Last Four Cranial Nerves

INTRODUCTION

• There are 12 pairs of cranial nerves.

I: Olfactory

II: Optic

III: Oculomotor

IV: Trochlear

V: Trigeminal

VI: Abducent

VII: Facial

VIII: Vestibulo-cochlear

IX: Glossopharyngeal

X: Vagus

XI: Accessory

XII: Hypoglossal.

• The first and second pair of cranial nerves arise from the forebrain and the remaining 10 pairs arise from the brainstem.

Functional Components of Cranial Nerves

• Cranial nerves contain functional components that perform specific motor or sensory activities.
• These components are classified into efferent (motor) and afferent (sensory) fibres.

Efferent (Motor) Components

1. General Somatic Efferent (GSE): These fibres supply skeletal muscles derived from somites, particularly the extraocular muscles and muscles of the tongue.

2. Special Visceral Efferent (SVE) or Branchial Efferent: These fibres supply skeletal muscles derived from the pharyngeal arches. Traditionally, the sternocleidomastoid and trapezius are also included in this category.

3. General Visceral Efferent (GVE): These are preganglionic parasympathetic fibres that regulate smooth muscles, cardiac muscle, and glands through autonomic ganglia.

Afferent (Sensory) Components

1. General Visceral Afferent (GVA): These fibres carry sensations from the viscera and sensory information from baroreceptors and chemoreceptors, mainly through the glossopharyngeal and vagus nerves.

2. Special Visceral Afferent (SVA): These fibres carry taste sensations through the facial, glossopharyngeal, and vagus nerves. Olfaction is also traditionally classified under this component.

3. General Somatic Afferent (GSA): These fibres transmit touch, pain, temperature, and proprioceptive sensations from the face, scalp, and mucous membranes, predominantly through the trigeminal nerve.

4. Special Somatic Afferent (SSA): These fibres carry visual, auditory, and vestibular sensations through the optic and vestibulocochlear nerves.

Functional Components of Individual Cranial Nerves

• Each cranial nerve may contain one or more functional components.
• Trochlear nerve (CN IV): It contains GSE fibres that supply the superior oblique muscle.
• Oculomotor nerve (CN III): It carries GSE fibres to extraocular muscles and GVE fibres for parasympathetic control of the sphincter pupillae and ciliary muscle.

GLOSSOPHARYNGEAL NERVE

• The glossopharyngeal nerve is the ninth cranial nerve (CN IX).
• It is a mixed nerve containing motor, sensory, and parasympathetic fibres.
• It supplies the stylopharyngeus muscle, parotid gland, posterior one-third of the tongue, pharynx, and middle ear. It also carries sensations from the carotid sinus and carotid body.

Functional Components and Nuclei

1. Special Visceral Efferent (SVE)

• These fibres arise from the nucleus ambiguus in the medulla oblongata.
• They supply the stylopharyngeus muscle, which develops from the third pharyngeal arch.

2. General Visceral Efferent (GVE)

• These preganglionic parasympathetic fibres arise from the inferior salivatory nucleus.
• They pass through the tympanic and lesser petrosal nerves and relay in the otic ganglion.
• Postganglionic fibres reach the parotid gland through the auriculotemporal nerve.

3. General Visceral Afferent (GVA)

• These fibres carry visceral sensory information from the pharynx, carotid sinus, and carotid body.
• Their cell bodies lie in the inferior ganglion, and their central processes terminate in the nucleus tractus solitarius.

4. Special Visceral Afferent (SVA)

• These fibres carry taste sensations from the posterior one-third of the tongue, including the circumvallate papillae.
• Their cell bodies lie in the inferior ganglion, and their central processes terminate in the gustatory part of the nucleus tractus solitarius.

5. General Somatic Afferent (GSA)

• These fibres carry general sensory information from a small region of the external ear and adjoining structures.
• Their cell bodies are associated mainly with the superior ganglion.
• Their central processes terminate in the spinal trigeminal nucleus.

Ganglia of Glossopharyngeal Nerve

– The glossopharyngeal nerve possesses two sensory ganglia containing the cell bodies of primary sensory neurons.

1. Superior (jugular) ganglion: It is small and lies within the jugular foramen. It contains mainly general somatic sensory neurons.
2. Inferior (petrosal) ganglion: It is larger and lies near the lower end of the jugular foramen. It contains visceral sensory and taste neurons.

Origin of Glossopharyngeal Nerve

• The nerve emerges from the posterolateral sulcus of the medulla oblongata through three or four rootlets.
• These rootlets appear immediately superior to those of the vagus nerve.

Course and Relations

Intracranial course

• The rootlets pass forwards and laterally towards the jugular foramen.
• They unite to form a single nerve trunk that leaves the cranial cavity through the anteromedial part of the jugular foramen.
• The nerve passes through a separate dural sheath and presents superior and inferior sensory ganglia near the foramen.

Extracranial course

• The nerve descends between the internal jugular vein and internal carotid artery.
• It passes forwards between the internal and external carotid arteries, deep to the styloid process and its attached muscles.
• It curves around the lateral surface of stylopharyngeus and gives a motor branch to this muscle.
• Finally, it enters the pharynx between the superior and middle constrictor muscles and divides into terminal branches.

Branches and Distribution

1. Tympanic Branch (Jacobson’s Nerve)

• It arises from the inferior ganglion and enters the tympanic cavity through the tympanic canaliculus.
• It forms the tympanic plexus and supplies the mucosa of the middle ear, mastoid air cells, and auditory tube.
• Its preganglionic parasympathetic fibres continue through the lesser petrosal nerve to the otic ganglion and ultimately supply the parotid gland.

2. Carotid Branch (Hering’s Nerve)

• It descends towards the carotid bifurcation.
• It carries sensory impulses from the carotid sinus (baroreceptors) and carotid body (chemoreceptors).
• It participates in reflex regulation of blood pressure, heart rate, and respiration.

3. Pharyngeal Branches

• They contribute to the pharyngeal plexus.
• They carry general sensations from the mucous membrane of the oropharynx.

4. Muscular Branch

• It supplies the stylopharyngeus muscle, the only skeletal muscle directly innervated by the glossopharyngeal nerve.
• The muscle elevates the pharynx during swallowing and speaking.

5. Tonsillar Branches

• They form a plexus around the palatine tonsil.
• They carry sensory fibres from the palatine tonsil, adjoining soft palate, and palatoglossal arch.

6. Lingual Branches

• They supply the mucous membrane of the posterior one-third of the tongue.
• They carry both general sensations and taste, including taste from the circumvallate papillae.

7. Communicating Branch to Vagus Nerve

• The glossopharyngeal nerve may communicate with the auricular branch of the vagus nerve.
• Some sensory fibres associated with the glossopharyngeal nerve contribute to innervation of a small region of the external ear.

VAGUS NERVE

• The vagus nerve is the tenth cranial nerve (CN X).
• It is the longest and most widely distributed cranial nerve.
• It is a mixed nerve containing motor, sensory, and parasympathetic fibres.
• It supplies structures in the head and neck, thorax, and abdomen.

Functional Components

Efferent (Motor) Components

1. Special Visceral Efferent (SVE)

• These fibres arise from the nucleus ambiguus in the medulla.
• They supply the skeletal muscles of the palate, pharynx, and larynx.

2. General Visceral Efferent (GVE)

• These are preganglionic parasympathetic fibres arising mainly from the dorsal motor nucleus of vagus.
• They supply the thoracic and abdominal viscera, mainly as far as the proximal two-thirds of the transverse colon.
• They regulate cardiac activity, smooth muscle contraction, and glandular secretion.

Afferent (Sensory) Components

3. General Visceral Afferent (GVA)

• These fibres carry visceral sensations from the pharynx, larynx, oesophagus, and thoracoabdominal viscera.
• They also carry sensory impulses from the aortic arch baroreceptors and aortic bodies.
• Their central processes terminate mainly in the nucleus tractus solitarius.

4. Special Visceral Afferent (SVA)

• These fibres carry taste sensations from the epiglottis and adjoining posterior region of the tongue.
• They terminate in the gustatory part of the nucleus tractus solitarius.

5. General Somatic Afferent (GSA)

• These fibres carry general sensations from parts of the external ear, external acoustic meatus, and posterior cranial fossa dura.
• They terminate in the spinal trigeminal nucleus.

Ganglia of Vagus Nerve

The vagus nerve possesses two sensory ganglia.

1. Superior (Jugular) Ganglion

• It is a small ganglion situated within the jugular foramen.
• It contains sensory neuron cell bodies carrying general somatic sensations from parts of the external ear and posterior cranial fossa dura.

2. Inferior (Nodose) Ganglion

• It is a larger ganglion situated immediately below the jugular foramen.
• It contains cell bodies of neurons carrying visceral sensations from the pharynx, larynx, and thoracoabdominal viscera.
• It also contains sensory neurons transmitting taste from the epiglottic region.

Note: Fibres traditionally described as the cranial root of the accessory nerve join the vagus near its inferior ganglion. These fibres are now generally regarded as belonging functionally to the vagus nerve.

Origin of Vagus Nerve

• The vagus nerve emerges through approximately 10 rootlets from the posterolateral sulcus of the medulla oblongata.
• These rootlets lie between the olive and inferior cerebellar peduncle.
• They unite to form a single nerve trunk.

Course and Relations

Intracranial course

• The rootlets unite and pass laterally towards the jugular foramen.
• The nerve leaves the cranial cavity through the jugular foramen along with the glossopharyngeal and accessory nerves.
• It presents superior and inferior sensory ganglia near the foramen.

In the neck

• The vagus nerve descends vertically within the carotid sheath.
• It lies between and posterior to the internal jugular vein and the internal carotid artery in the upper neck.
• In the lower neck, it occupies a similar position between the internal jugular vein and common carotid artery.

In the thorax

Right Vagus Nerve

• It enters the thorax anterior to the right subclavian artery.
• It gives rise to the right recurrent laryngeal nerve, which loops beneath the right subclavian artery.
• It passes posterior to the root of the right lung.
• Its fibres contribute predominantly to the posterior oesophageal plexus, which forms the posterior vagal trunk.

Left Vagus Nerve

• It enters the thorax between the left common carotid and left subclavian arteries.
• It crosses the left side of the aortic arch and gives rise to the left recurrent laryngeal nerve.
• It passes posterior to the root of the left lung.
• Its fibres contribute predominantly to the anterior oesophageal plexus, which forms the anterior vagal trunk.

In the abdomen

– Both vagal trunks enter the abdomen through the oesophageal hiatus of the diaphragm at T10.

Anterior Vagal Trunk

• It lies mainly on the anterior surface of the abdominal oesophagus.
• It gives hepatic and gastric branches and continues along the lesser curvature of the stomach.

Posterior Vagal Trunk

• It lies mainly behind the abdominal oesophagus.
• It gives gastric branches and a major coeliac branch that reaches the coeliac plexus.

Branches and Distribution

A. Branches in the Head and Neck

1. Meningeal branch.
2. Auricular branch.
3. Pharyngeal branches.
4. Carotid branches.
5. Superior laryngeal nerve.
6. Right recurrent laryngeal nerve.
7. Cervical cardiac branches.

B. Branches in the Thorax

1. Left recurrent laryngeal nerve.
2. Thoracic cardiac branches.
3. Pulmonary branches.
4. Oesophageal branches.

C. Branches in the Abdomen

Anterior Vagal Trunk

1. Hepatic branches.
2. Anterior gastric branches.
3. Greater anterior gastric nerve (anterior nerve of lesser curvature).

Posterior Vagal Trunk

1. Posterior gastric branches.
2. Coeliac branches.
3. Greater posterior gastric nerve (posterior nerve of lesser curvature).
4. Nerve of Grassi, when present as a distinct branch.

Branches In The Head And Neck

1. Meningeal Branch

• It arises near the superior ganglion and re-enters the cranial cavity through the jugular foramen.
• It supplies the dura mater of the posterior cranial fossa.

2. Auricular Branch (Arnold’s Nerve)

• It arises from the superior ganglion and passes posterior to the internal jugular vein.
• It enters the mastoid canaliculus and emerges through the tympanomastoid fissure.
• It supplies parts of the external acoustic meatus, external surface of the tympanic membrane, concha, and adjoining auricle.

3. Pharyngeal Branches

• They arise near the inferior ganglion and pass forwards between the internal and external carotid arteries.
• They contribute to the pharyngeal plexus.
• They supply:
  • All muscles of the pharynx except stylopharyngeus, which is supplied by the glossopharyngeal nerve.
  • All muscles of the soft palate except tensor veli palatini, which is supplied by the mandibular nerve.

4. Carotid Branches

• Small branches may contribute to the innervation of the carotid sinus and carotid body.
• However, their principal sensory innervation is through the carotid sinus branch of the glossopharyngeal nerve.

5. Superior Laryngeal Nerve

• It arises from the inferior ganglion of the vagus nerve.
• It passes downwards and forwards, deep to the internal carotid artery.
• It divides into internal and external laryngeal nerves.

A. Internal Laryngeal Nerve

• It is predominantly sensory.
• It passes through the thyrohyoid membrane along with the superior laryngeal vessels.
• It supplies general sensation to:
  • Laryngeal surface of the epiglottis.
  • Aryepiglottic folds and laryngeal vestibule.
  • Mucous membrane of the larynx above the vocal folds.
  • Adjoining mucosa of the piriform recess.

B. External Laryngeal Nerve

• It is predominantly motor and smaller than the internal laryngeal nerve.
• It descends along the inferior constrictor, close to the superior thyroid artery.
• It supplies:
  • Cricothyroid muscle, which tenses and lengthens the vocal folds.
  • Part of the inferior constrictor of the pharynx.

Recurrent laryngeal nerve

• The recurrent laryngeal nerve is a branch of the vagus nerve associated with the sixth pharyngeal arch.
• The right and left nerves differ in their origins and courses.

Origin and Course

Right Recurrent Laryngeal Nerve

• It arises from the right vagus nerve at the root of the neck.
• It hooks around the inferior surface of the right subclavian artery and ascends in the tracheoesophageal groove.

Left Recurrent Laryngeal Nerve

• It arises from the left vagus nerve in the superior mediastinum.
• It hooks beneath the arch of aorta, close to the ligamentum arteriosum.
• It then ascends through the tracheoesophageal groove towards the larynx.

Termination

• Both nerves enter the larynx deep to the inferior constrictor muscle, near the cricothyroid joint.
• Their terminal portions are called the inferior laryngeal nerves.

Distribution

1. Motor supply: They innervate all intrinsic muscles of the larynx except cricothyroid, which is supplied by the external laryngeal nerve.
2. Sensory supply: They carry general sensations from the laryngeal mucosa below the vocal folds.
3. Other branches: They supply the upper oesophagus and adjoining trachea.

Relation with Inferior Thyroid Artery

• The recurrent laryngeal nerve is closely related to the inferior thyroid artery near the lower pole of the thyroid gland.
• The nerve may pass anterior to, posterior to, or between the branches of the artery.
• Surgical significance: During thyroidectomy, the nerve must be carefully identified and preserved. Terminal branches of the inferior thyroid artery are controlled while maintaining the vascular supply to the parathyroid glands.

ELEVENTH CRANIAL NERVE

• The accessory nerve is the eleventh cranial nerve (CN XI).
• Traditionally, it is described as having two roots:
  1. Cranial root: It joins the vagus nerve and supplies muscles of the palate, pharynx, and larynx through vagal branches.
  2. Spinal root: It follows an independent course and supplies sternocleidomastoid and trapezius muscles.

Note: The cranial root is now generally considered functionally part of the vagus nerve. The spinal root constitutes the accessory nerve proper.

Functional Components

1. Special Visceral Efferent (SVE)

• These fibres arise from the nucleus ambiguus of the medulla oblongata.
• They form the traditionally described cranial root and join the vagus nerve.
• They are distributed to muscles of the soft palate, pharynx, and larynx through vagal branches.

2. General Somatic Efferent (GSE)

• These fibres arise from the spinal accessory nucleus, situated in the upper cervical spinal cord, approximately from C1 to C5/C6.
• They form the spinal root and provide motor innervation to sternocleidomastoid and trapezius muscles.

Origin of Accessory Nerve

Cranial Root

• It emerges through several rootlets from the posterolateral sulcus of the medulla, below the vagus nerve.
• These rootlets unite to form a short nerve trunk.

Spinal Root

• It arises through several rootlets from the lateral aspect of the upper five or six cervical spinal cord segments.
• The rootlets unite to form a trunk that ascends between the dorsal spinal nerve roots and denticulate ligament.

Course and Relations

Intracranial course

Cranial Root

• It passes laterally towards the jugular foramen.
• Traditionally, it is described as joining the spinal root briefly before separating to join the vagus nerve.

Spinal Root

• It ascends through the vertebral canal and enters the cranial cavity through the foramen magnum, posterior to the vertebral artery.
• It passes upwards and laterally towards the jugular foramen.
• It leaves the skull through the jugular foramen along with the glossopharyngeal and vagus nerves.

Extracranial course

• The spinal accessory nerve descends backwards and laterally, usually crossing anterior to the internal jugular vein.
• It passes deep to the posterior belly of digastric and styloid muscles.
• It enters the deep surface of sternocleidomastoid and supplies the muscle.
• It communicates with fibres from C2 and C3 spinal nerves.
• It emerges from the posterior border of sternocleidomastoid and crosses the posterior triangle obliquely.
• It passes over levator scapulae, superficial to the prevertebral fascia.
• Finally, it enters the deep surface of trapezius, usually a short distance above the clavicle, and communicates with fibres from C3 and C4.

Distribution

Spinal root

1. Sternocleidomastoid

• It provides motor innervation to sternocleidomastoid.
• The muscle also receives contributions from C2 and C3 spinal nerves, mainly for proprioception.

2. Trapezius

• It provides the principal motor supply to trapezius.
• The muscle receives additional fibres from C3 and C4 spinal nerves, particularly for proprioception.

Cranial root (Traditional Description)

– Its fibres join the vagus nerve and are distributed through:

1. Pharyngeal Branches

• They supply all muscles of the soft palate except tensor veli palatini, which is supplied by the mandibular nerve.
• They also supply the pharyngeal muscles except stylopharyngeus, which is supplied by the glossopharyngeal nerve.

2. Laryngeal Branches

• Fibres distributed through the recurrent laryngeal nerves supply all intrinsic muscles of the larynx except cricothyroid.
• Cricothyroid is supplied by the external laryngeal nerve.

HYPOGLOSSAL NERVE

• The hypoglossal nerve is the twelfth cranial nerve (CN XII).
• It is predominantly a motor nerve supplying all intrinsic and extrinsic muscles of the tongue except palatoglossus, which is supplied by the vagus nerve.
• It controls tongue movements essential for speech, mastication, and swallowing.

Functional Components

General Somatic Efferent (GSE) Fibres

• These fibres arise from the hypoglossal nucleus in the medulla oblongata.
• They supply all intrinsic and extrinsic muscles of the tongue except palatoglossus.

Proprioceptive Fibres

• Proprioceptive impulses from tongue muscles may travel through the hypoglossal nerve for part of their course.
• These sensory fibres are associated mainly with upper cervical spinal nerves rather than the hypoglossal motor nucleus.

Hypoglossal Nucleus

• It is an elongated motor nucleus, approximately 2 cm long, situated in the medulla oblongata.
• Its upper part lies beneath the hypoglossal triangle in the floor of the fourth ventricle.

Origin of Hypoglossal Nerve

• The nerve emerges through approximately 10–15 rootlets from the anterolateral sulcus of the medulla, between the pyramid and olive.
• The rootlets unite to form the hypoglossal nerve.

Course and Relations

Intracranial course

• The rootlets pass laterally through the subarachnoid space towards the hypoglossal canal.
• They may form two bundles that pierce the dura separately.
• These bundles unite within or near the hypoglossal canal before leaving the skull.

Extracranial course

• The nerve emerges from the hypoglossal canal and initially lies deep to the internal jugular vein and internal carotid artery.
• It descends between the internal carotid artery and internal jugular vein.
• Near the lower border of the posterior belly of digastric, it curves forwards and crosses the internal and external carotid arteries superficially.
• It passes deep to the posterior belly of digastric and stylohyoid.
• It continues forwards superficial to the hyoglossus muscle, while the lingual artery lies deep to this muscle.
• It passes deep to mylohyoid and the submandibular region before entering the tongue.
• Finally, it divides into muscular branches supplying the tongue.

Branches and Distribution

A. Branches of Hypoglossal Nerve Proper

• These branches contain motor fibres originating from the hypoglossal nucleus.
• They supply the following muscles:

Intrinsic muscles of tongue

1. Superior longitudinal.
2. Inferior longitudinal.
3. Transverse.
4. Vertical.

Extrinsic muscles of tongue

1. Genioglossus.
2. Hyoglossus.
3. Styloglossus.

Note: Palatoglossus is supplied by the vagus nerve through the pharyngeal plexus.

B. Branches Carrying C1 Spinal Nerve Fibres

• The hypoglossal nerve is joined by fibres from the C1 spinal nerve.
• These fibres accompany it for a short distance before forming separate branches.

1. Meningeal Branch

• It contains fibres mainly derived from C1, with possible contributions from C2.
• It re-enters the cranial cavity through the hypoglossal canal and supplies the dura mater of the posterior cranial fossa.

2. Superior Root of Ansa Cervicalis (Descendens Hypoglossi)

• It descends along the anterior wall of the carotid sheath.
• It joins the inferior root of ansa cervicalis, derived from C2 and C3 spinal nerves.
• The ansa cervicalis supplies sternohyoid, sternothyroid, and omohyoid muscles.

3. Nerve to Thyrohyoid

• It arises from C1 fibres accompanying the hypoglossal nerve.
• It passes near the greater horn of the hyoid bone and supplies the thyrohyoid muscle.

4. Nerve to Geniohyoid

• It arises from C1 fibres above the hyoid bone.
• It supplies the geniohyoid muscle.