Corticobulbar and corticospinal tracts
The pyramidal tract provides voluntary control of muscular movements. It consists of two distinct pathways, the corticobulbar tract and the corticospinal tract.
The corticospinal tract carries motor signals from the primary motor cortex in the brain, down the spinal cord, to the muscles of the trunk and limbs. Thus, this tract is involved in the voluntary movement of muscles of the body.
The corticobulbar tract carries efferent, motor, information from the primary motor cortex to the muscles of the face, head and neck. It does this by synapsing with motor cranial nerves in the brainstem. Therefore the corticobulbar tract is responsible for innervating the muscles of the face, head and neck, as well as the muscles involved in swallowing, phonation and facial expression.
This article will describe the anatomy and function of the corticobulbar and corticospinal tracts.
- Corticobulbar (corticonuclear) tract
- Corticospinal tract
- Clinical aspect
Corticobulbar (corticonuclear) tract
The corticobulbar tract is part of the pyramidal system. It is sometimes also called the corticonuclear tract.
This tract originates from upper motor neurons (UMN) located in the lateral aspect of the primary motor cortex. Specifically, this tract originates from the V-th layer of the cerebral cortex, where the giant pyramidal cells of Betz reside. Most of these cell bodies are found in Brodmann area 4 (primary motor), area 6 (premotor cortex), areas 3, 1, 2 (postcentral gyrus) and area 5 (parietal cortex). The arising fibers converge to pass within the corona radiata, the knee “genu” of the internal capsule, then continue inferiorly through the cerebral peduncle of the midbrain, to reach the brainstem.
While descending through the brainstem, the UMNs terminate on motor nuclei of certain cranial nerves within the midbrain, pons and medulla. These motor nuclei are considered the lower motor neurons (LMN) of the corticobulbar pathway, they carry efferent signals directly to the muscles of the face, head and neck. In the pons, fibers dedicated to the facial nerve leave the corticobulbar tract, cross the midline right above the facial nuclei and synapse with its motor nuclei. A similar pathway is followed by fibers connecting with the motor nuclei of the trigeminal nerve, at the level of the mid-pons. In the same manner, corticobulbar tract fibers project to the glossopharyngeal and accessory nerves by synapsing with their respective motor nuclei in the medulla oblongata. The corticonuclear tract also indirectly affects the nuclei of oculomotor, trochlear and abducens nerves within the midbrain, through intermediate structures such as the medial longitudinal fasciculus and reticular formation.
The corticobulbar tract provides voluntary control over the muscles of the face, head and neck. This is in contrast to the corticospinal tract which controls the movement of the torso and limbs.
Specifically, the corticobulbar tract carries upper motor neuron input to the motor nuclei of the trigeminal, facial, glossopharyngeal and accessory cranial nerves. This then supplies voluntary control over a number of head and neck functions;
- Trigeminal nerve (CN V): the motor component of trigeminal nerves supplies the muscles of mastication.
- Facial nerve (CN VII): the motor nucleus has both dorsal and ventral regions. Neurons in the dorsal region innervate the muscles of the upper face, while neurons in the ventral region innervate the muscles of the lower face.
- Glossopharyngeal nerve (CN IX): innervates the muscles of the pharynx and larynx.
- The accessory nerve (CN XI): supplies the sternocleidomastoid and trapezius muscles.
The corticobulbar tract also influences the activity of muscles supplied by the third, fourth and sixth cranial nerves, indirectly modulating their activity. It does this by synapsing with intermediate structures from the brainstem such as the medial longitudinal fasciculus and reticular formation.
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The corticospinal tract is a motor pathway that carries efferent information from the cerebral cortex to the spinal cord. It 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 neurons lie. These specialized upper motor neurons are called the pyramidal cells of Betz. Axons of Betz cells descend and converge to join a white matter sheet within each cerebral hemisphere called the “corona radiata''. The fibers then pass inferiorly through the anterior two-thirds of the posterior arm of the internal capsule (a white matter structure, located between the thalamus and the basal ganglia), through the cerebral peduncles of the midbrain, the pons and into the medulla.
In the anterior aspect of the lower medulla, the majority of corticospinal fibers decussate (pyramidal decussation). The crossed fibers form the lateral corticospinal tract while the uncrossed fibers enter the anterior corticospinal tract. The former is responsible for providing voluntary motor information to the muscles of the limbs while the latter supplies the axial muscles of the trunk. Both tracts run along the spinal cord, synapsing with lower motor neurons in the anterior gray horn on the same side. The lower motor neurons leave the spinal cord through the ventral root and form peripheral nerves which innervate the musculature of the body.
More details about the pyramidal tracts are provided below:
Anterior corticospinal tract (ACST)
The anterior corticospinal tract (also called the "Bundle of Turck”) emerges from the un-decussated fibers of the corticospinal tract, at the level of the bulbomedullary junction. The UMN axons descend in the anterior funiculus of the spinal cord. At their spinal cord level of innervation, the axons cross over to the contralateral side. Thus this is their point of decussation. The UMNs then synapse in the anterior horn with lower motor neurons. Their axons pass through the anterior root of the spinal nerve and terminate in the endplate of skeletal muscles. The neural impulses are conveyed to the muscle cells through the motor endplates of the neuromuscular junction (myotomes), resulting in muscle movement.
The anterior corticospinal tract is usually small, varying inversely in size with the lateral corticospinal tract. Lying close to the anterior median fissure, it is present only in the upper part of the spinal cord, gradually diminishing in size as it descends. The tract ends around the level of the mid-thoracic region.
The anterior corticospinal tract is primarily responsible for gross and postural movement of the trunk and proximal musculature (axial muscles).
Lateral corticospinal tract
The lateral corticospinal tract (LCST) is the largest descending motor pathway in the human body, it spans the entire length of the spinal cord, eventually supplying motor signals to all the skeletal muscles of our upper and lower limbs.
As the upper motor neurons (UMN) fibers pass through the caudal medulla, the majority of fibers (80%) decussate (pyramidal decussation) to the contralateral side and enter the lateral corticospinal tract. The tract 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. 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 individual peripheral nerves on their way to the skeletal muscles.
The lateral corticospinal tract is responsible for the voluntary movement of the contralateral upper and lower limbs.
The upper motor neurons of the LCST, the giant pyramidal cells of Betz, preserve a somatotopic organization, called the motor homunculus. This homunculus represents the somatotopic motor distribution of the human body, along the motor cortex. From medial to lateral we see the motor processing regions corresponding to the structures of the face, then hand and arm, next the shoulder, head, neck and trunk, the lower limb and most laterally, the genitalia. This somatotopic organization is preserved all along the corticospinal tracts, whereby the more medial part of the LCST is responsible for innervation of the cervical region and the lateral part of the tract sends efferent output to the lower thoracic, lumbar and sacral regions, respectively.
The conceptual approach to the motor system as comprising an upper and lower motor neuron is most important for clinical practice. Lesion of an upper motor neuron (UMNL) would cause central paralysis (spastic paralysis) while lesion to a lower motor neuron (LMNL) results in peripheral paralysis (flaccid paralysis). This is due to the inhibitory effect that UMNs have on LMNs. In the absence of upper control, the lower motor neuron exhibits a hyperreactivity. While a lesion in a LMN will cause interruption to skeletal muscle tonus control.
Upper motor neuron injury results in the following:
- Spastic weakness (paresis) when voluntary movement is attempted
- Increased muscle spasticity
- Exaggerated tendon reflexes
- Positive pathological pyramidal signs (e.g. Plantar reflex “Babinski”)
Lower motor neuron damage results in:
- Flaccid weakness (paresis) of the limb
- Loss of muscle tone
- Loss of tendon reflexes
- Atrophy of the muscle
- Fasciculation (random muscles twitches)
Another important clinical aspect of these two tracts is the level of the crossing. Decussation of the corticospinal tract occurs at the junction of the medulla oblongata and spinal cord while the corticobulbar tracts decussate above each relevant cranial nerve nuclei. Thus lower motor neurons of the musculature of the body receive motor input mostly from the contralateral hemisphere, the lower motor nuclei of cranial nerves receive bilateral innervation. With regards to the corticospinal tract this means that injury above the pyramidal decussation leads to contralateral motor deficits. Whereas damage below the pyramidal decussation will result in ipsilateral motor deficits.
The etiology can vary significantly. These injuries can be caused by trauma, tumors, infection or degenerative diseases.