Descending tracts of the spinal cord

Classification

Descending pathways can be classified based on their somatotopic organization (lateral vs. medial motor systems) or based on whether the control is voluntary or involuntary (pyramidal vs. extrapyramidal systems)

In the first classification, lateral tracts travel in the lateral columns of the spinal cord. They synapse on more laterally located motor neurons, in the ventral horn of the spinal cord. Medial motor systems descend in the anteromedial aspect of the spinal cord. They synapse on medial ventral horn neurons.

The lateral motor system includes the following tracts:

• Lateral corticospinal tract; responsible for voluntary movement of the limbs.
• Rubrospinal tract; augments the activity of the flexor muscles and inhibits the action of the extensor (antigravity) muscles.

The medial motor systems comprise the following pathways:

• Anterior corticospinal tract; controls the voluntary movement of the axial and girdle muscles.
• Vestibulospinal tract; controls body balance. 
• Reticulospinal tract; regulates the function of spinal reflex arcs and maintains muscle tone when standing and walking.
• Tectospinal tract; responsible for the blinking reflex and eye pursuit movements when following an object.

In the second classification, descending tracts are divided into pyramidal and extrapyramidal systems. The pyramidal system consists of the corticospinal and corticobulbar tracts. These are responsible for voluntary movement of the human body. 

Meanwhile, the extrapyramidal system consists of numerous tracts that descend from various cerebral cortex or brainstem structures. These are the rubrospinal, vestibulospinal, reticulospinal and tectospinal. This system is under unconscious control and is responsible for the modulation or regulation of the same movements.

Corticobulbar (corticonuclear) tract

The corticobulbar tract is part of the pyramidal system. It is sometimes also called the corticonuclear tract. The tract originates from upper motor neurons (UMN) located in the lateral aspect of the primary motor cortex. The arising fibers then converge to pass through the corona radiata, the internal capsule, the cerebral peduncle of the midbrain and into the brainstem. Here, the UMNs terminate on the motor nuclei of cranial nerves V, VII, IX, XI (and indirectly on cranial nerves III, IV and VI) within the brainstem. These motor nuclei act as the lower motor neurons (LMN) of the pathway.

By carrying motor signals from the primary motor cortex to the motor cranial nerves, the corticobulbar tract is able to provide voluntary control of the muscles of the face, head and neck, as well as the muscles involved in swallowing, phonation and facial expression. More precisely,  the motor component of the trigeminal nerve supplies muscles of mastication. The facial nerve supplies the muscles of facial expression. The glossopharyngeal nerve innervates the muscles of the pharynx and larynx. While the accessory nerve supplies the sternocleidomastoid and trapezius muscles.

Corticospinal tract

The corticospinal tract is a white matter motor pathway running from the cerebral cortex to the spinal cord. This pathway is responsible for the voluntary movements of the limbs and trunk.

The path starts in the motor cortex, where the bodies of the first-order neuron lie (pyramidal cells of Betz). These specialized neurons descend via the corona radiata, the internal capsule (a white matter structure, located between the thalamus and the basal ganglia), the cerebral peduncle of the midbrain, the pons and then into the medulla.

Once in the anterior aspect of the lower medulla, the majority of the corticospinal fibers decussate or cross over to the opposite side, this is known as pyramidal decussation. At this point, the corticospinal tract divides into two: the lateral corticospinal tract and the anterior corticospinal tract. The crossed fibers enter the lateral corticospinal tract, while the uncrossed fibers form the anterior corticospinal tract. Both tracts descend through the spinal cord and synapse with lower motor neurons in the anterior gray horn on the same side. The lower motor neurons then leave the spinal cord through the ventral root to form peripheral nerves which innervate the musculature of the body.

Lateral corticospinal tract

The lateral corticospinal tract (LCST) is the largest descending motor pathway. It begins in the cerebral cortex, receiving a range of inputs from the primary motor cortex, premotor cortex and supplementary motor areas. The tract also receives nerve fibers from the somatosensory area, which plays a role in regulating the activity of the ascending tracts.

After exiting the motor cortex, fibers from the upper motor neurons (UPN) converge in the corona radiata, then pass inferiorly to form a white matter structure of the brain known as the internal capsule. The internal capsule is located between the basal ganglia and thalamus. After passing through the anterior two-thirds of the posterior limb of the internal capsule, the fibers continue to pass down inferiorly, through the center of the cerebral peduncle of the midbrain, before entering the pons and medulla. As they pass through the caudal medulla, the majority of fibers (80%) decussate (pyramidal decussation) to the contralateral side. The crossed fibers form the lateral corticospinal tract while the uncrossed enter the anterior corticospinal tract.

The LCST then descends in the lateral funiculus along the entire spinal cord, synapsing with second-order, lower motor neurons (LMN) in the ventral horn at each level of the spinal cord. From here, impulses generated in the second-order motor neurons pass through the anterior roots of the spinal nerve, through the peripheral nerve plexuses (in the cervical, brachial and lumbosacral regions), and along the individual peripheral nerves on their way to the skeletal muscles. The neural impulses are conveyed to the muscle cells through the motor endplates of the neuromuscular junction (myotomes), resulting in muscle movement.

In the cerebral cortex, the fibers arise from the fifth layer of the cerebral cortex, composed mainly of specialized giant pyramidal cells of Betz (upper motor neuron). Their fibers preserve a somatotopic organization, called the motor homunculus, which represents a map of brain areas dedicated to motor processing for different anatomical divisions of the body. This somatotopic organization is preserved all along the corticospinal tracts, whereby the more medial part of the LCST is responsible for the cervical region and the more lateral aspects supply the lower thoracic, lumbar and sacral regions, respectively.

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Anterior corticospinal tract

The anterior corticospinal tract emerges from the un-decussated fibers of the corticospinal tract, at the level of the bulbomedullary junction. The tract travels inferiorly in the anterior funiculus of the spinal cord. The fibers of the anterior corticospinal tract cross over (decussate) at the spinal level they innervate, where they then synapse with lower motor neurons in the anterior horn. Their axons form peripheral nerves that extend out to the target muscle, synapsing at the motor endplate, also known as the neuromuscular junction. 

In contrast to the lateral corticospinal tract, which controls the fine movements of the arms and legs, the anterior corticospinal tract controls the actions of axial muscles (of the trunk).

Extrapyramidal tracts

The remaining descending tracts are grouped into the extrapyramidal system: reticulospinal, vestibulospinal, rubrospinal and tectospinal. These tracts facilitate the unconscious, reflexive control of muscles from various brainstem structures to postural or anti-gravity muscles.

Each tract has a different origin, which can easily be deduced from its name. For example, the term rubrospinal tract is formed by combining “rubro-” which means “red” in Latin, and “-spinal”  or spine. Thus the origin of this tract is the red nucleus and it terminates in the lower motor neurons of the spine.

Rubrospinal tract

The rubrospinal tract is a descending motor pathway that originates in the red nucleus, located on each side of the midbrain tegmentum at the level of superior colliculi. Their axons immediately cross the midline and descend through the pons and medulla oblongata to enter the lateral funiculus of the spinal cord. The fibers terminate by synapsing with internuncial neurons in the anterior gray column, within the laminae V, VI and VII, at all levels of the spinal cord.

Like all extrapyramidal tracts, the rubrospinal tract is involved in the mediation of involuntary movement. In particular, this tract is responsible for the regulation of flexion and extension tone of large group muscles, as well as fine motor control. In humans, this tract is relatively small, and its clinical importance is uncertain. It may participate in assuming pyramidal tract functions, after injury, and may play a role in decorticate posturing of the upper extremities, which is typically seen in lesions above the red nucleus.

Vestibulospinal tracts

The vestibulospinal tracts (VST) are essential pathways by which higher centers of the brain maintain the balance of the body. These tracts are vital for several reflex actions performed by the body in order to retain its axial position relative to gravity. Their activity depends upon the position of the head and neck, aided by labyrinthine receptors as well as sensory information from the cerebellum. 

There are two vestibulospinal tracts; the lateral vestibulospinal tract and the medial vestibulospinal tract. Both are responsible for antigravity muscle tone in response to the head being tilted to one side and are indirectly influenced by the cerebellum and the labyrinthine system. Broadly, the lateral vestibulospinal tract arises from the lateral vestibular nucleus (Deiters) and descends along the periphery of the anterior white column of the spinal cord. The medial vestibulospinal tract, however, arises from the medial vestibular nucleus and descends within the medial longitudinal fasciculus of the brainstem. The length of these two vestibulospinal tracts differs, with the lateral vestibulospinal tract descending along the entire spinal cord, while the medial vestibulospinal tract stops at the mid-thoracic.  

The vestibular system also receives substantial input from the semicircular canals, otolith organs and the fastigial nucleus, as well as proprioceptive information from the spinal cord in order to maintain the body’s equilibrium.

Reticulospinal tracts

The reticulospinal tract is an essential component of the extrapyramidal system. Together with the vestibulospinal tracts, they maintain the body’s balance and make postural adjustments. This pathway starts in the pontine reticular formation and extends along the entire length of the spinal cord.

The reticular formation (RF) is a set of interconnected nuclei dispersed throughout the brainstem. Besides its main vital functions of cardiovascular and respiratory control, consciousness, circadian rhythm, sleep-wake cycles, it plays an important role in coordination of voluntary movement. This is done with the help of the corticoreticulospinal pathway (system).

The reticulospinal tract is part of the larger corticoreticulospinal system. This system consists of the following structures:

• Corticoreticular fibers
• Pontine (medial) reticulospinal tract
• Medullary (lateral) reticulospinal tract

The corticoreticular fibers arise from the premotor cortex and supplementary motor area. These fibers descend into the brainstem and synapse bilaterally with the neurons of the pontine and medullary reticular formation. In doing so, these nuclei give rise to the reticulospinal tracts. The medullary reticulospinal tract arises from the medullary nuclei of the reticular formation, located in the rostral medulla of the brainstem. The pontine reticulospinal tract, however, arises from the pontine nuclei of the reticular formation, found in the ventral pons. 

After passing through the medulla, the pontine reticulospinal tract descends uncrossed within the anterior white column of the spinal cord. Its fibers terminate by entering the anterior gray horn of the spinal cord where they synapse with third-order neurons. The medullary reticulospinal tract, however, descends in the lateral white column of the spinal cord. They terminate the same way as the pontine reticulospinal tract. These tracts synapse with alpha and gamma motor neurons that supply paravertebral and limb extensor musculature.

Both the medial and lateral reticulospinal tracts control muscle tone and reflex activity. These pathways fit into the pattern of reciprocal inhibition, which means that during the contraction of flexor muscles, there is a simultaneous relaxation of the antagonistic extensor muscles. Muscle tone, balance maintenance and postural changes form a necessary background upon which voluntary movement is executed, which explains why these pathways have numerous synapses with the lower motor neurons.

Tectospinal tract

The tectospinal tract (or colliculospinal tract) connects the midbrain tectum and cervical regions of the spinal cord. Its function is to mediate reflex postural movements of the head in response to visual and auditory stimuli. Therefore, it has been assumed to manage postural change on the visual information received to the superior colliculus.

This tract originates in the superior colliculus, where it receives information from the retina and cortical visual association areas. The fibers then project to the contralateral side of the midbrain (decussating dorsal to the mesencephalic duct) and descend within the medial longitudinal fasciculus into the ventral funiculus of the cervical spinal cord. The tectospinal tract terminates on the neurons within laminae VI-VII.

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