Q 1.12. Facial Nerve Anatomy and Physiology

Overview:

• • The facial nerve, also known as the seventh cranial nerve, plays a crucial role in innervating various areas of the head and neck.
  • It contains motor, sensory, and parasympathetic (secretomotor) nerve fibers.

Nuclei:

• • The facial nerve comprises three nuclei:
    • Main motor nucleus: Responsible for voluntary control of facial muscles.
    • Parasympathetic nuclei: Involved in secretomotor supply to submandibular and sublingual salivary glands, as well as the lacrimal gland.
    • Sensory nucleus: Conveys taste sensation from the anterior two-thirds of the tongue.

Motor Pathway:

• • The upper motor neuron resides in the facial motor area of the precentral gyrus.
  • Axons from the upper motor neuron travel along the ipsilateral corticobulbar tract to the lower pons.
  • Most fibers cross to the other side and synapse with the lower motor neuron.
  • The main motor nucleus (lower motor neuron) divides into four subnuclei:
    • Dorsal: Innervates facial muscles of the ipsilateral upper quadrant.
    • Intermediate: Receives corticobulbar input from both hemispheres.
    • Lateral: Innervates muscles of the ipsilateral lower quadrant of the face.
    • Medial: Supplies auricular muscles, posterior belly of the digastric muscle, stapedius muscle, and stylohyoid muscle.

Parasympathetic Pathway:

• • The superior salivatory and lacrimal nuclei constitute the parasympathetic nuclei.
  • Located in the lower pons, posterolateral to the facial motor nucleus.
  • The superior salivatory nucleus supplies sublingual and submandibular salivary glands, as well as palatine and nasal glands.
  • Inputs to the lacrimal nucleus come from the hypothalamus (emotional response) and the sensory trigeminal nerve (reflex lacrimation due to eye irritation).

• Bell’s palsy: This is the most common cause of facial paralysis, characterized by sudden weakness or paralysis on one side of the face.
• Lyme disease: A tick-borne illness that can cause facial palsy, among other symptoms.
• Stroke: A cerebrovascular accident can affect the facial nerve if the stroke occurs in areas of the brain responsible for facial movements.
• Tumors: Parotid gland tumors, ear tumors, or skull base tumors can compress or invade the facial nerve.
• Trauma: Injury to the nerve, such as fractures of the temporal bone, can result in facial nerve damage.
• Viral infections: Certain viruses, like herpes simplex or varicella-zoster, can infect the facial nerve and lead to conditions such as Ramsay Hunt syndrome.
• Congenital anomalies: Developmental issues can result in facial nerve abnormalities present from birth.

Facial nerve damage in clinical practice – how to understand the main differential diagnosis?

1. Bell’s Palsy:

   • Cause: It’s like a temporary glitch in the facial nerve (cranial nerve VII) due to inflammation.
   • Symptoms: Suddenly, one side of your face feels lazy—it droops, like a sleepy emoji. But here’s the key: Bell’s palsy doesn’t directly involve the brain itself.
   • Affected Muscles: Both upper and lower facial muscles on one side are affected. It’s like the entire face takes a nap.
   • Other Symptoms: Typically, there are no other neurological deficits—just the facial droop.
   • Severity: Scary but usually not serious. Think of it as a minor hiccup in the nerve dance.

2. Stroke:

• • Cause: A stroke is a brain emergency! It happens when blood flow to a part of your brain gets cut off.
  • Symptoms: Beyond facial droop, strokes throw a wild party in your brain. Imagine sudden weakness or paralysis on one side of your body, trouble speaking, and even cognitive hiccups.
  • Affected Muscles: More often, only the lower facial muscles (like the mouth) are affected. The upper face may remain normal.
  • Other Symptoms: Strokes are party crashers—they bring along pals like weakness, numbness, or paralysis in other body parts (arms, legs, speech difficulties, etc.).
  • Severity: Potentially life-threatening. Call the emergency!

References:

1ncbi.nlm.nih.gov

2pubmed.ncbi.nlm.nih.gov

3researchgate.net