Corticospinal Tract

Overview:

Nerve cells and their networks throughout the body are known for their complicated and complex infrastructure that although seems very difficult to understand, are very ideally placed and interconnected to supply the body tissues with messages from the brain.

This brain has various centres and regions to process and interpret information that it keeps on receiving continuously, and as it is the centre of all information processing and command execution in the body (except for some reflex movements controlled by the spinal cord), it makes complete sense that all body responses would be directed through a specific pathway of nerve cells that carry the processed information from the higher centres of the brain to the target muscles or tissues or cells. Many motor functions are executed in the same way, as information is sent via specific pathways.

One of these pathways is the corticospinal tract, which, as the name suggests, arises in the cortical regions of the brain and extends down towards the spinal cord area. In this article, the structure and the components of the nervous system that help shape the corticospinal tract are mentioned in detail. The functions of CST and the clinical complications that result as a result of any problem regarding its functions are also briefly described.

Summary:

• The corticospinal tract is a network of nerve cells’ axons that transports data about motion from the brain areas around the cerebral cortex to the spinal cord
• The nerve cells of the corticospinal tract arise from cortical regions while moving downwards towards the brainstem and then into the spinal cord
• The precentral gyrus also identified as primary motor cortex, is a component of the brain’s neocortex that is accountable for conscious locomotion
• The Corticospinal tract regulates afferent input data, spinal reactions, as well as nerve cell interactions and functions; the most important of these is the reconciliation of self-initiated distal movement patterns
• The pyramidal tracts are vulnerable to harm or injury because they run nearly the entire length of the central nervous system. As previously stated, they are extremely vulnerable as they pass through the internal capsule, which is a common site of cerebrovascular accidents

Location:

The Corticospinal Tract (also shortly referred to as CST), further recognized as the Pyramidal Tract, is a network of axons that connects the spinal cord to the cerebral cortex.

The corticospinal tract is a network of nerve cells’ axons that transports data about motion from the brain areas around the cerebral cortex to the spinal cord. Approximately 50 percent of these axons emerge in the primary motor cortex, and many arise in non-primary motor parts of the brain and also some parietal lobe areas such as the somatosensory cortex.

The nerve cells of the corticospinal tract arise from such mentioned cortical regions while moving downwards towards the brainstem and then into the spinal cord. This is the area where they synapse on nerve cells that regulate skeletal muscle movement effectively.

Structure:

It is a component of the sloping spinal tract system, which stems from the cortex or brainstem (Figure 1).

The Brainstem is located between the bottom layer of the brain and the upper part of the spinal cord. The brainstem is the framework that attaches the brain’s cerebrum to the cerebellum and spinal cord. It is divided into three parts, which are listed in order of appearance when moving downward: the midbrain, the pons, and the medulla oblongata.

Upper motor neurons are the nerve cells that move in the corticospinal tract; they recombine on lower motor neurons in the spinal cord, which communicate directly with skeletal muscle to cause muscle activity. The figure below gives a visual concept of the corticospinal tract and the components of the nervous system it incorporates, which are briefly explained below.

Motor nerve cells:  

Motor nerve cells are the brain and spinal cord cells that help the body to move, talk, gulp down food and drinks, and inhale and exhale by transmitting signals from the brain to the muscles cells of the body that performs these tasks. The nerve fibres of motor neurons are of the maximum length in the body, with a singular axon stretching from the bottom surface of the spinal cord to the feet.

Motor neurons are classified as upper or lower motor nerve cells and establish a variety of strictly regulated, complicated circuit elements all through the body. This regulates both consensual and unintentional movements by innervating effector glands and muscles.  A two-nerve cell circuit is formed by the upper motor nerve cells and the lower motor nerve cells. 

Upper nerve cells emerge from the cerebral cortex and transport to the brain stem or spinal cord. Lower motor nerve cells originate in the spinal cord and travel all through the body to form a nerve cells network in the glands and muscle cells. 

Upper and lower motor nerve cells send and receive signals using different neurotransmitters. Upper motor nerve cells make use of glutamate, while acetylcholine is used by lower motor nerve cells.

Spinal Cord:

The spinal cord acts as a crucial conduit and reflex centre between the peripheral nerves and the brain, transmitting motor signals from the different parts of the brain to the muscle fibres, organs, and tissues, as well as sensory input from these regions directly to the brain. The spinal cord extends from the foramen magnum to the boundary located at the lowest surface of the first lumbar vertebra inside the vertebral canal. It has enlargements in two places: the cervical and lumbar areas.

Its upper extreme part is prolonged with the medulla, with the change occurring above the point of departure of the very first pair of cervical nerves.

The precentral gyrus:

The precentral gyrus (abbreviated as PCG), also identified as the motor strip or primary motor cortex, is a component of the brain’s neocortex that is accountable for conscious locomotion. This gyrus is situated in the most posterior region of the frontal lobe, directly present at the anterior side of the central sulcus, and extends from the peak area of the brain down to the sylvian fissure, which distinguishes the temporal lobe.

Skeletal muscles: 

Skeletal muscles are considered voluntary as the body has direct control over them with the help and coordination from the nervous system. Compressions and movements can be modified to produce strong, quick motions or comparatively tiny, precise actions. Skeletal muscles can extend and contract while being able to return to their original configuration.

These muscles connect to bone and have the primary duty of contracting to allow the body’s bone framework to relocate. Because of how they appear when observed, they are also recognized as striated muscles. The’stripy’ image is caused by the strips of Actin and Myosin that shape the Sarcomere, which is reported within the Myofibrils.

Decussation of pyramids or pyramidal decussation:

This is the area at the medulla-spinal cord intersection at which motor fibres from the medullary pyramids pass the centre line. The fibres then travel into the spinal cord as the corticospinal tract.

The cerebral peduncles: 

The corticospinal and corticobulbar tracts are significant downwards passages found in the cerebral peduncles. Cerebral peduncles are a big package of nerve cells that extends from the cerebrum to the pons and imitates a large stalk (Figure 2). On every part of the brainstem, there are two cerebral peduncles. The cerebral peduncles are defined differently depending on the source of inquiry, but a general definition is that they encompass the entire region of the midbrain that resides anterior to the cerebral aqueduct.

The corticospinal tract:

The corticospinal tract is one of the primary pathways for transmitting mobility-relevant data from the higher parts of the brain to the spinal cord, with nearly one million nerve fibres (average conduction velocity of about 60m/s utilizing glutamate as their transmitter material). About 50 percent of all these axons present in the corticospinal tract originate from nerve cells in the primary motor cortex, but some also arise in non-primary motor regions of the brain and also parietal lobe areas such as the somatosensory cortex.

The CST axons depart into the brainstem as a portion of huge fibre packets or bundles known as cerebral peduncles. The tract goes further down into the medulla oblongata. At this point, it shapes two big compilations of axons identified as the pyramids, which generate noticeable ridges on the outer surface of the brainstem. The decussated fibres shape the lateral corticospinal tract. These will join the spinal cord and therefore allow mobility on the part of the body which is contralateral to the side of the brain in which they are located.

Approximately ninety percent of the fibres in the corticospinal tract decussate, or intersect over to the opposite region of the brainstem, at the bottom surface of the pyramids in a package of axons, known as the pyramidal decussation. The remaining ten percent of corticospinal tract fibres do not decussate; instead, they do proceed to move down into the ipsilateral spinal cord; and this particular segment of the corticospinal tract is recognized as the anterior corticospinal tract.

The fibres of the two distinct branches of the corticospinal tract trigger and cause activity in various kinds of muscle fibres in a favourable manner.

The lateral corticospinal tract is mainly responsible for controlling body muscle activity in the arms and legs. The anterior corticospinal tract is engaged in the trunk, pectoral muscle, and neck muscular contractions. Roughly one-fifth of all corticospinal fibres finish at thoracic stages, one-fourth at lumbosacral stages, and fifty-five percent end at cervical stages. Several of the fibres that arise in the motor cortex end up in the ventral horn region of the spinal cord. 

Function:

Sending signals all along the corticospinal tract is implicated in a lot of motions and activities including having to walk and attempting to reach, but it is particularly necessary for perfect gestures or movements, such as written work, clicking, or fastening laces of shoes, etc.

In human beings, it shows the actual command of motor skills and is most straightforwardly in control of perfect, digital movement patterns.

The Corticospinal tract controls afferent input data, spinal reactions, and nerve cell interactions and functions, the most significant of these is the conciliation of self-initiated distal movement patterns. Primary motor cortex (M1) outcoming signals make a contribution to the CST such as through interconnection to excitatory monosynaptic alpha nerve cells; polysynaptic linkages onto gamma nerve cells (which are accountable for muscle spindle size regulation); and polysynaptic linkages through the use of interneurons inside the spinal cord.

When nerve cells are majorly impacted by only one axon, they are said to be “monosynaptic,” and whenever they are impacted through indirect means by several axons, they are said to be “polysynaptic.”

The nerve cells of the corticospinal tract play a very important role in motor function execution in the body, an example of this being them sending out several collateral fibres that move downwards to the spinal cord, making a connection in a variety of locations such as the basal ganglia, thalamus, several sensory nuclei, and so forth. Furthermore, corticospinal tract fibres discontinue in multiple locations throughout the spinal cord, which also includes the posterior horn (that is usually a part of the process that handles sensory input that is received).

These wide and varied linkages imply that the corticospinal tract’s capabilities are highly probable to be diversified as well and that trying to define it as having mobility as its only task is an underestimation of its capabilities and control.

Nerve cells emerge from the cortex, align, and slide down via the internal capsule (which is a white matter passageway, positioned between the basal ganglia and the thalamus). This is clinically relevant because the inner capsule is especially vulnerable to contraction from hemorrhagic ruptures, a process characterized as a ‘capsular stroke’. A descending tract lesion can result from such an incident. The nerve cells then move through the crus cerebri that is present in the midbrain, the pons, and then into the medulla upon passing through the internal capsule. The tract splits in the most inferior (caudal) portion of the medulla:

The lateral corticospinal tract fibres decussate or cross over to the other region of the Central nervous system (Figure 3). After that, they come down into the spinal cord, eventually reaching the ventral horn, at every segmental degree. Lower nerve fibers travel from the ventral horn to deliver the muscle fibres.

The anterior corticospinal tract continues to come down into the spinal cord ipsilaterally. It then intersects and discontinues in the cervical and upper thoracic segmental stages of the ventral horn. 

Complications involving corticospinal tract:

After a specific injury to the corticospinal tract, people commonly gain back the capacity to make simplistic movements (for example trying to reach) after some time, but they may not be able to bounce back the capacity to produce specific finger movements.

Supranuclear lesions:

It is another name for upper motor nerve cell lesions. Corticospinal Tract Injuries can result in some problems.

Because they run just about the entire length of the central nervous system, the pyramidal tracts are vulnerable to harm or injury.  As stated earlier, they are incredibly susceptible as they cross through the internal capsule, which is a regular sight of cerebrovascular accidents (CVA).

Side effects will occur on the contralateral part of the body when there is only a unilateral malformation of the right or left corticospinal tract. The following are the primary symptoms of an upper motor neuron lesion:

1. Hyperreflexia is accompanied by greater muscle reflexes.
2. Weakness of body muscles
3. Babinski sign:  hallux outgrowth in reaction to blunt excitation of the soles of feet. 
4. Clonus is defined as unintentional, repeated muscle contractions.
5. Hypertonia is associated with greater muscle tone.

However, other forms of injury can also lead to malfunctioning corticospinal tract controlled motor functions. Regarding spinal cord damage or harmful incident, both enforced by will (sensory and motor) and unintentional command can be compromised, with the point of restoration varying according to the intensity of the lesion. Motor shortcomings will be ipsilateral to the location of the lesion because the corticospinal tract has already decussated. 

Conclusion:

The Corticospinal Tract (also shortly referred to as CST), further recognized as the Pyramidal Tract, is a network of axons that connects the spinal cord to the cerebral cortex. It includes many structural and functional components of the neuromuscular system as it runs downwards towards the spinal cord, such as the decussation of pyramids, motor neurons, the cerebral peduncles, precentral gyrus, and skeletal muscle cells which are the target tissues that receive the motor information sent from the higher parts of the brain. In humans, it demonstrates actual command of motor skills and is most simply in command of perfect, digital movement patterns.  In case of injury, side effects will be exhibited on the contralateral part of the body when there is only a unilateral malformation of the right or left corticospinal tract.

Reference list

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