Corticobulbar tract

The corticobulbar (or corticonuclear) tract is a white matter pathway connecting the cerebral cortex to the brainstem. The 'bulb' is an archaic term for the medulla oblongata; in modern clinical usage, it sometimes includes the pons as well. The word 'bulbar' therefore refers to the nerves and tracts connected to the medulla, and also by association to the muscles thus innervated, those of the tongue, pharynx and larynx.

Anatomy

The corticobulbar tract originates in motor cortex of the frontal lobe, just superior to the lateral fissure and rostral to the central sulcus. The tract descends through the genu of the internal capsule and a few fibers in the posterior limb of the internal capsule, as it passes from the cortex down to midbrain. In the midbrain, the internal capsule becomes the cerebral peduncles. The white matter is located in the ventral portion of the cerebral peduncles, called the crus cerebri. The middle third of the crus cerebri contains the corticobulbar and corticospinal fibers. The corticobulbar fibers exit at the appropriate level of the brainstem to synapse on the lower motor neurons of the cranial nerves.

Function

The corticobulbar tract is composed of the upper motor neurons of the cranial nerves. The muscles of the face, head and neck are controlled by the corticobulbar system, which terminates on motor neurons within brainstem motor nuclei. This is in contrast to the corticospinal tract in which the cerebral cortex connects to spinal motor neurons, and thereby controls movement of the torso, upper and lower limbs.

The corticobulbar tract innervates cranial motor nuclei bilaterally with the exception of the lower facial nuclei which are innervated only unilaterally (below the eyes) and CN XII, which is innervated unilaterally as well. Both the lower part of CN VII and CN XII are innervated by the contralateral cortex. Among those nuclei that are bilaterally innervated a slightly stronger connection contralaterally than ipsilaterally is observed. The corticobulbar tract directly innervates the nuclei for cranial nerves V, VII, XI, and XII. It does not innervate nuclei for nerves III, IV, and VI because these are mediated by cortical projections and yoked together by the MLF, medial longitudinal fasciculus. It also contributes to the motor regions of nerves X and IX in the nucleus ambiguus.

See also

• Upper motor neuron
• Upper motor neuron lesion

External links

• Brainstem at UWisc 07CNXII
• http://thebrain.mcgill.ca/flash/i/i_06/i_06_cl/i_06_cl_mou/i_06_cl_mou.html
• http://pathology.mc.duke.edu/neuropath/nawr/motor-systems.html