Anatomy of Facial Nerve – Nuclei & Course

Anatomy of the Facial Nerve - Nuclei and Course

In this post, we'll cover the facial nerve's nucleus, its overall course, and a detailed look at its intratemporal segments.

✨ Nucleus of the Facial Nerve

The Facial Nerve is a mixed nerve with both motor and sensory/parasympathetic functions, originating from four nuclei in the brainstem:

1. Motor Nucleus of the Facial Nerve:

   • Located in the pons.
   • Controls muscles of facial expression.

2. Superior Salivatory Nucleus:

   • Located in the pons.
   • Provides preganglionic parasympathetic fibers for salivation (submandibular and sublingual glands) and lacrimation (lacrimal gland), as well as mucous secretion from nasal and palatine glands.

3. Nucleus of the Tractus Solitarius:

   • Receives taste sensation from the anterior two-thirds of the tongue via the chorda tympani nerve.
   • Also receives general visceral sensation.

4. Spinal Nucleus and Tract of the Trigeminal Nerve (CN V):

   • Receives somatosensory (general sensation) information from the skin of the external auditory canal, parts of the pinna, and potentially around the ear.

🧠 Supranuclear Innervation:

• Neurons from the motor cortex project differently to the facial motor nucleus:

  • To motor neurons controlling the upper face (Frontalis, Orbicularis Oculi): Bilateral projection (from both sides of the cortex).

  • To motor neurons controlling the middle and lower face: Contralateral projection (only from the opposite side of the cortex).

Why is forehead spared in Supranuclear lesions?

• The facial nucleus also receives fibers from the thalamus which provide involuntary control over facial muscles.

  • Clinical Importance: Emotional movements (smiling, crying) are often preserved in supranuclear palsies due to these alternate thalamic pathways.

🧠 Sensory/Parasympathetic Pathway from Nuclei:

• Fibers from the Superior Salivatory Nucleus (parasympathetic) and the Nucleus of the Tractus Solitarius (taste) and Spinal Nucleus of V (somatosensory) consolidate to form the sensory root, also known as the Nerve of Wrisberg (or Nervus Intermedius). This nerve runs together with the motor root.

🏃 Overall Course of the Facial Nerve

The facial nerve's path is traditionally divided into three main parts:

1. Intracranial Part (Cisternal):

From the pons to the entrance of the Internal Acoustic Meatus (IAM). Approximately 24 mm long.

1. Intratemporal Part:

The longest and most complex part, running within the temporal bone from the IAM to the Stylomastoid Foramen.

Approximately 28-30 mm long. This part is further subdivided into segments.

1. Extracranial Part:

From the Stylomastoid Foramen to its terminal branches in the face. Approximately 15-20 mm long.

🦴 Detailed Course of the Facial Nerve

Let's trace the nerve through each of its parts.

1. Intracranial Part (Cisternal Segment)

• Exit: Leaves the brainstem at the pontomedullary junction, approximately 1.5 mm anterior to the Vestibulocochlear Nerve (CN VIII) and caudal to the Trigeminal Nerve (CN V).

• Course: The motor root and the sensory root (Nervus Intermedius) run together anterolaterally through the Cerebellopontine Angle (CP Angle) cistern towards the opening of the IAM (porus acusticus internus).

• Entry: Enters the temporal bone via the Porus Acusticus Internus.

2. Intratemporal Part (Course within the Temporal Bone)

This is the longest part and runs within the bony Fallopian Canal. It is subdivided into four segments:

a. Meatal Segment:

• Length: 8-12 mm.

• Course: Runs within the Internal Acoustic Meatus (IAM) from the Porus Acusticus Internus (medially) to the Fundus (laterally).

• Relations within IAM:

Accompanied by the Cochlear Nerve, Superior and Inferior Vestibular Nerves, and the Labyrinthine Artery.

• At the Fundus (Lateral End of IAM): The fundus is divided into four quadrants by the horizontal Crista Falciformis and the vertical Bill's Bar.

  • Anterosuperior Quadrant: Facial Nerve (motor root + nervous intermedius) enters the Fallopian Canal (Meatal Foramen).
  • Posterosuperior Quadrant: Superior Vestibular Nerve.
  • Anteroinferior Quadrant: Cochlear Nerve.
  • Posteroinferior Quadrant: Inferior Vestibular Nerve.

b. Labyrinthine Segment:

• Length: 3-5 mm.

• Key Feature: This is the shortest and narrowest segment of the facial nerve (average diameter 0.68 mm), and the bony Fallopian Canal is also narrowest here (at the meatal foramen).

• Course: Starts at the Fundus of the IAM (Meatal Foramen) and runs anteriorly, superiorly, and laterally towards the Geniculate Ganglion. It forms an angle of about 120 degrees with the IAM.

c. Geniculate Ganglion (First Genu):

• Location: At the end of the Labyrinthine segment, where the nerve makes a sharp posterior turn (the First Genu). Lies superiorly within the temporal bone, beneath the floor of the middle cranial fossa.

• Nature: It's a sensory ganglion containing cell bodies, but it does NOT contain synapses.

• Fibers Passing Through:

  • Central processes of General Somatic Afferent neurons (pain/touch/temp from EAC, etc.).
  • Cell bodies and central processes of Special Visceral Afferent neurons (taste from anterior 2/3 tongue).
  • Preganglionic Parasympathetic fibers (for lacrimal gland and nasal/palatine glands).

• Branches Originating Here: The Greater Petrosal Nerve (carries parasympathetic fibers for lacrimation) is the first branch of the facial nerve, arising from the geniculate ganglion.

d. Tympanic Segment:

• Length: 9-12 mm.

• Width: Wider than the labyrinthine segment (1.2-1.6 mm).

• Course: Starts at the Geniculate Ganglion and runs posteriorly, inclining slightly inferiorly, along the medial wall of the Tympanic Cavity. It goes above the Cochleariform Process and the Oval Window.Ends at the Second Genu.

• Second Genu: The posterior turn (~90-125 degrees) situated posterior to the oval window, marking the transition to the mastoid segment.

• Landmarks: The relationship between the tympanic segment and the Cochleariform Process is remarkably constant.

e. Mastoid Segment (Vertical Segment):

• Length: 10-14 mm.

• Key Feature: This is the longest intratemporal segment and runs mostly vertically.

• Course: Starts at the Second Genu and runs vertically downwards in the posterior wall of the middle ear and the anterior wall of the mastoid process towards the Stylomastoid Foramen.

• Exit: Exits the temporal bone via the Stylomastoid Foramen. Mean depth from mastoid cortex at the foramen is about 13 mm.

3. Extracranial Part

• Length: 15-20 mm.

• Exit: Leaves the skull base via the Stylomastoid Foramen (between the styloid process and mastoid process).

• Course: Enters the Parotid Gland on its posterior-medial surface. Passes forwards and downwards behind the ramus of the mandible.

• Branching: Within the parotid gland, it divides into its main trunks and terminal branches.

  • Behind and superficial to the Retromandibular vein and External Carotid artery, it divides into:

    • Upper Temporofacial Trunk
    • Lower Cervicofacial Trunk

  • These trunks further branch to form the Pes Anserinus (Goose's Foot) and give rise to the five terminal branches, which diverge within the parotid and exit its anteromedial surface to supply the muscles of facial expression:

    • Temporal (Frontalis, Orbicularis Oculi upper part)
    • Zygomatic (Orbicularis Oculi lower part)
    • Buccal (Buccinator, muscles of upper lip, nose)
    • Marginal Mandibular (muscles of lower lip and chin)
    • Cervical (Platysma)