It runs from the motor cortex of the precentral gyrus through the genu of the internal capsule and the cerebral peduncles into the brainstem, where its fibres leave at each level to end on the motor nuclei of the cranial nerves in the pons and medulla. It moves the face, jaw, throat and tongue. [1][2]
A tract has no shape of its own in this atlas; lit: the parts it runs through, in order.
The 3D brain needs JavaScript. The steps below still say where the corticobulbar tract runs, in order.
The brain starts seen from the left, a little from the front and below, where the tract runs down the front of the brainstem to its last nuclei in the medulla. Step through lights one step at a time, about 1.2 s each, top to bottom; tap a step below to light it. The lit step is in the bright highlight colour, the steps before it stay lit in a paler tint of it and the steps still ahead are grey. Drag to turn, pinch or scroll to zoom. Step through is off because your device asks for reduced motion; tap each step instead.
The tract itself is not drawn, nor are the nuclei it ends on: each step lights the whole part it runs through. The atlas has no shape for any cranial-nerve nucleus, so the pons and medulla stand in for the nuclei inside them. [3][1]
The route, in 5 steps
The tract starts in the primary motor cortex (Brodmann area 4) of the precentral gyrus, in the part just above the lateral fissure and in front of the central sulcus, low on the side of the brain where the face, jaw and tongue are mapped; it also takes fibres from other parts of the cortex. Only the precentral gyrus is lit. [1][4]Where is the precentral gyrus?
Its fibres descend through the corona radiata and gather in the genu, the bend of the internal capsule between its anterior and posterior limbs, with only a few in the posterior limb, where the corticospinal fibres run. [1][5]Where is the internal capsule?
In the midbrain the capsule's fibres become the cerebral peduncles; the corticobulbar fibres share the middle third of the crus cerebri, the front part of each peduncle, with the corticospinal fibres, and run on beside them into the brainstem. [1][6]Where are the cerebral peduncles?
Fibres leave the tract at the level of each nucleus. In the pons they end on the trigeminal motor nucleus, in the upper pons, which works the chewing muscles through the trigeminal nerve, and on the facial motor nucleus, which drives the muscles of facial expression through the facial nerve. Not separate shapes in this atlas; lit: the part that holds them, the pons. [1][7][8]Where is the pons?
The last fibres end in the medulla: on the nucleus ambiguus, whose motor neurons work the soft palate, pharynx and larynx for speech and swallowing through the glossopharyngeal and vagus nerves, and on the hypoglossal nucleus near the midline, whose hypoglossal nerve moves the tongue. Not separate shapes in this atlas; lit: the part that holds them, the medulla oblongata. The tract ends here, in the brainstem. [1][9][10][11]Where is the medulla oblongata?
It is the other pyramidal tract beside the corticospinal tract: the corticospinal tract ends on motor neurons of the spinal cord for the trunk and limbs, the corticobulbar tract on motor nuclei in the brainstem for the face, head and neck. [1][12]
It reaches the motor nuclei of cranial nerves V, VII, IX and XII directly, and the motor part of X in the nucleus ambiguus; the nerves that move the eyes (III, IV and VI) are not on that list. Every nerve and its nuclei are on the cranial nerves page. [1]
Only about half of its fibres cross the midline, so most of these nuclei take commands from both hemispheres, a little more from the opposite one. Two take them from the opposite hemisphere only: the part of the facial nucleus for the muscles below the eyes, and the nucleus for the genioglossus, the muscle that pushes the tongue forward. [1]
Sources simplify that crossing differently: OpenStax A&P 2e describes the corticobulbar axons as reaching motor nuclei on the same side, while Wikipedia's account has about half of them crossing. [2][1]
“Bulbar” refers to the medulla oblongata, once called the bulb. Not all of its fibres end on motor nuclei: some end on the sensory relays of the brainstem (the gracile, cuneate and solitary nuclei and the trigeminal sensory nuclei), where they are thought to turn incoming signals up or down. [1]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); it has no shape for tracts, so a pathway is shown by the parts it runs through.