Mixed Cranial Nerves
Before we begin, let’s take a moment to think about your face.
No, this isn’t a trick, and, yes, yours is a very pretty face; but that’s not the point of this exercise - let us gaze beyond the looking glass and take a moment to think about all the structures in your head and neck and consider all the things they do! For example: can you hear daisies singing, and see the Cheshire Cat’s fleeting white smile? Can you smell the smoke billowing from the caterpillar’s pipe, and taste the cookie which demands of you “EAT ME”?
...Perhaps not. Frankly (and arguably fortunately) we are not in Wonderland; but vision, audition, olfaction, and gustation are all accomplished using structures in the head and neck, and these are just the special senses! What about all the skin, muscles, and glands? Yes, we know, there’s quite a lot going on.
But worry not, dear friends, because your plight has led you here! And where is Here, you might ask? We may not be in Wonderland, but you do have to begin somewhere, and Here seems as good a place as any! In fact, we suspect Wonderland might be far more confusing, so, ultimately, if you were choosing between the two you’ve chosen well.
Since many students struggle with anatomy on its own, we recognize that the task of learning neuroanatomy and the anatomy of the head and neck may feel insurmountable. This is why we are here, to help - let us be your guide! We are happy you decided to read this article, one of a series of articles aiming to help students by providing an introduction to the innervation of the head and neck. So count to three, take a leap of good faith, and follow us down the rabbit hole! We’ll leave a pillow at the bottom for you to land on, no problem.
- The Cranial Nerves
- The Trigeminal Nerve
- The Facial Nerve
- The Glossopharyngeal Nerve
- The Vagus Nerve
- Mixed Cranial Nerves & Reflexes
- Clinical Notes
- Related diagrams and images
The Cranial Nerves
As their name suggests, the cranial nerves (CN) (except for the spinal accessory nerve, which originates in the spinal cord) originate in the brain and contribute to the peripheral nervous system (PNS) by supplying the structures of the head and neck.
These 12 paired nerves, and their main branches, include:
- The olfactory nerve (CN I)
- The optic nerve (CN II)
- The oculomotor nerve (CN III)
- The trochlear nerve (CN IV)
- The trigeminal nerve (CN V)
- The abducens nerve (CN VI)
- The facial nerve (CN VII)
- Temporal branch
- Zygomatic branch
- Buccal branch
- Mandibular branch
- Cervical branch
- The vestibulocochlear nerve (CN VIII)
- The glossopharyngeal nerve (CN IX)
- The vagus nerve (CN X)
- The spinal accessory nerve (CN XI)
- The hypoglossal nerve (CN XII)
Some of these nerves have special sensory functions, some have somatic sensory functions, some have autonomic functions, some have somatic motor functions, and some have a combination of the aforementioned functions. The olfactory nerve, the optic nerve, the facial nerve, the vestibulocochlear nerve, the glossopharyngeal nerve, and the vagus nerve each play roles in special sensory functions (i.e. olfaction, vision, gustation, audition, and balance). The trigeminal nerve (all three branches: the ophthalmic, maxillary, and mandibular) and the glossopharyngeal nerve play roles in somatic sensory functions. The oculomotor nerve, the facial nerve, the glossopharyngeal nerve, and the vagus nerve have important autonomic functions. Finally, the oculomotor nerve, the trochlear nerve, the mandibular branch of the trigeminal nerve (V3), the abducens nerve, the facial nerve, the glossopharyngeal nerve, the vagus nerve, the spinal accessory nerve, and the hypoglossal nerve are responsible for motor functions.
To understand each nerve and each of its separate functions, we have separated the 12 cranial nerves into smaller groups. These groups separate the cranial nerves into those which are considered to have primarily motor functions; those considered to have primarily sensory functions; and those with a combination of both motor and sensory components.
This article will provide an introduction to the cranial nerves which are considered mixed nerves, which includes the trigeminal nerve, the facial nerve, the glossopharyngeal nerve, and the vagus nerve.
The Trigeminal Nerve
Types of Fibers & Innervation
The trigeminal nerve (CN V) is a hefty nerve that contains both general sensory (afferent) fibers and somatic motor (efferent) fibers. The general sensory component sends somatosensory input–including pain, touch, pressure, and temperature sensation–to the brain from the anterior two-thirds of the head, including the face. The smaller somatic efferent component innervates the skeletal muscles derived from the first branchial arch:
- the mylohyoid muscles
- tensor tympani
- tensor veli palatini
- anterior belly of the digastric muscle
- the muscles of mastication (the masseter, temporalis, medial pterygoid, and lateral pterygoid muscles)
Because of its size, the trigeminal nerve can be easily seen where it emerges from the pons near the middle cerebral peduncle.
The cell bodies that receive sensory input from the face can be found in the trigeminal ganglion, which is itself located lateral to the cavernous sinus, in a cleft of the petrous bone. The sensory component of the trigeminal nerve has three divisions: the ophthalmic division, the maxillary division, and the mandibular division.
The ophthalmic division of the trigeminal nerve (CN V1) transmits sensory signals from receptors on the:
- upper eyelid
- dorsal surface of the nose
- mucous membranes of the nasal and frontal sinuses.
The signals then travel along nerve fibers which enter the skull through the superior orbital fissure (along with the oculomotor, trochlear, and abducens nerves).
The maxillary division of the trigeminal nerve (CN V2) transmits sensory signals from receptors on the:
- lateral surface of the nose
- upper teeth
- hard palate, and upper cheek
- mucous membranes of the upper teeth, nose, and roof of the mouth
Signals generated by these receptors then travel along the nerve fibers into the skull via the foramen rotundum.
The mandibular division of the trigeminal nerve (CN V3) transmits sensory signals from receptors on the:
- lower jaw
- lower teeth
- parts of the posterior cheek
- external ear
- anterior two-thirds of the tongue
- floor of the mouth
It also supplies motor innervation to the muscles of mastication and a few other muscles in the lower face (listed previously). These fibers enter the skull via the foramen ovale.
Course of Sensory Fibers
Once the fibers comprising the branches of the trigeminal nerve enter the skull through their various foramina, they converge and enter the pons as a unit. From the pons, the fibers take one of two directions:
- they either synapse with the main sensory nucleus
- or they enter the trigeminal nerve tract, descend further down into the pons and medulla, and synapse on the spinal nucleus of the trigeminal nerve.
Some of the pain fibers from the posterior face descend even as far down the trigeminal tract as the second cervical spinal level (C2)!
Second-order neurons arise from the aforementioned nuclei and project to the ventral posteromedial (VPM) nucleus of the contralateral thalamus. Fibers from the spinal nucleus project via the ventral trigeminothalamic tract to the contralateral VPM; whereas fibers from the sensory nucleus project bilaterally to the VPM, passing either ipsilaterally via the dorsal trigeminothalamic tract or contralaterally via the ventral trigeminothalamic tract.
Third-order sensory fibers project from the VPM to the ipsilateral face region of the postcentral gyrus (i.e. the somatosensory cortex) in the parietal lobe of the cerebral cortex.
Course of Motor Fibers
The somatic motor fibers of the trigeminal nerve originate in the motor trigeminal nuclei, which each receive bilateral input from both the left and right cerebral cortices. Axons from motor neurons in this nucleus exit the brainstem close to where the sensory fibers enter. Like their sensory counterparts in the mandibular division of the trigeminal nerve, they travel through the foramen ovale to exit the skull. These motor fibers innervate:
- the muscles of mastication–the masseter, temporalis, and medial and lateral pterygoid muscles
- tensor tympani
- tensor veli palatini
- mylohyoid muscles
- anterior belly of the digastric muscle
The Facial Nerve
The facial nerve (CN VII) contains many different types of fibers, including general sensory (afferent) fibers, special sensory fibers, visceral/autonomic motor (efferent) fibers, and somatic motor fibers.
Types of Fibers & Innervation
General sensory fibers in the facial nerve transmit somatosensory signals to the brain from the external acoustic meatus, as well as the skin over the mastoid and lateral pinna. Special sensory fibers in the facial nerve are responsible for receiving and transmitting taste information from the anterior two-thirds of the tongue.
Visceral/autonomic motor fibers in the facial nerve are responsible for innervating the:
- lacrimal gland
- submandibular gland
- sublingual gland
- the mucous membranes of the nasal cavity and hard and soft palates
They allow for production of tears, saliva, etc., from these locations.
Somatic motor fibers in the facial nerve are responsible for innervating the:
- muscles of facial expression
- muscles in the scalp (which are derived from the second pharyngeal arch)
- the stapedius muscle in the ear
- the posterior belly of the digastric muscle
- the stylohyoid muscle
Course of Fibers
The motor root of the facial nerve originates in the facial (motor) nerve nucleus in the pons of the brainstem, which receives input from a number of other structures and brain regions, including the primary motor cortex and the ophthalmic division of the trigeminal nerve. The fibers travel towards the floor of the fourth ventricle, deviate around the abducens nucleus, and descend.
The facial nerve emerges from the lateral surface of brainstem at the pontine-medullary junction between the abducens (CN VI) and vestibulocochlear (CN VIII) nerves. The motor root travels with the nervus intermedius (a smaller sensory root containing parasympathetic fibers, general sensory fibers, and special sensory fibers) in the cerebellopontine angle and–accompanied by the vestibulocochlear nerve (CN VIII) and the labyrinthine artery and vein–enters the internal auditory meatus of the temporal bone.
The facial nerve roots then enter the facial canal in petrous part of temporal bone, where the small sensory and large motor roots fuse, forming a singular facial nerve. The combined nerve then enlarges at the geniculate ganglion in the facial canal of the temporal bone, which contains the neuronal cell bodies. At the geniculate ganglion, the facial nerve gives off the greater petrosal nerve, the first in a series of nerves which eventually carry preganglionic parasympathetic fibers to the lacrimal gland, stimulating tear production (i.e. lacrimation).
It passes beneath the trigeminal ganglion and reaches the foramen lacerum where it is joined by deep petrosal nerve to become the nerve of the pterygoid canal (otherwise known as the Vidian nerve). The greater petrosal nerves contain parasympathetic fibers for the pterygopalatine ganglion, as well as taste fibers.
As the facial nerve continues to travel along the bony canal, additional branches emerge:
- the nerve to stapedius
- the chorda tympani
- preganglionic parasympathetic fibers
The nerve to stapedius innervates the stapedius muscle, as its name suggests. The stapedius muscle attaches to the posterior surface of the stapes, one of the three ossicles of the middle ear. The stapedius muscle contracts in response to loud noises, preventing excessive oscillation of the stapes, thereby dampening its vibrations and controlling the amplitude of sound waves.
The chorda tympani leaves the facial nerve above the stylomastoid foramen and is responsible for transmitting taste sensation, via the aforementioned special sensory fibers, from the anterior two-thirds of the tongue. It passes through the posterior wall of the middle ear, crosses the neck of the malleus and emerges at the medial end of the petrotympanic fissure. It joins the posterior aspect of the lingual nerve at an acute angle and carries taste fibers for the anterior two-third of tongue and efferent preganglionic parasympathetic fibers to the submandibular ganglion which are responsible for innervating the submandibular gland, stimulating salivary secretions.
The facial nerve exits the skull via stylomastoid foramen; nearby, it gives off the posterior auricular nerve which is meant to supply the occipital belly of the occipitofrontalis muscle and some of the auricular muscles, and nerves to the posterior belly of the digastric and the stylohyoid. The nerve then enters the parotid gland, from whence it gives off five terminal branches–the temporal, zygomatic, buccal, marginal mandibular, and cervical branches–which emerge from around the parotid gland and innervate structures across the entire face.
Via these five branches, the motor component of the facial nerve innervates the muscles of facial expression, which play a crucial role in nonverbal communication.
The muscles of facial expression, innervated by the branches of the facial nerve, are divided into groups by their locations, such as:
- the orbital group
- the nasal group,
- the oral group
- a group of auricular muscles
- the occipitofrontalis
The muscles of facial expression are what allow you to make all those funny faces you’ve been making while trying to digest all this information.
The Glossopharyngeal Nerve
Types of Fibers & Innervation
The glossopharyngeal nerve (CN IX) has two motor components: the first innervates the stylopharyngeus muscles of the pharynx, whereas the second is a preganglionic parasympathetic component associated with salivation. The glossopharyngeal nerve also has a visceral afferent component which transmits information from baroreceptors in the carotid sinus to the medulla; a special afferent component which transmits information from the carotid body regarding changes in blood gas levels and pH to the medulla; and a somatic sensory component which transmits sensory information from the back of the ear to the medulla.
Course of Motor Fibers
The somatic efferent fibers of the glossopharyngeal nerve originate from cell bodies in the rostral part of the nucleus ambiguus (whereas the vagal efferents arise from the caudal part). The fibers exit the brain through the jugular foramen and innervate the stylopharyngeus muscle, which contracts to elevate the pharynx during speech and swallowing.
The visceral efferent component of the glossopharyngeal nerve originates in the inferior salivatory nucleus of the reticular formation in the medulla. These fibers are preganglionic parasympathetic in nature, and project from neurons in the inferior salivatory nucleus to the otic ganglion. The axons of short postganglionic parasympathetic neurons in the otic ganglion subsequently project to and innervate the parotid gland, stimulating salivation.
Course of Sensory Fibers
The visceral afferent component of the glossopharyngeal nerve is responsible for the carotid sinus baroreceptor reflex. The fibers originate in the carotid sinus, where baroreceptors are triggered in response to changes in blood pressure. Signals regarding these pressure changes are transmitted to cell bodies in the inferior (petrosal) ganglion, and then along their axons to synapse in the solitary nucleus of the brainstem, followed by the dorsal motor nucleus of the vagus and the nucleus ambiguus.
A special afferent component of the glossopharyngeal nerve is responsible for the carotid body chemoreceptor reflex. Chemoreceptors in the carotid body respond to changes in pH (a measure of hydrogen ion concentration, or blood acidity) and blood gas levels (arterial concentrations of oxygen and carbon dioxide): sensory afferent neurons with their cell bodies in the inferior ganglion detect this information and transmit it to the solitary nucleus in the brainstem.
Another group of special afferent fibers in the glossopharyngeal nerve are responsible for taste sensation from sensory receptors in the posterior third of the tongue. Fibers from these receptors transmit signals to cell bodies in the inferior ganglion, which send axons to synapse in the solitary nucleus. The solitary nucleus then sends activating signals to the VPM of the thalamus, which subsequently activates neurons in the postcentral gyrus.
The general sensory afferent component of the glossopharyngeal nerve receives sensory input from the:
- tympanic membrane
- mucosa of the inner ear
- skin of the external ear
- eustachian tube
- upper pharynx
Nerve fibers transmit these signals from cell bodies in the superior ganglion to the spinal nucleus of the trigeminal nerve and eventually to the thalamus and cortex. After synapsing in the trigeminal spinal nucleus, the signals follow trigeminothalamic pathways.
The Vagus Nerve
Types of Fibers & Innervation
Like the glossopharyngeal nerve, the vagus nerve (CN X) has two types of motor components: a somatic efferent component, which innervates muscles derived from the fourth and fifth branchial arches; and a visceral efferent component, which innervates various organs throughout the body.
It also contains a small somatic afferent component which transmits sensory signals from a small part of the back of the ear; a small visceral afferent component which transmits signals regarding changes in blood pressure to the brain; and a special afferent component which transmits taste information and changes in blood oxygen levels to the brain.
Course of Motor Fibers
The somatic efferent component of the vagus nerve originates in the nucleus ambiguus. The fibers travel ventro-laterally from the nucleus ambiguus, eventually exiting the skull through the jugular foramen. Peripheral branches of this component of the vagus nerve include:
- the pharyngeal nerve, which innervates the pharynx and soft palate
- the superior laryngeal nerve, which descends near the pharynx and supplies the inferior constrictor muscle, cricothyroid muscle, and superior cardiac nerve
- the recurrent laryngeal nerve, which supplies the muscles of the larynx, playing a vital role in speech production
The visceral efferent component of the vagus nerve originates in the dorsal motor nucleus by the floor of the fourth ventricle. Its axons, a group of preganglionic parasympathetic fibers, join the vagus nerve and innervate various organs throughout the body. This component of the vagus has numerous influences, including:
- increasing bronchoconstriction
- increasing the rate of peristalsis
- slowing the heart rate
- increasing bronchial, gastric, pancreatic, and intestinal secretions
Course of Sensory Fibers
The visceral afferent fibers of the vagus nerve originate from the same regions innervated by the visceral efferent fibers (i.e. the plexi surrounding the abdominal and thoracic viscera, and the mucosal linings of the esophagus, larynx, pharynx, and soft palate). They also, however, arise from the aortic arch, receiving information from stretch receptors attuned to blood pressure changes. The cell bodies of these afferent fibers are located in the inferior (or nodose) ganglion, and their central processes enter the brain via the medulla and synapse on neurons in the solitary nucleus. These sensory inputs mediate feelings of hunger, thirst, and satiety, and play roles in numerous reflexes, including the baroreceptor reflex.
The special afferent component of the vagus nerve has two main responsibilities. The first is to play a role in respiratory function by transmitting information regarding blood oxygen and carbon dioxide content via chemoreceptors. The fibers sensing this information arise from the aortic body, and transmit it to their cell bodies located in the inferior ganglion. The central processes of these neurons then pass along the signals to neurons in the reticular formation of the medulla, nearby to the solitary nucleus.
The second function of the special afferent component of the vagus is to transmit taste sensation from taste buds at the very base of the tongue, located in the epiglottis and posterior pharynx. Once again, cell bodies are located within the inferior ganglion, with central processes entering the lower brainstem to synapse in the solitary nucleus. The taste signals are then transmitted to the VPM nucleus in the thalamus, followed by the ventrolateral part of the somatosensory gyrus.
The somatic afferent component of the vagus nerve is very small: it functions to receive pain, temperature, pressure, and tactile sensation from a small region of skin behind the ear and within the external auditory canal. It also receives signals from:
- the epiglottis
- the pharynx
- the part of the larynx at and above the vocal folds
- parts of the dura mater,
- external auditory canal
- outer tympanic membrane
The cell bodies receiving these signals are located in the superior (jugular) ganglion, and their central processes enter the medulla. These fibers, however, terminate on the spinal trigeminal nucleus, and from that point forward become part of the trigeminal tract.
Mixed Cranial Nerves & Reflexes
The corneal reflex, also called the blink reflex, is the involuntary response of blinking the eyelids when the cornea is stimulated. The trigeminal nerve comprises the afferent (sensory) limb of the corneal reflex, while the facial nerve comprises the efferent (motor) limb.
Stimulation of sensory receptors in the cornea sends signals along the trigeminal nerve and into the brainstem. The trigeminal nerve axons descend via the spinal trigeminal tract and synapse with neurons in the pars caudalis of the spinal trigeminal nucleus. Axons from these neurons subsequently project to the contralateral VPM thalamic nucleus.
On the way, however, collateral axons from pars caudalis neurons are sent bilaterally to synapse with neurons in the facial nerve motor nuclei. As part of the facial nerve, the axons of motor neurons in these nuclei exit the skull via the stylomastoid foramen, and innervate the orbicularis oculi muscles in the eyelids as part of the zygomatic branch of the facial nerve. Innervation of the orbicularis oculi muscles leads the eyes to blink. Because both left and right facial nerve motor nuclei receive input from sensory stimulation of the trigeminal nerve on either side, the corneal reflex is both direct (in the stimulated eye) and indirect (in the opposite eye, also called a consensual reflex). The blink does, however, tend to be stronger on the stimulated side.
Stimulation of the cornea, of course, is also ultimately perceived as painful; this occurs due to transmission of the noxious information via ascending fibers in the anterior trigeminothalamic tract.
The mandibular reflex, otherwise known as the jaw jerk reflex, is a version of the muscle stretch reflex mediated through the trigeminal nerve.
Tapping on the chin stretches muscle spindle fibers in the temporalis and masseter muscles, which triggers action potentials in A-alpha (primary) muscle spindle fibers and A-beta (secondary) muscle spindle fibers. These afferent fibers travel along the sensory root of the trigeminal nerve to both synapse on cell bodies in the mesencephalic nucleus, and send collaterals bilaterally to synapse on motor neurons in the trigeminal motor nuclei. As part of the motor root of the trigeminal nerve, axons of these motor neurons innervate the temporalis and masseter muscles, resulting in contraction of these muscles and closure of the jaw.
The gag reflex has an afferent limb mediated by the glossopharyngeal nerve and an efferent limb mediated by the glossopharyngeal and vagus nerves. The gag reflex allows for constriction and elevation of the pharynx in response to irritation in the back of the throat, at the base of the tongue and/or in the soft palate in the back of the roof of the mouth, functioning to push out the object that is irritating the area. These regions between the mouth and pharynx are called the fauces; for this reason, the gag reflex may also be referred to as the faucial reflex.
When there is stimulation of A-delta fibers and C fibers in the fauces, signals are sent along the glossopharyngeal nerve to cell bodies in its superior ganglion. The signals are then transmitted via interneurons to the nucleus ambiguus, the origination of the efferent limb of the reflex. Efferent signals then travel along the glossopharyngeal nerve to innervate the stylopharyngeus muscle, and along the vagus nerve to innervate the pharyngeal constrictor muscles and other muscles which move the palate.
Carotid Body Chemoreceptor Reflex
Increase in carbon dioxide levels, decrease in oxygen levels, or alterations in pH in the blood stimulate afferent fibers in the glossopharyngeal nerve, ultimately activating reticulospinal neurons in the reticular formation. Fibers then descend in the spinal cord to synapse on ventral horn cells in the cervical spinal cord, specifically in cervical levels 3, 4, and 5. Axons from these ventral horn cells form the phrenic nerve, which innervates the muscles of the diaphragm and causes reflex contractions of the diaphragmatic muscles. This increases respiratory rate, which ultimately reduces the amount of carbon dioxide in the blood.
The baroreceptor reflex functions to maintain a person’s blood pressure and cardiac output when mean arterial pressure changes. For example, when a person suddenly stands up from a sitting or lying position, blood pressure drops; this leads to decreased firing by receptors in the carotid body and aortic arch. Signals originating in the carotid body are transmitted by the glossopharyngeal nerve, whereas signals originating in the aortic arch are transmitted by the vagus nerve. The decreased signalling rate ultimately results in disinhibition of the sympathetic nervous system, which leads to an increase peripheral vascular tone, cardiac rate, and cardiac output.
Infant ReflexesA number of infantile reflexes are mediated by the trigeminal, facial, glossopharyngeal, and vagus nerves, as well as the hypoglossal nerve. The snout, sucking, and rooting reflexes, known as the primitive reflexes, typically disappear within the first few months of life; although they have been observed to reappear in some individuals with dementia, or degeneration or dysfunction of the frontal lobe. In infants, however, these reflexes are essential for survival by facilitating feeding.
The trigeminal nerve makes up the afferent limb of the primitive reflexes, and is activated by touching around or in the mouth. Signals travel along afferent trigeminal fibers to the spinal trigeminal ganglion in the brain stem, terminating in the spinal trigeminal nucleus and principal sensory nucleus. Fibers from these nuclei, as they travel to the VPM nucleus of the thalamus, give off collaterals which either travel directly or indirectly via interneurons to the facial nucleus, nucleus ambiguus, accessory nucleus, and hypoglossal nucleus. This leads to innervation of the infant’s facial muscles via the facial nucleus; orientation of the head toward or away from the stimulus via the accessory nucleus; and contraction of the laryngeal and pharyngeal muscles to allow for sucking via the hypoglossal nucleus.
Trigeminal neuralgia, otherwise known as tic douloureux, is a painful condition caused by irritation of the trigeminal nerve. It can occur due to infection or inflammation of the nerve, a tumor compressing the nerve, or a vascular lesion affecting blood supply to the nerve. Trigeminal neuralgia is usually associated with a specific branch of the trigeminal nerve, and therefore tends to localize to the region of the ipsilateral side of the face supplied by that branch.
Surgical Removal of the Parotid Gland
The five terminal branches of the facial nerve–the temporal, zygomatic, buccal, marginal mandibular, and cervical branches–are closely anatomically related to the parotid gland: they emerge from the parotid gland’s upper, anterior, and lower borders. Because of this close association, removal of the parotid gland (i.e. in the removal of an adenoma or neoplasm) without damaging these branches is a particularly delicate procedure. Damage to any of these five branches would result in weakness or paralysis of the muscles supplied.
Facial Nerve Palsy
Facial nerve palsy can be associated with a variety of etiologies and syndromes. Additional symptoms depend on the level at which the lesion occurs. Although most facial nerve palsies are considered idiopathic, common causes include infection, trauma, iatrogenic injury, and neoplasia. The incidence of facial palsy in neonates is reported to be 0.6–1.8 per 1000 live births, but is primarily associated with forceps delivery. The incidence in adults ranges between 17-35 per 100000.
Vascular damage to the facial nerve usually occurs at the supranuclear, pontine, and (rarely) cerebellopontine angle. Upper motor neuron (UMN) lesions occur in strokes and can easily be differentiated with lower motor neuron (LMN) lesions by their presentation. A LMN lesion causes paralysis of the whole side of face, while an UMN lesion results in sparing of the forehead. The muscles in the forehead remain unaffected because they receive input from both the left and right cerebral hemispheres: input from the ipsilateral hemisphere maintains the function of the muscles in the upper face even when input from the contralateral hemisphere is lost. This is unlike the muscles in the lower part of the face, which receive input from the contralateral hemisphere only.
Lesions at the level of the geniculate ganglion typically result in weakness or paralysis of the muscles on the entire ipsilateral side of the face. Because the greater petrosal nerve and chorda tympani have not yet branched off of the facial nerve at that level, lacrimation, salivation, and taste sensation in the anterior two-thirds of the tongue are also likely to be affected.
If the facial nerve itself is damaged prior to dividing into the temporal, zygomatic, buccal, marginal mandibular, and cervical branches, the muscles of facial expression in the entire side of the face supplied by the damaged nerve may be weakened or paralyzed. This is most commonly associated with viral inflammation of the facial nerve before it exits the stylomastoid foramen. If the lesion occurs distally to the branching of the greater petrosal nerve and chorda tympani, lacrimation, salivation, and taste sensation in the anterior two-thirds of the tongue will be unaffected.
When the stapedius muscle, the nerve to stapedius, or the facial nerve is damaged, paralysis of the stapedius muscle may lead to hyperacusis. In this condition, loss of inhibition of oscillation of the stapes results in its excessive vibration: as a result, sounds that would otherwise be considered of normal volume are perceived as being uncomfortably loud.
Bell’s palsy is the most common form of peripheral facial nerve palsy. Although there is usually no detectable cause (i.e. idiopathic), some evidence suggests that latent infection with herpes simplex virus type 1 (HSV-1) plays a role, causing inflammation of the nerve and subsequent symptoms. It presents with sudden onset of impairment of facial expression, typically on one side. It is frequently preceded by periauricular paraesthesia or otalgia and may be associated with dry eyes, xerostomia, tinnitus, and hyperacusis.
Ramsay Hunt Syndrome results from reactivation of the varicella zoster virus in the geniculate ganglion. It presents as a triad of facial nerve palsy, vertigo, and vesicles in the ipsilateral external ear, palate or anterior tongue. Treatment typically consists of steroids and antivirals.
Facial nerve paralysis secondary to acute otitis media is more common in young children. The most common cause of otitis media is the gram-positive bacteria Streptococcus pneumoniae, and the majority of cases resolve with antibiotics.
Facial nerve paralysis is also a feature of skull-base osteomyelitis, a condition which occurs primarily in elderly / immunocompromised patients. The characteristic features are severe pain, aural discharge, and progressive cranial neuropathies.
As in an infant injured during a forceps delivery, facial nerve palsy in an adult can also be due to any trauma affecting the temporal bone.
Inferior Medial Pontine Syndrome
Inferior medial pontine syndrome, also called Foville syndrome, typically occurs when there is occlusion of the paramedian branches of the basilar artery and subsequent ischemia of the medial aspect of the pons. This can result in damage to a number of structures, including:
- the corticospinal fibers, resulting in contralateral hemiplegia
- the medial lemniscus, resulting in contralateral diminution or potential loss of vibration, proprioception, and fine touch sensation;
- the PPRF, abducens nucleus, or abducens nerve, resulting in ipsilateral paralysis of the lateral rectus muscle and subsequently diplopia, or a potential loss of conjugate gaze toward the side of the lesion via interruption of communication between the abducens nucleus of one side of the brain with the oculomotor nucleus on the opposite side.
If the lesion is in the caudal pons and extends laterally, in may involve:
- the lateral lemniscus, resulting in hyperacusis
- the middle cerebellar peduncle, resulting in ataxia
- the motor nucleus of the facial cranial nerve, resulting in ipsilateral facial paralysis
- the spinal trigeminal nucleus and tract, resulting in ipsilateral loss of pain and temperature sensation in the face
- the anterolateral system resulting in contralateral loss of pain and temperature sensation in the body.
A lesion at this level resulting in corticospinal deficits on one side of the body with motor cranial nerve deficits on the opposite side of the face is referred to as a middle alternating hemiplegia.
If the lesion is in the rostral pons and extends medially, it may involve:
- the part of the medial lemniscus that contains fibers carrying sensory information from the upper extremity, leading to contralateral loss of vibration, proprioception, and fine touch sensation in the upper extremity;
- the trigeminal motor nucleus, resulting in ipsilateral paralysis of the muscles of mastication;
- the anterolateral system and parts of the spinal trigeminal tract and nucleus, resulting in contralateral loss of pain and temperature sensation in the body and ipsilateral loss of pain and temperature sensation in the face, respectively.
Glossopharyngeal neuralgia, also called glossopharyngeal tic, is a rare condition in which a person experiences idiopathic pain (i.e. pain without identifiable cause) localized to the parts of the mouth with sensory innervation from the glossopharyngeal nerve (the tonsillar area, posterior pharynx, and posterior tongue). The pain may be exacerbated by speaking or swallowing.
Lateral Pontine Syndrome
Occlusion of the long circumferential branches of the basilar artery and subsequent ischemia of the lateral aspect of the pons is associated with lateral pontine syndrome. This results in damage to a number of structures, including:
- the middle and superior cerebellar peduncles, resulting in ataxia and gait instabilities, with a tendency to fall toward the side of the lesion
- the vestibular and cochlear nuclei and nerves, resulting in vertigo, nausea or vomiting, nystagmus, deafness, or tinnitus
- the facial motor nucleus, resulting in ipsilateral paralysis of the muscles of facial expression
- the trigeminal motor nucleus, resulting in ipsilateral paralysis of the muscles of mastication
- descending hypothalamospinal fibers, resulting in ptosis, miosis, and anhidrosis (a.k.a. Horner syndrome)
- the anterolateral system and parts of the spinal trigeminal tract and nucleus, resulting in contralateral loss of pain and temperature sensation in the body and ipsilateral loss of pain and temperature sensation in the face, respectively
- the PPRF, resulting in loss of conjugate gaze toward the side of the lesion.
The precise constellation of symptoms observed depends significantly on whether the lesion occurs in the rostral or caudal regions of the pons. Lesions of the lateral pons and their associated clinical presentations are often referred to as Gubler syndrome, or Miller-Gubler syndrome; but basilar pontine lesions specifically involving the trigeminal root may also be referred to as mid pontine base syndrome.
Lateral Medullary Syndrome
Lateral medullary syndrome, otherwise known as Wallenberg syndrome, results when the posterior inferior cerebellar artery (PICA), supplying the dorsolateral medulla, is occluded. It can also occur when the vertebral artery, which supplies the PICA, is occluded. Such occlusion results in loss of blood flow, or ischemia, to the structures receiving blood supply from the PICA. Among these is the spinal trigeminal tract and nucleus, damage to which leads to loss of pain and temperature sensation in the side of the face ipsilateral to the lesion. Also damaged in this syndrome are the nucleus ambiguus, and the roots of the glossopharyngeal and vagus nerves, resulting in dysphagia, paralysis of the soft palate, hoarseness, and reduction or loss of the gag reflex.
Other symptoms of Wallenberg syndrome include:
- contralateral loss of pain and temperature sensation in the body, caused by damage to the anterolateral system
- ipsilateral Horner syndrome (i.e. miosis, ptosis, anhidrosis, and facial flushing), caused by damage to the descending hypothalamospinal tract
- nausea, diplopia, nystagmus, vertigo, and a tendency to fall toward the lesioned side, caused by damage to the vestibular nuclei
- ataxia on the side of the lesion, caused by damage to the restiform body and spinocerebellar tract.
The term syringobulbia refers to the formation of a cavity within the brainstem, typically the medulla. This may occur in addition to or as an extension of syringomyelia, a cavitation in the spinal cord, or it may occur completely on its own. While the cavity in syringomyelia usually forms in the middle of the spinal cord, the cavity in syringobulbia tends to be off to one side of the midline. Enlargement of this cavity can affect the surrounding structures.
Pressure to or damage of the hypoglossal nucleus or nerve is associated with weakened tongue muscles, leading to deviation of the tongue toward the side of the lesion upon protrusion. Pressure or damage to the nucleus ambiguus can cause weakness in the pharyngeal muscles, muscles of the palate, and vocal muscles, and presents with deviation of the uvula away from the side of the lesion. Nystagmus can result if the vestibular nuclei are affected, and damage to or pressure on the spinal trigeminal tract, nucleus, or fibers as they cross the midline can result in loss of pain and temperature sensation on the ipsilateral face.
A 67 year-old man visits his primary care physician with the complaint of fever, headache, fatigue, and a painful red rash on his face. He tells you the rash appeared only a day ago, but it was preceded by a few days of burning pain in the same region. On inspection, you note that the rash is erythematous, with a mix of fluid-filled blisters and ulcerated, crusting lesions. It is sensitive to touch, and only presents on the left upper third of his face, including his left eyelid.
Trigeminal neuralgia can have a variety of causes, one of which can be herpes zoster, otherwise known as shingles. Herpes zoster occurs in those with a history of infection with the varicella-zoster virus (VZV)–an enveloped, double-stranded DNA virus–which causes chickenpox. Chickenpox is one of the most common viral exanthems of childhood, and it is extremely virulent (i.e. infectious): by adulthood, over 95% of people will have contracted it. In a usual first infection (again, typically occurring in childhood) chickenpox is characterized by a pruritic (i.e. itchy) full-body rash of blisters, commonly described as having a “dew-drop on a rose petal” type of appearance. These blisters display what is referred to as temporal heterogeneity: new blisters erupt while old blisters simultaneously ulcerate and crust over.
In those with a history of chickenpox, the virus can enter and establish latency in the dorsal root ganglia of the spinal cord, including the trigeminal ganglia. Because of this, the virus can periodically reactivate, resulting in shingles, a painful skin rash which appears in a dermatomal distribution. Shingles occurs most frequently in those who are elderly or immunocompromised.
When trigeminal neuralgia is caused by reactivation of latent herpes zoster infection in the trigeminal ganglia, the infection and subsequent rash and other symptoms tend to primarily affect the ophthalmic branch of the trigeminal nerve. If this is the case, the condition is called ophthalmic zoster.
As of 1995, a live-attenuated chickenpox vaccine became available to the public for use in children 12 months of age and older. Although the current recommendations are that all children be vaccinated between 12 and 18 months of age with a booster vaccination between 11 and 12 years of age, adolescents and adults who have never been infected are also eligible to receive this vaccination.