| Corticobulbar tract | |
|---|---|
 Corticobulbar tract. | |
 Components and location of the corticobulbar tract. | |
| Details | |
| Identifiers | |
| Latin | tractus corticonuclearis, tractus corticobulbaris |
| NeuroNames | 1319 |
| Anatomical terms of neuroanatomy | |
Thecorticobulbar (orcorticonuclear)tract is a two-neuronwhite matter motor pathway connecting themotor cortex in thecerebral cortex to themedullary pyramids, which are part of thebrainstem'smedulla oblongata (also called "bulbar") region, and are primarily involved in carrying themotor function of the non-oculomotor cranial nerves, like muscles of the face, head and neck. The corticobulbar tract is one of thepyramidal tracts, the other being thecorticospinal tract.
The corticobulbar tract originates in the primarymotor cortex of thefrontal lobe, just superior to thelateral fissure androstral to thecentral sulcus in theprecentral gyrus (Brodmann area 4).[1] The corticobulbar tract however also includes fibres from disparate regions from across the cerebral cortex (not limited to the frontal lobes).[2]
The tract descends through thecorona radiata and then thegenu of theinternal capsule (with a few fibers in the posterior limb of theinternal capsule) to themidbrain. In the midbrain, the internal capsule becomes thecerebral peduncles. The white matter is located in the ventral portion of the cerebral peduncles, called thecrus 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. In addition to endings in these motor neurons, fibers of the corticobulbar tract also end in the sensory nuclei of the brainstem includinggracile nucleus,cuneate nucleus,solitary nucleus, and all trigeminal nuclei.[1]
Only 50% of the corticobulbar fibers decussate, in contrast to those of thecorticospinal tract where most decussate:[1] cranial nerve nuclei innervating skeletal muscle thereby generally receive bilateral first-order neuron innervation (i.e. from both hemispheres).[2]
The corticobulbar tract is composed of theupper motor neurons of thecranial nerves. The muscles of the face, head and neck are controlled by the corticobulbar system, which terminates onmotor neurons within brainstem motornuclei. This is in contrast to thecorticospinal tract in which the cerebral cortex connects to spinal motor neurons, and thereby controls movement of the torso, upper and lower limbs. Fibers that end in the sensory nuclei of the brainstem are thought to enhance or inhibit sensory transmission across various sensory nuclei. This allows for the selective attention or inattention towards various stimuli.[1]
The corticobulbar tract innervates cranial motor nuclei bilaterally with the exception of the lower facial nuclei (which innervates facial muscles below the eyes) and the genioglossus muscle, which are innervated only unilaterally 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 nervesV,VII,IX, andXII. The corticobulbar tract also contributes to the motor regions of cranial nerveX in thenucleus ambiguus.