Corticobulbar tract
Encyclopedia
The corticobulbar 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.
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.
. The muscles of the face, head and neck are controlled by the corticobulbar system, which terminates on motor neuron
s 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
.
The corticobulbar tract is the major input of the lower motor neurons in the brainstem, that is why a lesion to the corticubulbar tract results in lower facial symptoms because the input is mostly unilateral.
White matter
White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...
pathway connecting the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
to the brainstem. The 'bulb' is an archaic term for the medulla oblongata
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
; in modern clinical usage, it sometimes includes the pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...
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 cortexMotor cortex
Motor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions.-Anatomy of the motor cortex :The motor cortex can be divided into four main parts:...
of the frontal lobe
Frontal lobe
The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...
, just superior to the lateral fissure and rostral
Rostral
Rostral can refer to:* Anatomical location or direction, towards the rostrum .* Rostral bone, in ceratopsian dinosaurs* Rostral column, a monumental scaled column, frequently decorated with ship bows* Rostral scale, in scaled reptiles...
to the central sulcus
Central sulcus
-External links:* via the Neuroscience Information Framework...
. The tract descends through the genu
Genu
Latin for "knee", genu is a term commonly encountered in studies of anatomy. It is used to refer to several anatomical structures, including :* conditions affecting the knees, such as genu valgum and genu varum...
of the internal capsule
Internal capsule
The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus. The internal capsule contains both ascending and descending axons....
and a few fibers in the posterior limb of the internal capsule
Internal capsule
The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus. The internal capsule contains both ascending and descending axons....
, 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 nervesCranial nerves
Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial nerves...
. The muscles of the face, head and neck are controlled by the corticobulbar system, which terminates on motor neuron
Motor neuron
In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...
s within brainstem motor nuclei. This is in contrast to the corticospinal tract
Corticospinal tract
The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....
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
Trigeminal nerve
The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...
, VII
Facial nerve
The facial nerve is the seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity...
, XI, and XII
Hypoglossal nerve
The hypoglossal nerve is the twelfth cranial nerve , leading to the tongue. The nerve arises from the hypoglossal nucleus and emerges from the medulla oblongata in the preolivary sulcus separating the olive and the pyramid. It then passes through the hypoglossal canal...
. It does not innervate nuclei for nerves III, IV
Trochlear nerve
The trochlear nerve is a motor nerve that innervates a single muscle: the superior oblique muscle of the eye....
, and VI because these are mediated by cortical projections and yoked together by the MLF, medial longitudinal fasciculus
Medial longitudinal fasciculus
The medial longitudinal fasciculus is a pair of crossed fiber tracts , one on each side of the brainstem. These bundles of axons are situated near the midline of the brainstem and are composed of both ascending and descending fibers that arise from a number of sources and terminate in different...
. It also contributes to the motor regions of nerves X
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...
and IX
Glossopharyngeal nerve
The glossopharyngeal nerve is the ninth of twelve pairs of cranial nerves . It exits the brainstem out from the sides of the upper medulla, just rostral to the vagus nerve...
in the nucleus ambiguus
Nucleus ambiguus
The nucleus ambiguus is a region of histologically disparate cells located just dorsal to the inferior olivary nucleus in the lateral portion of the upper medulla...
.
The corticobulbar tract is the major input of the lower motor neurons in the brainstem, that is why a lesion to the corticubulbar tract results in lower facial symptoms because the input is mostly unilateral.
External links
- 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