The facial motor nucleus is a collection of neurons in the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...

that belong to the

facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of tas ...

(

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and ...

VII). These

lower motor neuron Lower motor neurons (LMNs) are motor neurons located in either the anterior grey column, anterior nerve roots (spinal lower motor neurons) or the cranial nerve nuclei of the brainstem and cranial nerves with motor function (cranial nerve lower mo ...

s innervate the

muscles of facial expression The facial muscles are a group of striated skeletal muscles supplied by the facial nerve (cranial nerve VII) that, among other things, control facial expression. These muscles are also called mimetic muscles. They are only found in mammals, alth ...

and the

stapedius The stapedius is the smallest skeletal muscle in the human body. At just over one millimeter in length, its purpose is to stabilize the smallest bone in the body, the stapes or strirrup bone of the middle ear. Structure The stapedius emerges from ...

Structure

The

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucle ...

is situated in the caudal portion of the ventrolateral

pontine tegmentum Pontine may refer to: * Having to do with the pons, a structure located in the brain stem (from ''pons'', "bridge") * Pontine Marshes, a region of Italy near Rome * Pontine Islands The Pontine Islands (, also ; it, Isole Ponziane ) are an ar ...

. Its axons take an unusual course, traveling dorsally and looping around the

abducens nucleus The abducens nucleus is the originating nucleus from which the abducens nerve (VI) emerges—a cranial nerve nucleus. This nucleus is located beneath the fourth ventricle in the Anatomical terms of location, caudal portion of the pons, Anatomical t ...

, then traveling ventrally to exit the ventral pons medial to the

spinal trigeminal nucleus The spinal trigeminal nucleus is a nucleus in the medulla that receives information about deep/crude touch, pain, and temperature from the ipsilateral face. In addition to the trigeminal nerve (CN V), the facial (CN VII), glossopharyngeal (CN IX), ...

. These axons form the motor component of the

, with

parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of t ...

and sensory components forming the

intermediate nerve The intermediate nerve, nervus intermedius, nerve of Wrisberg or Glossopalatine nerve, is the part of the facial nerve (cranial nerve VII) located between the motor component of the facial nerve and the vestibulocochlear nerve (cranial nerve VIII) ...

. The nucleus has a dorsal and ventral region, with neurons in the dorsal region innervating muscles of the upper face and neurons in the ventral region innervating muscles of the lower face.

Function

Because it innervates muscles derived from

pharyngeal arch The pharyngeal arches, also known as visceral arches'','' are structures seen in the embryonic development of vertebrates that are recognisable precursors for many structures. In fish, the arches are known as the branchial arches, or gill arch ...

es, the facial motor nucleus is considered part of the

special visceral efferent Special visceral efferent fibers (SVE) are the efferent nerve fibers that provide motor innervation to the muscles of the pharyngeal arches in humans, and the branchial arches in fish. Some sources prefer the term "branchiomotor" or "branchial eff ...

(SVE) cell column, which also includes the

trigeminal motor nucleus The trigeminal motor nucleus contains motor neurons that innervate muscles of the first branchial arch, namely the muscles of mastication, the tensor tympani, tensor veli palatini, mylohyoid, and anterior belly of the digastric. This nucleus is l ...

nucleus ambiguus The nucleus ambiguus ("ambiguous nucleus" in English) is a group of large motor neurons, situated deep in the medullary reticular formation named by Jacob Clarke. The nucleus ambiguus contains the cell bodies of neurons that innervate the muscles ...

, and (arguably) the

spinal accessory nucleus The spinal accessory nucleus lies within the cervical spinal cord (C1-C5) in the posterolateral aspect of the anterior horn. The nucleus ambiguus is classically said to provide the "cranial component" of the accessory nerve. However, the very exist ...

Cortical input

Like all lower motor neurons, cells of the facial motor nucleus receive

cortical input Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...

from the

primary motor cortex The primary motor cortex (Brodmann area 4) is a brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary region of the motor system and works in association with other motor areas including premotor co ...

in the

frontal lobe The frontal lobe is the largest of the four major lobes of the brain in mammals, and is located at the front of each cerebral hemisphere (in front of the parietal lobe and the temporal lobe). It is parted from the parietal lobe by a groove betwe ...

of the

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

Upper motor neuron Upper motor neurons (UMNs) is a term introduced by William Gowers in 1886. They are found in the cerebral cortex and brainstem and carry information down to activate interneurons and lower motor neurons, which in turn directly signal muscles t ...

s of the cortex send axons that descend through the

internal capsule The internal capsule is a white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain. It carries information past the basal ganglia, separating the caudate nucleus and the thalamus from the putamen and the g ...

and synapse on neurons in the facial motor nucleus. This pathway from the cortex to the brainstem is called the

corticobulbar tract In neuroanatomy, the corticobulbar (or corticonuclear) tract is a two-neuron white matter motor pathway connecting the motor cortex in the cerebral cortex to the medullary pyramids, which are part of the brainstem's medulla oblongata (also call ...

. The neurons in the dorsal aspect of the facial motor nucleus receive inputs from both sides of the cortex, while those in the ventral aspect mainly receive contralateral inputs (i.e. from the opposite side of the cortex). The result is that both sides of the brain control the muscles of the upper face, while the right side of the brain controls the lower left side of the face, and the left side of the brain controls the lower right side of the face.

Clinical significance

As a result of the corticobulbar input to the facial motor nucleus, an upper motor neuron lesion to fibers innervating the facial motor nucleus results in

central seven Central facial palsy (colloquially referred to as central seven) is a symptom or finding characterized by paralysis or paresis of the lower half of one side of the face. It usually results from damage to upper motor neurons of the facial nerve. T ...

. The syndrome is characterized by spastic paralysis of the contralateral lower face. For example, a left corticobulbar lesion results in paralysis of the muscles that control the lower right quadrant of the face. By contrast, a lower motor neuron lesion to the facial motor nucleus results in paralysis of facial muscles on the same side of the injury. If a cause, such as trauma or infection, cannot be identified (this situation is called

idiopathic An idiopathic disease is any disease with an unknown cause or mechanism of apparent wikt:spontaneous, spontaneous origin. From Ancient Greek, Greek ἴδιος ''idios'' "one's own" and πάθος ''pathos'' "suffering", ''idiopathy'' means approxi ...

palsy) this condition is known as

Bell's palsy Bell's palsy is a type of facial paralysis that results in a temporary inability to control the facial muscles on the affected side of the face. In most cases, the weakness is temporary and significantly improves over weeks. Symptoms can vary fr ...

. Otherwise it is described by its cause.

Mechanism of Facial Nerve Upper vs Lower Motor Neuron Lesions

Any lesion occurring within or affecting the corticobulbar tract is known as an upper motor neuron lesion. Any lesion affecting the individual branches (temporal, zygomatic, buccal, mandibular and cervical) is known as a lower motor neuron lesion. Branches of the facial nerve leaving the facial motor nucleus (FMN) for the muscles do so via both left and right posterior (dorsal) and anterior (ventral) routes. In other words, this means lower motor neurons of the facial nerve can leave either from the left anterior, left posterior, right anterior or right posterior facial motor nucleus. The temporal branch travels out from the left and right posterior components. The inferior four branches do so via the left and right anterior components. The left and right branches supply their respective sides of the face (ipsilateral innervation). Accordingly, the posterior components receive motor input from both hemispheres of the cerebral cortex (bilaterally), whereas the anterior components receive strictly contralateral input. This means that the temporal branch of the facial nerve receives motor input from both hemispheres of the cerebral cortex whereas the zygomatic, buccal, mandibular and cervical branches receive information from only contralateral hemispheres. Now, because the anterior FMN receives only contralateral cortical input whereas the posterior receives that which is bilateral, a corticobulbar lesion (UMN lesion) occurring in the left hemisphere would eliminate motor input to the right anterior FMN component, thus removing signaling to the inferior four facial nerve branches, thereby paralyzing the right mid- and lower-face. The posterior component, however, although now only receiving input from the right hemisphere, is still able to allow the temporal branch to sufficiently innervate the entire forehead. This means that the forehead will not be paralyzed. The same mechanism applies for an upper motor neuron lesion in the right hemisphere. The left anterior FMN component no longer receives cortical motor input due to its strict contralateral innervation, whereas the posterior component is still sufficiently supplied by the left hemisphere. The result is paralysis of the left mid- and lower-face with an unaffected forehead. On the other hand, a lower motor neuron lesion is a bit different. A lesion on either the left or right side would affect both the anterior and posterior routes on that side because of their close physical proximity to one another. So, a lesion on the left side would inhibit muscle innervation from both the left posterior and anterior routes, thus paralyzing the whole left side of the face ( Bell’s palsy). With this type of lesion, the bilateral and contralateral inputs of the posterior and anterior routes, respectively, become irrelevant because the lesion is below the level of the medulla and the facial motor nucleus. Whereas at a level above the medulla a lesion occurring in one hemisphere would mean that the other hemisphere could still sufficiently innervate the posterior facial motor nucleus, a lesion affecting a lower motor neuron would eliminate innervation altogether because the nerves no longer have a means to receive compensatory contralateral input at a downstream decussation. Thus, the main distinction between an UMN and LMN lesion is that in the former, there is hemiplegia of the contralateral mid- and lower-face, whereas in the latter, there is complete hemiplegia of the ipsilateral face.

Additional images

File:Gray696.png, The cranial nerve nuclei schematically represented; dorsal view. Motor nuclei in red; sensory in blue. File:Gray697.png, Nuclei of origin of cranial motor nerves schematically represented; lateral view. File:Lower pons horizontal KB.svg, Cross section of the lower pons showing the facial motor nucleus and part of the root of the facial nerve.

References

{{DEFAULTSORT:Facial Motor Nucleus Cranial nerve nuclei