HYPOGLOSSAL NERVE (CN XII) - KUHS Essay Note

INTRODUCTION

UNIQUE FEATURES

• Only cranial nerve that is purely GSE (somatic motor) - no sensory or autonomic fibres in the main nerve.
• Its rootlets are attached in line with the ventral root of the 1st cervical spinal nerve.
• It picks up C1 fibres as it exits the skull, which are distributed as separate branches (not true CN XII fibres).
• It is the only cranial nerve to pass through the hypoglossal canal (anterior condylar canal).

FUNCTIONAL COMPONENTS AND NUCLEI

General Somatic Efferent (GSE) Fibres

• Arise from the hypoglossal nucleus - a longitudinal column of cells in the paramedian medulla oblongata.
• Located just beneath the hypoglossal trigone on the floor of the 4th ventricle.
• The nuclear column extends from the caudal medulla up to the pontomedullary junction.
• Supplies all the intrinsic muscles (superior/inferior longitudinal, transverse, vertical) and extrinsic muscles (genioglossus, hyoglossus, styloglossus) of the tongue.
• Exception: Palatoglossus is supplied by the vagus nerve (CN X) via the pharyngeal plexus (cranial root of accessory nerve).

Supranuclear (Corticobulbar) Control

• Corticobulbar fibres originate from the lower precentral gyrus (perisylvian area).
• Fibres controlling the genioglossus are predominantly crossed (contralateral control).
• Other tongue muscles have bilateral supranuclear control.
• Corticobulbar fibres cross the midline at the pontomedullary junction and enter the hypoglossal nucleus from its lateral aspect.

COURSE AND RELATIONS

Origin (Intramedullary Course)

• Fibres travel ventrolaterally through the medullary reticular formation, passing lateral to the medial longitudinal fasciculus (MLF), medial lemniscus, and pyramids.
• Emerge from the medulla as 10-15 rootlets from the anterolateral sulcus (preolivary/antero-lateral sulcus between the pyramid and the olive).
• The rootlets are located medial to CN IX, X, XI.

Posterior Cranial Fossa

• Rootlets run laterally, passing behind the vertebral artery.
• Coalesce into 2 roots that pierce the dura mater separately.
• Both roots pass through the hypoglossal canal (anterior condylar canal) in the occipital bone.
• The two roots unite into a single trunk within the canal.

Neck (Extracranial Course)

BRANCHES AND DISTRIBUTION

A. Branches of the Hypoglossal Proper (True CN XII)

• All intrinsic muscles of the tongue: superior & inferior longitudinal, transverse, vertical
• All extrinsic muscles of the tongue: genioglossus, hyoglossus, styloglossus
• Exception: Palatoglossus - supplied by CN X via pharyngeal plexus

B. Branches Containing C1 Fibres (Not True CN XII)

CLINICAL CORRELATION

Testing the Hypoglossal Nerve

1. Inspect the tongue at rest in the mouth - look for wasting, fasciculations, wrinkling of mucosa.
2. Ask the patient to protrude the tongue.
3. Ask the patient to move the tongue side to side and assess strength (pushing against the cheek).

Interpretation of Findings

• The genioglossus of the healthy side pushes the tongue tip forward and across; without the opposing genioglossus on the paralyzed side, the tongue deviates toward that side.

LESIONS OF THE HYPOGLOSSAL NERVE

1. Lower Motor Neuron (LMN) Lesion - Nuclear or Infranuclear

• Ipsilateral wasting (atrophy) of the tongue
• Ipsilateral fasciculations and fibrillations
• Wrinkling / furrowing of the mucous membrane on the affected side
• Tongue deviates toward the side of the lesion on protrusion
• Dysarthria and dysphagia (difficulty speaking and swallowing) if severe

2. Upper Motor Neuron (UMN) Lesion - Supranuclear

• Contralateral tongue deviation (tongue deviates away from the lesion, toward the paralyzed side)
• No wasting, no fasciculations (key distinction from LMN)
• Spastic tongue

IMPORTANT CLINICAL SYNDROMES

Medial Medullary Syndrome (Dejerine's Syndrome)

1. Ipsilateral LMN tongue palsy (CN XII involvement) - tongue deviates toward the lesion
2. Contralateral hemiplegia - sparing the face (pyramid involvement)
3. Contralateral loss of position and vibration sense (medial lemniscus involvement) - pain and temperature spared (spinothalamic tract unaffected)

Collet-Sicard Syndrome

• Damage to CN IX, X, XI, XII together (e.g., skull base fracture involving hypoglossal canal and jugular foramen)
• Features: hemiparalysis of tongue + vocal cord palsy + pharyngeal palsy + loss of taste on posterior 1/3 of tongue + trapezius/SCM paralysis

Villaret Syndrome

• CN IX, X, XI, XII + cervical sympathetic chain
• Adds ipsilateral Horner syndrome (ptosis, miosis, anhidrosis)

Jackson Syndrome

• CN X, XI, XII involved at the medullary level

Tapia Syndrome

• CN X (recurrent laryngeal branch) + CN XII
• Reported after anesthesia, laryngoscopy, cervical surgery

Godtfredsen Syndrome

• CN VI + CN XII palsy - associated with nasopharyngeal carcinoma or clival lesion

UNIQUE/HIGH-YIELD POINTS FOR KUHS

1. CN XII is the only purely GSE cranial nerve with NO sensory component.
2. It exits the skull through the hypoglossal canal (anterior condylar canal) - the only nerve to do so.
3. The palatoglossus is NOT supplied by CN XII - it is supplied by CN X (vagus via pharyngeal plexus) - a classic exam trap.
4. The ansa cervicalis (loop supplying infrahyoid/strap muscles) is formed by C1 (via CN XII) + C2, C3 descending cervical root.
5. In LMN palsy: tongue deviates toward the side of the lesion.
6. In UMN palsy: tongue deviates away from the side of the lesion (toward weak side).
7. LMN palsy shows fasciculations and wasting; UMN does not.
8. Medial medullary (Dejerine's) syndrome = ipsilateral LMN tongue palsy + contralateral hemiplegia + contralateral proprioceptive loss.
9. Hypoglossal nerve fibres in the medulla run lateral to the MLF, medial lemniscus, and pyramids.
10. The corticobulbar projections to CN XII are predominantly crossed (for genioglossus); other tongue muscles have bilateral representation.

DIAGRAM SUMMARY

HYPOGLOSSAL NUCLEUS (floor of 4th ventricle, paramedian medulla)
         |
    GSE fibres (10-15 rootlets)
         |
  Anterolateral sulcus of medulla
  (between pyramid and olive)
         |
  2 roots → Hypoglossal canal (anterior condylar canal)
         |
  Single trunk in neck
         |
  ┌──────────────────────────────────────────┐
  |                                          |
  Branches of CN XII proper       C1 branches (hitchhiker fibres)
  (lingual branches)               - Meningeal branch
  - All intrinsic muscles          - Descending hypoglossi
  - Genioglossus                     → Ansa cervicalis
  - Hyoglossus                     - Nerve to thyrohyoid
  - Styloglossus                   - Nerve to geniohyoid
  
  [NOT palatoglossus → CN X]