down into the medulla where it forms two large collections of axons known as the pyramids; the pyramids create visible ridges on the exterior surface of the brainstem.
• At the base of the pyramids, approximately 90% of the fibers in the corticospinal tract decussate, or cross over to the other side of the brainstem, in a bundle of axons called the pyramidal decussation.
• The fibers that have decussated form the lateral corticospinal tract; they will enter the spinal cord, and thus cause movement, on the side of the body that is contralateral to the hemisphere of the brain in which they originated.
• The other 10% of the corticospinal tract fibers will not decussate; they will continue down into the ipsilateral spinal cord; this branch of the corticospinal tract is known as the anterior (or ventral) corticospinal tract. Most of the axons of the anterior corticospinal tract will decussate in the spinal cord just before they synapse with lower motor neurons.
• The fibers of these two different branches of the corticospinal tract preferentially stimulate activity in different types of muscles.

• Lateral corticospinal tract primarily controls the movement of muscles in the limbs
• Anterior corticospinal tract is involved with movement of the muscles of the trunk, neck, and shoulders.

Of all corticospinal fibres approximately 20% terminate at thoracic levels, 25% at lumbosacral levels and 55% at cervical levels. Many of the fibres that originate from the motor cortex then terminate in the ventral horn of the spinal cord.

Function

Corticospinal Tract (CST)

The CST has many functions which include control of afferent inputs, spinal reflexes and motor neuron activity, the most important being the mediation of voluntary distal movements

• Outputs from the primary motor cortex (M1) contribute to the CST, making connections to: excitatory monosynaptic alpha motor neurons; polysynaptic connections onto gamma motor neurons (responsible for the control of muscle spindle length); polysynaptic connections via interneurons within the spinal cord.
• When the neurons are influenced directly by only one axon, they are called “monosynaptic,” and when indirectly, by many axons, they are known as “polysynaptic.”

Image: Illustrative examples of corticospinal tract tractography (imaging of the neural tracts) reconstruction in a participant. Tracts are projected on a T1-weighted MRI scan in coronal plane to allow view along the full tract length.

Recent developments have increased the understanding of the origin and termination of the CST neurons:

• 30%-40% arise from the primary motor cortex.
• Rest of the fibers arise from the supplementary motor area (SMA), premotor cortex (PMA), parts of the somatosensory areas (S1 and S2) and parts of the posterior parietal cortex.

Due to the various origins that contribute to the CST, it is considered that this tract not only forms part of the motor system, but also has a large sensory role also.

• The fibers originating from the sensory cortex terminate in the dorsal horn of the spinal cord.
• Here they synapse with interneurons that receive input from somatosensory receptors and are thought to regulate information from peripheral receptors within the spinal cord.
• Therefore, the CST may act as a ‘gate’, modulating or inhibiting information that is deemed useful or irrelevant.

Clinical relevance

When the upper motor neurons of the corticospinal tract are damaged, it can lead to a collection of deficits sometimes called upper motor neuron syndrome. 

• A lesion of the CST cranial to the decussation of the pyramids will result in deficits on the contralateral side.
• A lesion of the CST caudal to the decussation of the pyramids will result in deficits on the ipsilateral side.

The image below depicts the motor homunculus. Dependant on what aspect of this is damaged will result in motor deficits on the contralateral side of the body.Stroke/Traumatic Brain Injury

Spinal Cord Injury

• Following a spinal cord injury, both voluntary (sensory and motor) and involuntary control can be impaired and the extent of recovery dependent on the severity of the lesion (Freund et al, 2013). As the CST has already decussated, motor deficits will be ipsilateral to the site of the lesion.
• The ASIA outcome measure, which assesses both motor and sensation, will provide an indication of the level of the spinal cord lesion and whether or not it is complete or incomplete.
• Crozier et al (1991) concluded that 89% of those who were ASIA B-E with pinprick preservation went on to ambulate. This is due to close proximity of the spinothalamic tract to the lateral corticospinal tract and their shared blood supply.

Assessment

The effect of a lesion to the CST causes more than just muscle weakness. It also affects synergistic movement patterns that affect things such as dexterity, ambulation and activities of daily living.

There are a number of outcome measures that can be used dependent on what you want to assess. These include:

• Fugl-Meyer Assessment of Motor Recovery after Stroke (FMA)
• Oxford Muscle Grading System
• Stroke Rehabilitation Assessment of Movement (STREAM)
• Action Research Arm Test (ARAT)
• Cherokee Arm and Hand Activity Inventory
• Functional Ambulation Category
• Motor Assessment Scale
• Rivermead Mobility Index
• Rivermead Motor Assessment
• ASIA (Spinal Cord)

Read more about outcome measures in stroke rehabilitation by Salter et al (2013)

Stinear et al (2007) suggested that Corticospinal Tract integrity could be used to identify the likely extent of motor recovery and may enable appropriate selection of rehabilitation strategies for individuals recovering from stroke . In a further study conducted by Stinear et al (2012) they trialed the use of the PREP(predicting motor recovery) algorithm to assess the likelihood of upper limb recovery. By utilising the SAFE score (sum of the shoulder abduction and finger extension) 72 hours after stroke, Transcranial magnetic stimulation, motor
evoked potentials in affected upper limb or the Asymmetry Index (measured with diffusion-weighted MRI) they were able to predict whether there could be a complete- no recovery. It was suggested from these findings that clinicians using the PREP algorithm may be able to predict the likely extent of upper limb recovery and may be able to therefore manage of patient expectations from an earlier period.

Treatment

Following a lesion to part of the corticospinal tract, such as a stroke, their function is impaired resulting in contralateral motor deficits. Although people begin to experience motor recovery to some extent, complete recovery is rarely achieved.

Following damage to the corticospinal tract, there is a cascade of events that occur at both a cellular and network-level resulting in motor map reorganisation. This phenomenon is known as neuroplasticity, and it can be enhanced by rehabilitative training such as motor control and learning which is achieved by repetitive practice. Other treatment techniques may include:

• Gait Re-Education
• Mirror Therapy
• Constraint Induced Movement Therapy (CIMT)
• Task Specific Training

It is believed that during these activities that axonal remodelling may not only happen in the lesioned corticospinal tract but also the corticorubral tract from the ipsilesional hemisphere as the rubrospinal or the reticulospinal tract. It is thought that these deep brain areas provided support for the CST.

Another proposed mechanism is an increased production of trophic factors as well as an increased density of trophic receptors on the neural surface, producing an environment more suitable for neural remodeling.