The Nervous System · Part Two · How It Senses and Moves
Lesson 29 / 61
Cranial Nerves VII to XII: Where the Nuclei Sit and How Each Nerve Fails
Taste, balance, voice, swallowing and the tongue, with a number attached to every deficit.
Cranial nerves VII to XII leave the pons and the medulla carrying facial expression, hearing and balance. The same six carry taste, swallowing, voice, tongue movement and the parasympathetic command of the heart and gut. Report and command travel together in the same trunks. No route from a surface input to brainstem regulation is shorter. The level of a lesion is read in the forehead, the uvula, the voice and the tongue. Those four readings report one integrated state, the organization the Unified Model of Tone tracks.
Nerves
CN VII to CN XII, six pairs
Anterior tongue taste
CN VII, anterior two thirds
Residual deficit at one year
29 percent of 2,570 palsies
Vagus stimulation therapy
In clinical use since 1997
Cranial nerves VII to XII
Six pairs leaving the brainstem below the trigeminal. The facial and vestibulocochlear nerves exit at the cerebellopontine angle and enter the internal acoustic meatus of the temporal bone. The glossopharyngeal, vagus and accessory nerves leave through the jugular foramen. The hypoglossal nerve takes its own canal in the occipital bone.
Where these six nerves converge
Taste, blood pressure, lung stretch and gut stretch all arrive at the solitary nucleus. Swallowing and phonation leave through the nucleus ambiguus, with the dorsal motor nucleus of the vagus alongside it for the heart, the airway and the gut. Each of these six nerves fails by degree rather than all at once, so a deficit here is a surviving fraction rather than an absence. The crowding of the medullary floor itself is the subject of the brainstem.
01The facial nerve trunk
The facial nerve carries motor command, gland control and taste in a single trunk
The facial nerve, CN VII, supplies the muscles of facial expression and the stapedius muscle that damps sound. Its second root, the nervus intermedius, carries taste from the anterior two thirds of the tongue. The same root carries parasympathetic secretomotor fibers to the salivary, lacrimal, nasal and palatine glands. The trunk emerges at the cerebellopontine angle, at the lateral pontomedullary junction, alongside the eighth nerve.
Taste from the front of the tongue runs in the chorda tympani, and the cell bodies of those fibers sit in the geniculate ganglion Tokita 1978. Eight human temporal bones from four patients with familial dysautonomia showed an extreme paucity of geniculate ganglion cells against thirteen normal controls. The same specimens carried reduced superior and inferior vestibular neuron counts at P less than 0.001.
Where the seventh nerve has no sheath
The seventh nerve crosses the angle and threads the canal with the least structural protection it will ever have. Sixty facial nerves from thirty embalmed adult human cadavers, serially sectioned every 2 mm, showed no real fascicular organization in the pontocerebellar cistern or in the internal acoustic meatus Captier 2005. Perineurium and epineurium appear only from the geniculate ganglion onward.
Beyond the ganglion the extratemporal nerve divides into 2 to 6 fascicles in some specimens and 7 to 15 in others, and the authors concluded that this variability makes surgical repair difficult. The Unified Model of Tone treats the geniculate ganglion as a coordinate rather than a landmark. Taste cell bodies, the first fascicular sheathing and the branch to the lacrimal gland all appear at that one point. A lesion above it and a lesion below it strip different combinations from the same nerve.
02Findings
What the research shows
03Reading a facial palsy
A facial palsy is read twice, once for level and once for degree
A facial palsy declares its level in the forehead. An upper motor neuron lesion spares the forehead and orbicularis oculi. A lower motor neuron lesion of the facial motor nucleus or the peripheral nerve produces complete weakness on that side. The corneal reflex tests the same nucleus from the sensory side. Its afferent arm runs in the trigeminal nerve and its efferent arm in the seventh, which makes a one-sided failure of blink one of the earliest readable signs in the brainstem. The trigeminal side of that arc is set out in cranial nerves I to VI.
Why an upper motor neuron lesion spares the forehead
Cortical drive to the face is not spread evenly across the facial nucleus. Tracer injection in the macaque mapped six cortical face representations onto four subdivisions of that nucleus Morecraft 2001. M1, M4, LPMCv and LPMCd innervate primarily the contralateral lower facial muscles through the lateral subnucleus. M3 projects bilaterally to the dorsal and intermediate subnuclei that supply frontalis and orbicularis oculi, and M2 projects bilaterally to the medial subnucleus supplying the auricular muscles. Redundancy is concentrated on the upper face.
The vascular geography follows the same map. M2 and M3 sit on the medial wall in anterior cerebral artery territory, while M1 and LPMCv sit in middle cerebral artery territory Morecraft 2004. Contralateral lower facial paralysis with intact upper facial movement after a middle cerebral artery infarct is that split showing itself in a patient. Forehead sparing is a wiring fact with a tracer study behind it.
Corticofacial projections arise from limbic proisocortices as well as frontal isocortices, which Morecraft and colleagues identified as a possible substrate for the clinical dissociation between emotional and volitional facial movement. A patient can fail to smile on command and smile at a joke, using the same muscles and the same motoneurons. The Unified Model of Tone reads the facial motor nucleus as a shared final path with several independent drives arriving on it, so a resting face reports which drive currently holds the pool.
Recovery follows the conducting fraction
The Copenhagen Facial Nerve Study followed 2,570 peripheral facial palsies of different causes for up to a year without treatment. Normal mimical function returned in 71 percent, with sequelae slight in 12 percent, mild in 13 percent and severe in 4 percent Peitersen 2002. Eighty-five percent of recoveries appeared within three weeks and the rest across three to five months. Contracture followed in 17 percent and associated movements in 16 percent.
The diagnostic label matters less than the conduction behind it. In a prospective cohort of 196 patients, all but one paresis recovered completely, while 30 percent of the complete paralyses did not Linder 2010. Ninety-four percent recovered fully when electroneuronography showed denervation below 90 percent, and the worst outcomes fell to patients with 100 percent denervation. The 29 percent that keeps a deficit belongs to the surviving axon count.
The Unified Model of Tone reads the 90 percent mark as the point where a graded quantity gets forced into a category. Health in this nerve is a conducting fraction large enough that the map the face rebuilds matches the one it lost, and the movement comes back. Illness is a map rebuilt around a shortage, and contracture and synkinesis are what that rebuilding costs.
04Hearing and equilibrium
The vestibulocochlear nerve carries hearing and balance through the canal the facial nerve uses
CN VIII is purely sensory, and its two divisions fail separately. The cochlear division carries hearing and fails as conductive or sensorineural loss. The vestibular division carries equilibration and fails as vertigo and unsteadiness. The nerve arises laterally at the cerebellopontine angle and passes with the facial nerve into the internal acoustic meatus of the temporal bone, the cochlear fibers running anterior and the vestibular fibers posterior.
Conductive hearing loss reflects defective transmission of sound waves to the endolymph. Sensorineural loss arises from disease of the cochlea, the eighth nerve or the central auditory pathway, including the presbycusis of age. Nelson and Hinojosa examined 21 presbycusic human temporal bones against ten normal controls Nelson 2006. Outer hair cell loss and spiral ganglion cell loss appeared in every case, inner hair cell loss in 18 of 21 and stria vascularis loss in 10 of 21. The extent of ganglion cell degeneration tracked the slope of the audiogram, so presbycusis is four elements failing together rather than one lesion. Hearing loss of all causes reached 1.57 billion people in 2019, one in five people alive, and 62.1 percent of them were older than 50 GBD 2019 Hearing Loss Collaborators 2021. The ear in infancy is set out in hearing and the ear.
Vestibular schwannoma incidence rose sixfold as imaging improved
Vestibular schwannoma incidence in Denmark rose from 3.1 per million per year in 1976 to a peak of 22.8 in 2004, then settled to 19.4 per million per year in 2008 Stangerup 2010. The count came to 2,283 tumors in a national population of 5.0 to 5.5 million. Mean tumor size at diagnosis fell from 30 mm in 1979 to 10 mm in 2008, and hearing acuity at diagnosis improved alongside it. An earlier Danish count across 1977 to 1995 caught the front of the same rise, from 5 to 10 cases per million per year Howitz 2000. Its authors credited more precise diagnostic procedures and greater awareness rather than a change in biology. A lesion count at the cerebellopontine angle measures how hard the angle was searched, and the VII and VIII deficits found there now are smaller than the ones that used to reach diagnosis.
One blood supply serves the seventh nerve, the eighth nerve and the inner ear
A single vascular lesion in labyrinthine artery territory delivers vertigo, unsteadiness and hearing loss together, because that one branch of the vertebrobasilar system feeds VII, VIII and the inner ear. Acute hearing loss of vascular cause is mostly associated with infarction in anterior inferior cerebellar artery territory Kim 2016. Combined loss of auditory and vestibular function is the commonest of at least eight recognized AICA presentations. Around 11 percent of isolated cerebellar infarcts imitate an acute peripheral vestibulopathy, and the head impulse test separates posterior inferior cerebellar artery infarction from benign inner-ear disorders.
A facial lesion above the branch to stapedius removes the muscle that damps the ossicular chain, and the patient hears a peculiar echoing hyperacusis. The seventh nerve sets how loud the eighth nerve's world arrives.
05The solitary nucleus
The solitary nucleus receives taste and visceral report from four cranial nerves and routes it onward
Four cranial nerves feed one nucleus. Taste and visceral sensation reach the solitary nucleus and its tract from cranial nerves VII, IX and X and from the cranial part of the accessory nerve. The taste fibers arrive as central processes from three ganglia. The geniculate ganglion of VII carries the anterior two thirds of the tongue. The inferior ganglion of IX carries the posterior third, and the inferior ganglion of X the epiglottis.
The nucleus divides along its length. A rostral gustatory portion handles taste. A caudal cardiorespiratory portion receives carotid and aortic baroreceptors and chemoreceptors, lung stretch and gut wall information, then routes most of it to the hypothalamus and the cingulate gyrus. Little of that traffic reaches awareness, because the work is reflex. Raising blood pressure with phenylephrine in the rat produced c-fos expression in exactly the solitary zone where primary baroreceptor afferents terminate Chan 1998. In the same rats the barosensitive GABAergic cells sat in the caudal ventrolateral medulla, in a band running about 1.4 mm rostrally from the calamus scriptorius. That band lies immediately adjacent to the solitary zone that had received the afferents. Inflow and the first relay of outflow are neighbors, and the barosensitive solitary neurons reach the pressor cells of the rostral ventrolateral medulla through that caudal relay rather than by a direct line.
Where the solitary nucleus sends what it receives
The solitary nucleus sends its output with topography, subnucleus by subnucleus. Axonal transport work in the rat found a small cluster in the intermediate third, the subnucleus centralis Ross 1985. That cluster, with neurons lying near the solitary tract, selectively innervated the retrofacial nucleus and the nucleus ambiguus. Other subnuclei reach the rostral dorsal motor nucleus of the vagus and the C1 adrenaline neurons that project to the thoracic spinal cord. Diffusion and resting-state mapping in 20 Human Connectome Project participants across 21 regions found the same architecture in people, with the nucleus predominantly processing afferent input and a lateralization toward left-sided processing Forstenpointner 2022. The traced anatomy is what makes the solitary nucleus the afferent gateway of autonomic regulation: named inputs, named subnuclei, named targets in the medulla. The loop arithmetic that turns this inflow into a held blood pressure runs a little further down the medulla and is set out in autonomic regulation.
Skin on the outer ear reaches the solitary nucleus
Stimulating skin on the outer ear reaches that nucleus in living people. The cymba conchae is a patch of external ear innervated exclusively by the auricular branch of the vagus. Stimulating it electrically in 12 healthy adults activated the ipsilateral solitary nucleus, the bilateral spinal trigeminal nucleus and the dorsal raphe Frangos 2015. Activation also appeared in the locus coeruleus and in the contralateral parabrachial area, amygdala and nucleus accumbens. Earlobe stimulation served as the control and did not produce that pattern.
Body wall input reaches the same organs the vagus governs. In anesthetized animals, somatic afferent stimulation reflexly changes cerebral cortical blood flow, heart rate, gastric motility and adrenal medullary secretion Sato 1997. Urinary bladder contractility and splenic immune function answer the same way. Whether the sympathetic or the parasympathetic limb carries the reply depends on the organ, the site stimulated and the mode of stimulation.
Two nerves sharing one canal share one fate. At the jugular foramen three do: the ninth, tenth and eleventh leave the skull together at the base of the neck. Chiropractic care applies graded mechanical and sensory input at that base. The Unified Model of Tone reads site dependence as a property of the afferent lying under the contact, not of the current. Stimulating the cymba conchae in 12 healthy adults reached the solitary nucleus, the locus coeruleus and the parabrachial area. The same current on the earlobe, skin the vagus does not innervate, reached none of them. What an input reaches is set by which afferents lie under it, and what it then produces is measured in the organ rather than at the nucleus.
06The vagus in the throat
Cranial nerves IX and X run the throat, and their sensory arm keeps food out of the airway
Cranial nerves IX and X share the visceral floor of the medulla. The glossopharyngeal nerve supplies sensation to the oropharynx and posterior tongue, motor fibers to stylopharyngeus, and parasympathetic input through the inferior salivatory nucleus and the otic ganglion to the parotid gland. The vagus, CN X, governs phonation and swallowing through the nucleus ambiguus. It carries preganglionic fibers from the dorsal motor nucleus to the heart, the tracheobronchial tree and the gut as far as the splenic flexure, the longest reach in the cranial system.
The brainstem swallowing network runs the solitary nucleus and the nucleus ambiguus as one circuit, with the reticular formation linking them synaptically to cranial motoneuron pools on both sides Ertekin 2003. Muscles fire in sequence from the perioral muscles caudally to the cricopharyngeal sphincter, and that order does not alter. Ertekin and Aydogdu read the fixed order as evidence for a central pattern generator, one the cortex triggers and modulates rather than drives. Three fourths of oropharyngeal dysphagia is caused by neurological disease. The same sequence in the newborn is set out in the cranial nerves of feeding.
Aspiration begins on the sensory side
The laryngeal adductor reflex runs on afferents in the superior laryngeal nerve, a branch of the vagus. With that reflex absent, thin liquids penetrated the larynx in 46 of 46 patients and were aspirated in 43 of 46, against 26 of 76 and 13 of 76 in controls Aviv 2002. Purees penetrated in 39 of 46 and were aspirated in 32 of 46. Pharyngeal squeeze was impaired in 41 of the 46, against 17 of 76 controls. The muscles were never the failure.
A vagal lesion reads out in the voice, and the reading depends on the test
A unilateral vagal palsy deviates the uvula toward the normal side, produces a hoarse voice from laryngeal paralysis, and impairs the gag reflex. Across 27 studies and 25,000 patients, temporary recurrent laryngeal nerve palsy after thyroid surgery averaged 9.8 percent and permanent palsy 2.3 percent Jeannon 2009. Individual studies reported temporary palsy as high as 26 percent, and the spread tracked how the larynx was examined rather than how it was operated on.
The tenth nerve as an input channel
The right vagus innervates the sinoatrial node and the left branch reaches atrioventricular conduction. At this cardiac junction Otto Loewi first showed that a nerve secretes a chemical, naming the substance Vagusstoff Sourkes 2009. Its identification as acetylcholine, and its reading outward to the rest of the nervous system, took the middle third of the twentieth century. Vagus nerve stimulation therapy, approved by the US Food and Drug Administration in 1997 Wheless 2004, coils its leads around the left cervical vagus.
In the randomized active-control trial, 94 patients on high stimulation averaged a 28 percent reduction in seizure frequency against 15 percent in 102 patients on low stimulation, at p equal to 0.04 Handforth 1998. The control was low stimulation rather than an inactive device, because stimulation of the vagus in the neck alters the voice and the patient feels it. High stimulation produced more voice alteration and dyspnea than low. No changes appeared in physiologic indicators of gastric, cardiac or pulmonary function, which is what the cardiac asymmetry of the two vagal trunks predicts. The open-label extension was unblinded and analyzed by declining N, after 21 subjects exited for lack of efficacy. It held complete data on 164 of the 199 enrolled Amar 1999. In those patients mean reduction rose from 28 percent at 3 months on high stimulation to 37 percent at 15 months, with 39 percent passing 50 percent reduction. The Unified Model of Tone stakes its claim on the controlled result rather than the curve. Intermittent input on one cervical nerve moves the seizure threshold at all, which is the finding the model needs. Whether that effect grows across months cannot be answered from an unblinded extension losing its non-responders. The full trial record, and the blinding problem that runs through the field, is set out on the vagus nerve.
07Accessory and hypoglossal
The accessory and hypoglossal nerves lose function in graded amounts, measured in microvolts and in events per hour
The cranial accessory arises from a caudal extension of the nucleus ambiguus and joins the vagus. The spinal accessory, formed by rootlets from the upper four or five cervical segments, innervates the sternocleidomastoid and trapezius muscles that govern the posterior triangle of the neck. The hypoglossal nerve leaves the skull by its own canal and turns forward under the tongue. Neither nerve announces its injury as a yes or a no.
The surface landmarks usually given for the accessory path do not hold. Bilateral dissection of 25 adult cadavers found the course and the exit points differing significantly from previously recorded values Symes 2005. The cervical plexus contribution varied too, and the authors concluded that an accurate surface anatomy could not be delineated. Iatrogenic injury with pain, paralysis and scapular winging follows from trusting the diagram over the tissue.
How far into the triangle the knife went sets how much the trapezius loses. Across 60 neck dissections in 51 patients, trapezius surface EMG fell from a median 54.3 microvolts before surgery to 11.6 microvolts after Lima 2011. Across those 60 dissections the mean decrease per patient was 70 percent. After level IIb dissection the value stood at 12.5 microvolts, against 8.9 microvolts after IIb plus level V.
The hypoglossal nerve holds the airway open
The paired genioglossi hold the pharyngeal airway open through the night, and the twelfth nerve is what drives them. The hypoglossal nerve is purely motor to the intrinsic and extrinsic tongue muscles, except palatoglossus. The two genioglossi protrude the tongue when they work together, which is postural work as much as speech and swallowing. In a single-group cohort of 126 patients with obstructive sleep apnea, stimulation delivered to the hypoglossal nerve cut the median apnea-hypopnea index by 68 percent at 12 months Strollo 2014. The index fell from 29.3 to 9.0 events per hour, and the oxygen desaturation index fell 70 percent alongside it. In the randomized therapy-withdrawal phase the index climbed back from 7.6 to 25.8 events per hour, while the group that kept its stimulation held at 8.9.
Localizing a lower cranial nerve lesion
A lower motor neuron lesion deviates the tongue toward the weak side, with atrophy and fasciculations. An upper motor neuron lesion deviates it to the opposite side. The last four cranial nerves are rarely damaged in isolation, so when they fail together the lesion usually sits in the posterior fossa. The jugular foramen and the medullary floor are set out in the brainstem.
The earlobe and the cymba conchae take the same current, and only one of them reaches the brainstem.
08Tone
How this system expresses tone
Tone shows up here as what six nerves can still hold together. The surviving fraction sets the ceiling on recovery, what arrives at the brainstem sets the reflex, and what shares a route sets which deficits appear together.
Constraint
How far into the neck triangle a dissection goes sets what the trapezius keeps: from a median 54.3 microvolts, 12.5 after level IIb and 8.9 after IIb plus V.
Input quality
Swallowing fails from the sensory side. With the laryngeal adductor reflex absent, thin liquids were aspirated by 43 of 46 patients, against 13 of 76 controls.
Coupling
One labyrinthine artery feeds the seventh nerve, the eighth nerve and the inner ear, so one vascular lesion delivers hearing loss and vertigo together, the commonest of eight AICA presentations.
Set point: the twelfth nerve defends airway caliber overnight, and losing that defense reads as 29.3 events per hour. Oscillation: the ninth and tenth nerves run the swallow in a fixed order from the perioral muscles to the cricopharyngeal sphincter, and the order does not alter. Prediction: limbic and frontal cortices reach the seventh nerve's motor nucleus by separate roads, so a spontaneous smile survives when a commanded one fails. Time course: the left cervical vagus cut seizure frequency 28 percent at three months in the controlled trial, and 37 percent at fifteen in the open-label extension. Load: seventh nerve regrowth is paid for, with contracture in 17 percent and associated movements in 16 percent of 2,570 facial palsies. Gain: the seventh nerve sets stapedius damping of the ossicular chain, and a lesion above that branch leaves an echoing hyperacusis.
09Across the library
How this page relates to the rest of the library
How much of the tenth nerve's traffic runs upward, what heart rate variability indexes, and why a completely inactive control device was never available to the stimulation trials.
The two-neuron chain and the segmental map that vagal and glossopharyngeal outflow joins, including the ganglia where the second neuron waits.
How closely the medullary floor is packed, and why the last four cranial nerves failing together points to the posterior fossa rather than to any one nerve.
The same swallow sequence in the newborn, where suck, swallow and breathe have to interleave from the first hour of life.
The bony exit shared by the ninth, tenth and eleventh nerves, and what crowding at that exit does to feeding and settling in an infant.
What sustains dizziness after the eighth nerve's report is corrupted, and how a peripheral vestibulopathy is told apart from a cerebellar infarct at the bedside.
10Frequently asked
Questions about this topic
Why does a stroke spare the forehead?
Cortical drive to the face is not distributed evenly. Tracer work in the macaque showed M1, M4 and the lateral premotor face areas projecting mainly to the contralateral lateral subnucleus of the facial nucleus, which supplies the muscles around the mouth. M2 and M3 project to both sides, and M3 reaches the subdivisions supplying frontalis and orbicularis oculi. One hemisphere can be lost and the forehead still receives drive from the other. A peripheral facial nerve lesion removes every input at once, so that side goes weak.
What percentage of people recover completely from Bell's palsy?
The Copenhagen Facial Nerve Study followed 2,570 untreated peripheral facial palsies of different causes, Bell's palsy among them. Normal mimical function returned in 71 percent. Sequelae were slight in 12 percent, mild in 13 percent and severe in 4 percent, so roughly 29 percent carried some residual deficit. Contracture appeared in 17 percent and associated movements in 16 percent. Eighty-five percent of recovery arrived within three weeks, and the remainder across three to five months. Outcome tracks how much of the nerve is still conducting.
Which cranial nerves pass through the jugular foramen?
The glossopharyngeal, vagus and accessory nerves leave the skull through the jugular foramen, alongside the sigmoid sinus draining into the internal jugular vein. The facial and vestibulocochlear nerves take a different exit, the internal acoustic meatus, after crossing the cerebellopontine angle. The hypoglossal nerve has its own canal in the occipital bone. Because three nerves share one foramen, a lesion sitting there produces a combination rather than a single sign: a hoarse voice, an impaired gag, and weakness of the sternocleidomastoid and trapezius on the same side.
What does the solitary nucleus do?
The solitary nucleus is where taste and visceral report arrive in the medulla, taking fibers from cranial nerves VII, IX and X and from the cranial part of the accessory nerve. Its rostral portion handles taste. Its caudal portion receives carotid and aortic baroreceptors and chemoreceptors, lung stretch and gut wall information. Human diffusion and resting-state mapping confirms the nucleus predominantly processes afferent input. Its subnuclei then project topographically to the nucleus ambiguus, the dorsal motor nucleus of the vagus and the adrenaline cells that reach the thoracic cord.
Can stimulating the outer ear reach the vagus nerve?
Stimulation of the outer ear has been imaged reaching the vagal projection. In 12 healthy adults, electrical stimulation of the cymba conchae activated the ipsilateral solitary nucleus, the spinal trigeminal nucleus and the locus coeruleus. The cymba conchae is a patch of external ear innervated exclusively by the auricular branch of the vagus. Activation also reached the contralateral parabrachial area, amygdala and nucleus accumbens. Earlobe stimulation served as the control and did not produce that pattern. What such stimulation achieves clinically is a separate question.
Why does a one-sided vagus lesion make the voice hoarse?
The vagus supplies the laryngeal muscles through the nucleus ambiguus, and its recurrent laryngeal branch reaches the vocal fold. A unilateral lesion paralyzes one fold, so the two no longer meet cleanly and the voice turns hoarse. The uvula deviates toward the normal side and the gag reflex weakens. After thyroid surgery, temporary palsy of that branch averages 9.8 percent and permanent palsy 2.3 percent across 27 studies and 25,000 patients. Individual studies report up to 26 percent, and the spread tracks how the larynx was examined.
How much facial nerve has to survive for a face to recover?
Recovery tracks conducting fibers rather than the diagnostic label. Electroneuronography compares the response on the palsied side against the healthy side and reports the loss as a percentage. In a prospective cohort of 196 patients with peripheral facial palsy, 94 percent recovered fully when denervation stayed under 90 percent. The worst outcomes fell to patients at 100 percent denervation, and 30 percent of the complete paralyses never recovered fully. All but one paresis recovered completely. Ninety percent denervation is the point where a graded quantity turns into a category.
Which way does the tongue deviate after a hypoglossal lesion?
A lower motor neuron lesion of the hypoglossal nerve deviates the tongue toward the weak side, with atrophy and fasciculations of that half. An upper motor neuron lesion deviates it to the opposite side, without wasting. The nerve is purely motor to the intrinsic and extrinsic tongue muscles, except palatoglossus, and the paired genioglossi protrude the tongue when they act together. One weak genioglossus lets the intact one push the tip across the midline. The same muscles hold the airway open during sleep.
11The sources
References
Sources: primary literature, linked inline.