Facial motor nucleus
Encyclopedia
The facial motor nucleus is a collection of neuron
s in the brainstem that belong to the facial nerve
(cranial nerve VII). These lower motor neuron
s innervate the muscles of facial expression and the stapedius
.
. Its axons take an unusual course, traveling dorsally and looping around the abducens nucleus
, then traveling ventrally to exit the ventral pons medial to the spinal trigeminal nucleus
. These axons form the motor component of the facial nerve
, with parasympathetic
and sensory components forming the nervus intermedius
.
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.
(SVE) cell column, which also includes the trigeminal motor nucleus
, nucleus ambiguus
, and (arguably) the spinal accessory nucleus
.
in the frontal lobe
of the brain
. Upper motor neuron
s of the cortex send axons that descend through the internal capsule
and synapse on neurons in the facial motor nucleus. This pathway from the cortex to the brainstem is called the corticobulbar tract
.
Interestingly, 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.
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
palsy) this condition is known as Bell's palsy
. 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 contalateral 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.
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s in the brainstem that belong to the facial nerve
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...
(cranial nerve VII). These lower motor neuron
Lower motor neuron
Lower motor neurons are the motor neurons connecting the brainstem and spinal cord to muscle fibers, bringing the nerve impulses from the upper motor neurons out to the muscles...
s innervate the muscles of facial expression and the stapedius
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....
.
Anatomy
The nucleus is situated in the caudal portion of the ventrolateral pontine tegmentumPons
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...
. Its axons take an unusual course, traveling dorsally and looping around the abducens nucleus
Abducens nucleus
The abducens nucleus is the originating nucleus from which the abducens nerve emerges - a cranial nerve nucleus. This nucleus is located beneath the fourth ventricle in the caudal portion of the pons, medial to the sulcus limitans....
, then traveling ventrally to exit the ventral pons medial to the spinal trigeminal nucleus
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. The facial, glossopharyngeal, and vagus nerves also convey pain information from their areas to the spinal trigeminal nucleus.This nucleus...
. These axons form the motor component of the facial nerve
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...
, with parasympathetic
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...
and sensory components forming the nervus intermedius
Nervus intermedius
The nervus intermedius, or intermediate nerve, is the part of the facial nerve located between the motor component of the facial nerve and the vestibulocochlear nerve . It contains the sensory and parasympathetic fibers of the facial nerve...
.
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.
Classification
Because it innervates muscles derived from pharyngeal arches, the facial motor nucleus is considered part of the special visceral efferentSpecial visceral efferent
Special visceral efferent refers to efferent nerves which supply muscles which derived from the branchial arches.Some sources prefer the term "branchiomotor", or "branchial efferent"....
(SVE) cell column, which also includes the trigeminal motor nucleus
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.-External links:*...
, 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...
, and (arguably) the spinal accessory nucleus
Spinal accessory nucleus
The spinal accessory nucleus lies within the cervical spinal cord in the ventral horn. The nucleus ambiguus is classically said to provide the "cranial component" of the accessory nerve....
.
Cortical input
Like all lower motor neurons, cells of the facial motor nucleus receive cortical input from the primary motor cortexPrimary motor cortex
The primary motor cortex is a brain region that in humans is located in the posterior portion of the frontal lobe. Itworks in association with pre-motor areas to plan and execute movements. M1 contains large neurons known as Betz cells, which send long axons down the spinal cord to synapse onto...
in 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...
of the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. Upper motor neuron
Upper motor neuron
Upper motor neurons are motor neurons that originate in the motor region of the cerebral cortex or the brain stem and carry motor information down to the final common pathway, that is, any motor neurons that are not directly responsible for stimulating the target muscle...
s of the cortex send axons that descend through 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 synapse on neurons in the facial motor nucleus. This pathway from the cortex to the brainstem is called the corticobulbar tract
Corticobulbar tract
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...
.
Interestingly, 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.
Effects of lesions
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. 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
Acute facial nerve paralysis
Facial nerve paralysis is a common problem that involves the paralysis of any structures innervated by the facial nerve. The pathway of the facial nerve is long and relatively convoluted, and so there are a number of causes that may result in facial nerve paralysis...
on the same side of the injury. If a cause, such as trauma or infection, cannot be identified (this situation is called idiopathic
Idiopathic
Idiopathic is an adjective used primarily in medicine meaning arising spontaneously or from an obscure or unknown cause. From Greek ἴδιος, idios + πάθος, pathos , it means approximately "a disease of its own kind". It is technically a term from nosology, the classification of disease...
palsy) this condition is known as Bell's palsy
Bell's palsy
Bell's palsy is a form of facial paralysis resulting from a dysfunction of the cranial nerve VII that results in the inability to control facial muscles on the affected side. Several conditions can cause facial paralysis, e.g., brain tumor, stroke, and Lyme disease. However, if no specific cause...
. 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 contalateral 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.