VI. Abducent Nerve (Mixed Cranial Nerve – Motor and Sensory)

• Origin:
  The abducent nerve arises from the corpus trapezoideum, which is an anatomical structure located in the ventral part of the brainstem (pons region).
• Pathway:
  • After originating, it exits the cranial cavity via the foramen orbitorotundum.
  • It enters the orbital cavity, the space containing the eye.
• Function:
  • It supplies motor innervation to two specific muscles in the eye:
    1. Lateral rectus muscle: This muscle moves the eyeball outward (abduction).
       • This action is why the nerve is called the abducent nerve, as it controls the abduction of the eye.
    2. Retractor oculi muscle: This muscle helps retract the eyeball into the socket, offering protection.

The Facial Nerve (Cranial Nerve VII)

The facial nerve is a mixed cranial nerve that originates from the lateral part of medulla oblongata and exits the skull through the stylomastoid foramen after passing through the facial canal.

Key Features

Branches within the Facial Canal

1. Great Petrosal Nerve:
   • Parasympathetic fibers to lacrimal gland and carries taste fibers from the palate.
2. Stapedial Branch:
   • Supplies the stapedius muscle in the middle ear.
3. Chorda Tympani:
   • Carries taste fibers to the anterior two-thirds of the tongue.

Branches at Stylomastoid Foramen

• Caudal auricular, middle auricular, stylomastoid, and digastricus branches supply local muscles and skin.

Terminal Branches

1. Auriculopalpebral Nerve:
   • Innervates cranial auricular, orbicularis oculi, and eyelid muscles.
2. Dorsal Buccal Nerve:
   • Supplies buccinator, orbicularis oris, and muscles of the nose.
3. Ventral Buccal Nerve:
   • Innervates the lower lip and depressor muscles.

Clinical Relevance

Damage to the facial nerve leads to facial paralysis, affecting eyelids, ears, lips, and facial muscles.

Summary Table: Key Components of the Facial Nerve

The Cochleovestibular Nerve (Cranial Nerve VIII)

The Cochleovestibular Nerve is a sensory nerve with two parts:

1. Cochlear Part: Responsible for hearing.
2. Vestibular Part: Responsible for equilibrium (balance).

Cochlear Part (Hearing)

• Origin: Axons of spiral ganglion in the modiolus of cochlea (organ of Corti).
• Pathway:
  1. Axons pass through the internal acoustic meatus with the vestibular part.
  2. Enter brainstem at the caudal border of the pons.
  3. Terminate in the cochlear nuclei.
  4. Second-order neurons cross in the pons, ascend via lateral lemniscus, and end at the medial geniculate body.
  5. Third-order neurons project to the hearing center in the temporal lobe.

Vestibular Part (Equilibrium)

• Origin: Axons of vestibular ganglionic cells in the petrous bone.
• Pathway:
  1. Axons pass through the internal acoustic meatus.
  2. Carry impulses from semicircular canals to the vestibular nucleus in the medulla (floor of the 4th ventricle).
• Destinations of Vestibular Impulses:
  • To Cerebellum: Via posterior cerebellar peduncle for balance coordination.
  • To Eye Motor Nerves: Controls eye movements for stability.
  • To Spinal Cord: Forms the vestibulospinal tract, influencing muscle tone and posture.

Clinical Relevance

Lesions of the vestibular nerve can lead to:

• Vertigo: Sensation of whirling motion.
• Ataxia: Loss of muscle coordination.
• Nystagmus: Involuntary eye movements.

Summary Table: Cochleovestibular Nerve

Glossopharyngeal Nerve (Cranial Nerve IX)

The glossopharyngeal nerve is a mixed cranial nerve that emerges from the lateral medulla oblongata and exits the skull via the jugular foramen. It contains two ganglia (proximal and distal, often fused) and divides into terminal branches after giving off key branches.

Key Branches

1. Tympanic Nerve:
   • Forms the tympanic plexus with the carotid sympathetic plexus.
   • Gives rise to the lesser petrosal nerve, which joins the otic ganglion.
   • Provides secretomotor fibers to the parotid gland.
2. Carotid Sinus Branch:
   • Innervates the carotid sinus and carotid body.
   • Functions as a chemo- and baroreceptor.

Terminal Branches

1. Pharyngeal Branch:
   • Forms the pharyngeal plexus with vagus and sympathetic fibers.
   • Innervates the pharyngeal mucosa and muscles.
2. Lingual Branch:
   • Provides taste fibers to the posterior third of the tongue, soft palate, and isthmus faucium.

Vagus Nerve (Cranial Nerve X)

The vagus nerve is a mixed cranial nerve with motor, sensory, and autonomic functions. It originates from the lateral medulla oblongata, exits the skull via the jugular foramen, and extends from the neck to the abdomen, innervating multiple structures.

Course of the Nerve

1. Cervical Course:
   • Travels with the accessory nerve (XI) before separating.
   • Joins the cervical sympathetic nerve to form the vagosympathetic trunk within the carotid sheath.
   • Descends into the thorax, separating from the sympathetic cord.
2. Thoracic Course:
   • Right vagus: Passes under the right axillary artery and divides behind the aortic arch.
   • Left vagus: Passes under the left axillary artery, crosses the aorta, and divides at the upper esophagus.
3. Abdominal Course:
   • Esophageal continuations form the hepatic, reticulo-omasal, and celiac plexuses, innervating the stomach, intestines, and other viscera.

Branches of the Vagus Nerve

Cervical Branches

1. Auricular Nerve:
   • From the jugular ganglion, joins the facial nerve.
   • Supplies the skin of the external ear and external auditory meatus.
2. Pharyngeal and Esophageal Branches:
   • Form the pharyngeal plexus (with sympathetic fibers).
   • Supply the pharynx and cranial esophagus.
3. Cranial (Superior) Laryngeal Nerve:
   • Travels with the glossopharyngeal nerve.
   • Enters the larynx through the thyroid foramen, supplying the laryngeal structures.
4. Aortic Nerve:
   • Left vagus: Detaches before the thoracic inlet to supply the aorta.
   • Right vagus: Detaches at the level of the 2nd rib.

Thoracic Branches

1. Recurrent Laryngeal Nerve:
   • Right recurrent laryngeal nerve:
     • Detaches behind the 1st rib.
     • Loops around a small artery, ascending cranially to supply the larynx, trachea, and esophagus.
   • Left recurrent laryngeal nerve:
     • Detaches at the 4th intercostal space.
     • Loops around the aortic arch, ascending to supply the larynx and pharyngeal muscles.
2. Cardiac Nerves:
   • Contribute to the cardiac plexus.
   • Right cardiac nerve: Detaches at the 3rd rib.
   • Left cardiac nerve: Detaches at the 5th rib.
3. Pulmonary Branches:
   • Join sympathetic fibers to form the pulmonary plexus, innervating the lungs.

Abdominal Branches

• Esophageal continuations contribute to:
  • Hepatic and reticulo-omasal plexuses: Innervate the stomach and right side of its compartments.
  • Celiac plexus: Innervates the intestines and abdominal viscera (e.g., kidneys, adrenal glands).

Clinical Effects of Vagus Nerve Lesions

• Dysphagia (difficulty swallowing).
• Voice changes (due to laryngeal involvement).
• Loss of palate movement (unilateral lesion causes uvula deviation).

Accessory Nerve (Cranial Nerve XI)

Alternate Name: Also called the spinal accessory nerve due to its spinal root origin.

• Type: Motor
• Roots of Origin:
  • Cranial Root: Arises from the dorsolateral medulla oblongata.
  • Spinal Root: Arises from fibers of the upper five cervical spinal segments; enters the skull via the foramen magnum and joins the cranial root.
• Course and Branches:
  • The combined trunk exits the skull through the jugular foramen, posterior to the vagus nerve (X).
  • Cranial Fibers:
    • Join the vagus nerve.
    • Supply laryngeal, pharyngeal, and soft palate muscles.
  • Spinal Fibers:
    • Descend into the neck.
    • Receive branches from the 2nd and 3rd cervical nerves.
    • Branch into:
      • Sternomastoid branch: Supplies the sternomastoid muscle.
      • Trapezius branch: Supplies the trapezius muscle.
• Function: Motor control of the sternomastoid and trapezius muscles, and voluntary muscles of the larynx, pharynx, and soft palate.

Hypoglossal Nerve (Cranial Nerve XII)

• Type: Motor
• Origin: Rootlets arise from the ventrolateral medulla oblongata.
• Course and Branches:
  • Rootlets form a trunk that exits through the hypoglossal foramen.
  • Branches:
    • A twig joins the first cervical nerve (C1).
    • Passes across the external carotid artery.
    • Supplies all extrinsic and intrinsic muscles of the tongue, including:
      • Extrinsic muscles: Genioglossus, hyoglossus, and styloglossus.
      • Intrinsic muscles: Responsible for tongue shape and movement.
• Function: Motor control of tongue movements for speech, swallowing, and food manipulation.