M01.02.024 Opthalmic Nerve

The ophthalmic nerve (CN V1) is the first of three branches of the trigeminal nerve (CN V). It is responsible for sensory innervation of the upper face and scalp. In this article, we will explore the anatomical course, sensory functions, and autonomic functions of the ophthalmic nerve, with a focus on its clinical relevance for USMLE Step 1.

Anatomical Course of the Ophthalmic Nerve

Trigeminal Nerve and Its Origins

• Nuclei of the Trigeminal Nerve: The trigeminal nerve originates from four nuclei that extend from the midbrain to the medulla:
  • Three sensory nuclei:
    1. Mesencephalic nucleus
    2. Principal sensory nucleus
    3. Spinal nucleus
  • Motor nucleus: Supplies the mandibular division (V3) only.
• Sensory Root: The sensory nuclei combine to form the sensory root, which expands into the trigeminal ganglion located in the middle cranial fossa, lateral to the cavernous sinus.
• Motor Root: The motor root passes inferiorly along the trigeminal cave and only innervates the mandibular division (V3).
• Branches of the Trigeminal Nerve:
  • After the trigeminal ganglion, three divisions emerge:
    1. Ophthalmic (V1)
    2. Maxillary (V2)
    3. Mandibular (V3)

Pathway of the Ophthalmic Nerve

• Origin: The ophthalmic nerve arises from the trigeminal ganglion and passes laterally to the cavernous sinus.
• Main Branches: The nerve exits the skull through the superior orbital fissure and divides into three primary branches:
  1. Frontal nerve
  2. Lacrimal nerve
  3. Nasociliary nerve

Table: Branches of the Ophthalmic Nerve and Their Functions

Sensory Function of the Ophthalmic Nerve

The ophthalmic nerve (CN V1) is primarily responsible for sensory innervation in the upper face and head. Key structures innervated by CN V1 include:

Structures Innervated by CN V1:

• Forehead and scalp
• Frontal, ethmoid, and sphenoid sinuses
• Upper eyelid and conjunctiva
• Cornea
• Dorsum of the nose
• Lacrimal gland
• Parts of the meninges (via recurrent tentorial branch)

Table: Sensory Innervation by CN V1 Branches

Mapping Cutaneous Innervation

The cutaneous innervation of the face and scalp by CN V1 is well-defined, with minimal overlap between branches.

Autonomic Functions of the Ophthalmic Nerve

While CN V1 does not contain autonomic fibers itself, it serves as a pathway for sympathetic and parasympathetic fibers from other parts of the nervous system:

• Sympathetic Fibers: These hitchhike on the nasociliary nerve’s long ciliary nerves to reach the dilator pupillae in the eye, contributing to pupil dilation.
• Parasympathetic Fibers: These travel via the zygomatic nerve (from CN V2) and then the lacrimal branch of CN V1 to reach the lacrimal gland, stimulating tear production.

Clinical Relevance

Corneal Reflex

The corneal reflex involves the involuntary blinking of the eyelids in response to stimuli (e.g., touch, temperature) on the cornea.

• Afferent Limb: The ophthalmic nerve (CN V1) detects the stimulus.
• Efferent Limb: The facial nerve causes contraction of the orbicularis oculi muscle.

Absent corneal reflex may indicate damage to either the ophthalmic or facial nerve.

Points to Remember for USMLE Step 1

• CN V1 is sensory: It supplies sensory innervation to the upper face, including the forehead, eyes, and scalp.
• No autonomic fibers: While CN V1 itself does not carry autonomic fibers, it transports parasympathetic and sympathetic fibers to the lacrimal gland and eye.
• Key branches: Remember the three main branches—frontal, lacrimal, and nasociliary.
• Corneal reflex: An essential clinical test that involves CN V1 (afferent) and CN VII (efferent).