the hypoglossal nerve is the twelfth paired cranial nerve

8/9/2019 The Hypoglossal Nerve is the Twelfth Paired Cranial Nerve

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The hypoglossal nerve is the twelfth paired cranial nerve. Its name is derived from ancient greek,

hypo meaning under, and glossal meaning tongue. The nerve has a purely somatic motor

function, innervating the majority of the muscles of the tongue.In this article, the anatomical course, motor functions and clinical relevance of the nerve will be

examined.

Anatomical Course

Fig 1.0The extracranial anatomical course of the hypoglossal nerve

The hypoglossal nerve arises from the hypoglossal nucleusin the medulla oblongataof the

brain. It then passes laterally across the posterior cranial fossa, within the subarachnoid space.

The nerve exits the cranium via the hypoglossal canal.Now extracranial, the nerve receives a branch of thecervical plexus that conducts fibres from

C1/C2 spinal nerve roots. These fibres do not combine with the hypoglossal nerve they merely

travel within its sheath.

It then passes inferiorly to the angle of the mandible, crossing the internal and external carotidarteries, and moving in an anterior direction to enter the tongue.

Motor Function

The hypoglossal nerve is responsible for motor innervation of the vast majority of the muscles of

the tongue (except forpalatoglossus). These muscles can be subdivided into two groups:i) Extrinsic muscles

Genioglossus (makes up the bulk of the tongue)

Hyoglossus

Styloglossus

Palatoglossus (innervated by vagus nerve)

ii) Intrinsic muscles

Superior longitudinal
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Inferior longitudinal

Transverse

VerticalTogether, these muscles are responsible for all movements of the tongue.

Role of the C1/C2 Roots

The C1/C2 roots that travel with the hypoglossal nerve also have a motor function. They branch

off to innervate the geniohyoid(elevates the hyoid bone) and thryohyoid(depresses the hyoid

bone) muscles.Another branch containing C1/C2 fibres descends to supply the ansa cervicalisa loop of

nerves that is part of the cervical plexus. From the ansa cervicalis, nerves arise to innervate the

omohyoid, sternohyoid and sternthyroid muscles. These muscles all act to depress thehyoid

bone.

Fig 1.1Overview of the motor functions of the hypoglossal nerve

Clinical Relevance

Examination of the Hypoglossal Nerve
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Fig 1.2Right hypoglossal nerve palsy, characterised by deviation of the tongue to the right.

The hypoglossal nerve is examined by asking the patient to protrude their tongue. Other

movements such as asking the patient to push their tongue against their cheek and feeling for thepressure on the opposite side of the cheek may also be used if damage is suspected.

Palsy of the Hypoglossal NerveDamage to the hypoglossal nerve is a relatively uncommon cranial nerve palsy. Possible causes

include tumoursand penetrating traumatic injuries. If the symptoms are accompanied by acute

pain, a possible cause may be dissection of the internal carotid artery.Patients will present with deviation of the tongue towards thedamaged sideon protrusion, as

well as possible muscle wasting and fasciculations (twitching of isolated groups of muscle

fibres) on the affected side.

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The vagus nerve is the 10th

cranial nerve (CN X). It is a functionally diverse nerve, offering

many different modalities of innervation. Due to its widespread functions, pathology of thevagus nerve is implicated in a vast variety of clinical cases.

In this article we shall look at its anatomical course, motor, sensory and autonomic functions.

The vagus nerve is associated with the derivatives of the fourth pharyngeal arch.

Sensory: Innervates the skin of the external acoustic meatus and the internal surfaces of thelaryngopharynx and larynx. Provides visceral sensation to the heart and abdominal viscera.

Special Sensory: Provides taste sensation to the epiglottis and root of the tongue.

Motor:Provides motor innervation to the majority of the muscles of the pharynx, soft palate andlarynx.

Parasympathetic: Innervates the smooth muscle of the trachea, bronchi and gastro-intestinal

tract and regulates heart rhythm.
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Anatomical CourseThe vagus nerve has the longest course of all the cranial nerves, extending from the head to theabdomen. Its name is derived from the Latin vagary- meaning wandering. It is sometimes

referred to as the wandering nerve.In the HeadThe vagus nerve originates from the medulla of the brainstem. It exits the cranium via

thejugular foramen, with the glossopharyngeal and accessory nerves (CN IX and XI

respectively).Within the cranium, the auricular brancharises. This supplies sensation to the posterior part of

the external auditory and canal external ear.

In the NeckBy Truth-seeker2004 [CC-BY-SA-3.0], via Wikimedia Commons

Fig 1.0Overview of the major branches of the vagus nerve

In the neck, the vagus nerve passes into the carotid sheath, travelling inferiorly with the internal

jugular vein and common carotid artery. At the base of the neck, the right and left nerves have

differing pathways:

The right vagus nervepasses anterior to the subclavian artery and posterior to the sternoclavicular

joint, entering the thorax.
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The left vagus nervepasses inferiorly between the left common carotid and left subclavian arteries,

posterior to the sternoclavicular joint, entering the thorax.

Several branches arise in the neck:

Pharyngeal branchesProvides motor innervation to the majority of the muscles of the pharynx and

soft palate.

Superior laryngeal nerve - Splits into internal and external branches. The external laryngeal nerve

innervates the cricothyroid muscle of the larynx. The internal laryngeal provides sensory innervation

to the laryngopharynx and superior part of the larynx.

Recurrent laryngeal nerve (right side only)Hooks underneath the right subclavian artery, then

ascends towards to the larynx. It innervates the majority of the intrinsic muscles of the larynx.

In the Thorax

Fig 1.1The origin of the recurrent laryngeal nerves

In the thorax, the right vagus nerve forms theposterior vagal trunk, and the left forms

theanterior vagal trunk. Branches from the vagal trunks contribute to the formation of the

oesophageal plexus, which innervates the smooth muscle of the oesophagus.Two other branches arise in the thorax:

Left recurrent laryngeal nerveit hooks under the arch of the aorta, ascending to innervate the

majority of the intrinsic muscles of the larynx. Cardiac branchesthese innervate regulate heart rate and provide visceral sensation to the organ.

The vagal trunks enter the abdomen via the oesophageal hiatus, an opening in the diaphragm.

In the AbdomenIn the abdomen, the vagal trunks terminate by dividing into branches that supply the oesophagus,stomach and the small and large bowel (up to the splenic flexure).
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Sensory FunctionsThere are somaticand visceralcomponents to the sensory function of the vagus nerve.

Somatic refers to sensation from the skin and muscles. This is provided by the auricular nerve,

which innervates the skin of the posterior part of the external auditory canal and external ear.

Viscera sensation is that from the organs of the body. The vagus nerve innervates:

Laryngopharynxvia the internal laryngeal nerve.

Superior aspect of larynx(above vocal folds)via the internal laryngeal nerve.

Heartvia cardiac branches of the vagus nerve.

Gastro-intestinal tract(up to the splenic flexure)via the terminal branches of the vagus nerve.

Fig 1.2The three parts of the pharynx, and their borders. The laryngopharynx is innervated by the vagus nerve.

Special Sensory FunctionsThe vagus nerve has a minor role in taste sensation. It carries afferent fibres from the root of thetongueand epiglottis.

(This is not to be confused with the special sensation of the glossopharyngeal nerve, which

provides taste sensation for the posterior 1/3 of the tongue).

Motor FunctionsThe vagus nerve innervates the majority of the muscles associated with the pharynx and larynx.

These muscles are responsible for the initiation of swallowing and phonation.
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Fig 1.2Lateral view of the deep structures of the pharynx. Visible are the circular muscles of the pharynx, and the stylopharyngeus.

Muscles of the PharynxMost of the muscles of the pharynx are innervated by the pharyngeal branchesof the vagusnerve:

Superior, middle and inferior pharyngeal constrictor muscles

Palatopharyngeus

Salpingopharyngeus

An additional muscle of the pharynx, thestylopharyngeus, is innervated by the glossopharyngeal

nerve.Muscles of the LarynxInnervation to the intrinsic muscles of the larynx is achieved via the recurrent laryngeal

nerve and external branch of the superior laryngeal nerve.

Recurrent laryngeal nerve: Thyro-arytenoid

Posterior crico-arytenoid

Lateral crico-arytenoid

Transverse and oblique arytenoids

Vocalis

External laryngeal nerve:

CricothyroidOther MusclesIn addition to the pharynx and larynx, the vagus nerve also innervates the palatoglossusof the

tongue, and the majority of the muscles of thesoft palate.

Parasympathetic Functions
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In the thorax and abdomen, the vagus nerve is the main parasympathetic outflow to the heart and

gastro-intestinal organs.

The HeartCardiac branches arise in the thorax, conveying parasympathetic innervation to the sino-atrial

and atrio-ventricular nodes of the heart (For more heart anatomy, seehere).These branches stimulate a reduction in the resting heart rate. They are constantly active,

producing a rhythm of 6080 beats per minute. If the vagus nerve was lesioned, the resting

heart rate would be around 100 beats per minute.

Gastro-Intestinal SystemThe vagus nerve provides parasympathetic innervation to the majority of the abdominal organs.It sends branches to the oesophagus, stomach and most of the intestinal tractup to the splenic

flexure of the large colon.

The function of the vagus nerve is to stimulate smooth muscle contraction and glandular

secretions in these organs. For example, in the stomach, the vagus nerve increases the rate of

gastric emptying, and stimulates acid production.

Clinical Relevance: Disorders of the Vagus NerveCardiovascular

Many pharmacological agents can be used to potentiate vagal tone on the heart therefore slowingthe heart rate. Beta-blockers, muscarinic agonists and cardiac glycosides such as Digoxin are just

a few that can be used.

Vasovagal syncope can ensue during a period of emotional stress for example causing a sudden

drop in blood pressure and heart rate. Further to this a carotid massage can compress the carotid

sinus leading to the perception of a high blood pressure. This will cause CN X to increase itsfiring leading to a decreased activity of the SA node and AV node. Overall a decreased rate and

strength of contraction will ensue and the person may experience syncope.

Many congenital heart defects such as a patent ductus arteriosuscan irritate the left recurrent

laryngeal nerve, leading to dysphonia (hoarse voice).Gastro-Intestinal

Lesions to the CN X are rare. A lesion to the pharyngeal branches can lead to dysphagia

(difficulty swallowing) due to the involvement with the muscles of the pharynx. As CN X

innervates the Palatopharyngeus and Salpingopharyngeus muscles a lesion here will cause the

Palatoglossal arch to drop leading to Uvula deviation away from the affected side. The CN IX issensory to the oropharynx and laryngopharynx with CN X being the motor efferents involved in

the Gag reflex therefore a lesion in this area will cause a loss of the Gag reflex.

Once upon a time a Vagotomy could be done to reduce excess stomach acid production.

However with advancements in pharmacological therapy this is no longer necessary.
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Other

As stated above a lesion to one of the RLNs will cause dysphonia. A lesion to both RLNs will

cause aphonia (loss of voice) and a stridor (inspiratory wheeze). Paralysis of the RLNs usuallyoccur due to cancer of the larynx or thyroid gland or due to surgical complications.

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be used as a substitute for professional diagnosis and treatment.By visiting this site you agree to the foregoing terms and conditions. If you do not agree to the foregoing terms and conditions, you should not

enter this site.

The glossopharyngeal nerve, CN IX, is the ninth paired cranial nerve. In thisarticle, we shall look at the anatomical course of the nerve, and the motor,sensory and parasympathetic functions of its terminal branches.

OverviewEmbryologically, the glossopharyngeal nerve is associated with thederivatives of the third pharyngeal arch.

Sensory: Innervates the oropharynx, carotid body and sinus, posterior 1/3 ofthe tongue, middle ear cavity and Eustachian tube.Special Sensory: Provides taste sensation to the posterior 1/3 of the tongue.Parasympathetic: Provides parasympathetic innervation to the parotid gland.Motor: Innervates the stylopharyngeus muscle of the pharynx.
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Anatomical CourseThe glossopharyngeal nerve originates in the medulla oblongataof the brain.It emerges from the anterior aspect of the medulla, moving laterally in theposterior cranial fossa. The nerve leaves the cranium via thejugular

foramen. At this point, the tympanic nervearises. It has a mixed sensoryand parasympathetic composition.

Fig 1.0 Lateral view of the neck, showing the innervation of the stylopharyngeus muscle.

Immediately outside the jugular foramen lie two ganglia (collections of nervecell bodies). They are known as the superiorand inferior (or petrous)ganglia - they contain the cell bodies of the sensory fibres in theglossopharyngeal nerve.

Now extracranial, the glossopharyngeal nerve descends down the neck,anterolateral to the internal carotid artery. At the inferior margin ofthestylopharyngeus, several branches arise to provide motor innervation tothe muscle. It also gives rise to the carotid sinus nerve, which providessensation to the carotid sinus and body.The nerve enters the pharynx by passing between thesuperior and middlepharyngeal constrictors. Within the pharynx, it terminates by dividing intoseveral branches lingual, tonsil and pharyngeal.

Sensory FunctionsThe glossopharyngeal nerve provides sensory innervation a variety ofstructures in the head and neck.
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Fig 1.1 Overview of the branches of the glossopharyngeal nerve.

The tympanic nerve arises as the nerve traverses the jugular foramen. Itpenetrates the temporal bone and enters the cavity of the middle ear. Here, itforms the tympanic plexus a network of nerves that provide sensoryinnervation to the middle ear,internal surface of the tympanicmembrane and Eustachian tube.At the level of the stylopharyngeus, thecarotid sinus nerve arises. Itdescends down the neck to innervates both the carotid sinus and carotidbody, providing information regarding blood pressure and oxygenationrespectively.The glossopharyngeal nerve terminates by splitting into several sensory

branches:

Pharyngeal branch combines with fibres of the vagus nerve to form thepharyngeal plexus. It innervates the mucosa of the oropharynx.

Lingual branch provides the posterior 1/3 of the tongue with general andtaste sensation
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Tonsillar branch forms a network of nerves, known as the tonsillarplexus, which innervates the palatine tonsils.

Special SensoryThe glossopharyngeal nerve provides taste sensation to the posterior 1/3 ofthe tongue, via its lingual branch (Note: not to be confused with the lingualnerve).

Motor FunctionsThe stylopharyngeusmuscle of the pharynx is innervated by theglossopharyngeal nerve. This muscle acts to shorten and widen the pharynx,and elevate the larynx during swallowing.

Parasympathetic Functions

Fig 1.2 Path of the parasympathetic fibres to the parotid gland.

The glossopharyngeal nerve provides parasympathetic innervation tothe parotid gland.These fibres originate in the inferior salivatory nucleus ofCN IX. These fibres travel with the tympanic nerve to the middle ear. Fromthe ear, the fibres continue as thelesser petrosal nerve, before synapsing at

the otic ganglion.The fibres then hitchhike on theauriculotemporal nerve to the parotidgland, where they have a secretomotor effect.Remember although the facial nerve splits into its five terminal branches inthe parotid gland, it is the glossopharyngeal nerve that actually supplies thegland.

Clinical Relevance Gag Reflex
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The glossopharyngeal nerve supplies sensory innervation to the oropharynx,and thus carries theafferent information for the gag reflex. When a foreignobject touches the back of the mouth, this stimulates CNIX, beginning thereflex. The efferent nerve in this process is the vagus nerve, CNX.An absent gag reflex signifies damage to the glossopharyngeal nerve.

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The External Ear


The Nasal Cavity


The Vagus Nerve (CN X)


Ulnar Claw vs Hand of Benediction


Instant Anatomy Book Review


The medical information on this site is provided as an information resource only, and is not to be used or relied on for any diagnostic ortreatment purposes. This information is intended for medical education, and does not create any doctor-patient relationship, and should not

be used as a substitute for professional diagnosis and treatment.By visiting this site you agree to the foregoing terms and conditions. If you do not agree to the foregoing terms and conditions, you should not

enter this site.

Oliver Jones

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the hypoglossal nerve is the twelfth paired cranial nerve

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