Corticobulbar & Corticospinal Pathways
Movement is an important characteristic of animals. For years, this ability has enabled individuals to avoid danger, communicate and express creative and artistic ideations. The brain is able to regulate these functions through specific pathways. Cells of the cerebral hemispheres project varying nerve fibres that travels through the brainstem and terminate in the medulla oblongata (corticobulbar or corticonuclear tract) and in the spinal cord (corticospinal or pyramidal tract).
The prefix cortico- refers to structures originating in or related to the cerebral cortex. While the suffix -spinal is pretty self-explanatory (relating to the spine), -bulbar and –nuclear have more subtle meanings. The former is an archaic term used to refer to the medulla oblongata, while the latter refers to cranial nerve nuclei. Therefore, the corticospinal tracts are those fibers that terminate at various levels in the spinal cord. However, the terms corticobulbar and corticonuclear are used interchangeably to refer to the fibers that terminate at or adjacent to cranial nerve nuclei and other tracts in the mesencephalic part (midbrain, pons and medulla oblongata) of the brain.
Through these tracts, the cortex is able to modify spinal reflexes and control movement, posture and muscle tone. This article will focus on the anatomy of the corticobulbar and corticospinal pathways with specific reference to their general and special motor involvement. Additionally, the article will also look at clinical significance of damage to either aforementioned pathways.
The corticobulbar and corticospinal tracts are the axonal projections of the soma of giant pyramidal cells of Betz located in layer V of the cerebral cortex. 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). These fibers enter the genu and posterior limb of the internal capsule after passing through the subcortical white matter. The genu contains corticobulbar fibers from the primary cortex, while the posterior limb houses the corticospinal tract and a few other tracts of the corticobulbar tract. The fibers of the corticospinal tract are arranged in a somatotopic (specific areas of the body are mapped to specific areas of the cortex; see human homunculus) manner. Therefore, those fibers specific for upper limb muscle function are anterior to those concerned with the trunk and lower limbs.
After leaving the internal capsule, the fibers travel ventrally to enter the crus cerebri (part of the basis pedunculi or peduncles of the midbrain; peduncle = stalk-like structure). The crus cerebri, along with the tegmentum (area ventral to the cerebral aqueduct and posterior to the substantia nigra) form the basis pedunculi. The crus cerebri is an externally convex structure that has both corticospinal and corticobulbar fibers in the middle two-thirds of the structure. For completion, the lateral one-sixth and medial one-sixth of the crus cerebri has temporopontine and frontopontine fibers, respectively.
As the tracts descend through the crus cerebri, they travel ventrally and become separated by transverse pontocerebellar fibers. It is at this level, the two tracts begin to diverge. The corticonuclear fibers decussate and go towards their respective cranial nerve nuclei. However, the corticospinal tract maintains its course caudally where it forms the pyramids.
The corticobulbar (or corticonuclear) pathway is responsible for regulating brain stem activity. The nomenclature encompasses the impact of the cortex on the overall activity of the pons and medulla oblongata as well as the cranial nerve nuclei located there. This article will focus on the impact of the cortices on the activity of the motor component of cranial nerve nuclei, namely oculomotor (CN III), trochlear (CN IV), motor division of trigeminal (mCN V), abducens (CN VI), facial (CN VII), glossopharyngeal (IX), vagus (CN X), spinal accessory (CN XI) and hypoglossal (CN XII) nerves.
As previously stated, the cell bodies of the corticonuclear fibers are located in Brodmann areas 3, 1, 2 (postcentral gyrus), areas 6, 8 (frontal eye fields) and area 4 (precentral gyrus). In addition to area 4 fibers that occupy the genu of the internal capsule, fibers originating from areas 8, 6 tend to occupy the caudal region of the anterior limb of the internal capsule. Additionally, those derived from areas 3, 1, 2 occupy the rostral part of the posterior limb. The fibers then follow the above stated course before branching off to the designated cranial nerve nuclei.
The corticobulbar tract indirectly regulates CN III, IV and VI via its impact on the rostral interstitial nucleus of the medial longitudinal fasciculus and the paramedian pontine reticular formation. The former is located in the midbrain at the level of the superior colliculus, lateral to the CN III and cerebral aqueduct; it regulates vertical gaze. The latter is located in the pons, superior to CN VI, medial to the mCN V, lateral to the cerebral aqueduct and caudal to CN IV; it influences horizontal gaze.
The other motor cranial nerves are regulated either directly or indirectly by fibers from the precentral gyrus. Most of the cranial nerve nuclei receive predominantly contralateral innervation. However, some nuclei (such as mCN V, CN IX, CN X & CN XII) receive some accessory ipsilateral projections from the corticobulbar tract. In addition to receiving direct bilateral input from the corticobulbar tract, the contralateral mCN V also receives indirect innervation via the reticular formation. The nucleus ambiguous acts as an intermediate nucleus between the corticonuclear tract and CN IX & X.
CN VII can be divided into regions that supply the upper and lower face. The corticonuclear fibers directly innervate the upper division of each nucleus bilaterally. However, it mostly innervates the contralateral lower division of the nucleus, with only few fibers going to the ipsilateral inferior part of the nucleus. Indirect innervation is achieved with the help of adjacent fibers of the reticular formation.
Unlike the corticonuclear tract, fibers of the corticospinal tract have a distinct somatotopic arrangement. As previously stated, the corticospinal tract is derived from area 4 and area 3, 1, 2. The somatotopic representation of the primary motor area (area 4) shows proximal areas (face, upper limbs) laterally and distal areas (trunk and lower limbs) medially. As the fibers progress into the posterior limb of the internal capsule, those fibers destined for proximal limbs are anterior to those destined for the trunk and lower limbs. At the level of the crus cerebri, the fibers are rearranged such that the upper limbs are represented medially, followed by the trunk then lower limbs laterally. This medial to lateral arrangement of the corticospinal tract continues caudally as the tract traverses the pons, pyramids of the medulla oblongata and spinal cord. It should be noted that the arrangement is not as clear in the pons as it is in the crus cerebri, medulla oblongata and spinal cords.
At the level of the foramen magnum, a large number of corticospinal fibers cross the midline and continue their descent in the lateral funiculus of the spinal cord as the lateral corticospinal tract. Those fibers that did not decussate at the pyramidal decussation continue caudally in the ventral funiculus as the ventral corticospinal tract.
In the lateral funiculus, the lateral corticospinal tract is medial to the dorsal spinocerebellar tract (in the cervical and thoracic regions) and anterolateral to the dorsal horn. It can be found throughout the spinal cord, gradually getting smaller, before terminating ipsilaterally at the level of S4. Unlike the larger lateral corticospinal tract, the smaller ventral corticospinal tract ends at about the level of T6. Its course in the ventral funiculus is adjacent to the ventral median fissure of the spinal cord. Eventually, the fibers of the ventral corticospinal tract will decussate via the ventral white commissure to terminate at the opposite neurons. Overall, the fibers originating from area 4 terminate in the interneurons of Rexed laminae VI – IX, while those originating from area 3, 1, 2 (postcentral gyrus) terminate primarily in the interneurons of Rexed laminae IV & V.