Trigeminal Nerve (CN 5)
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Key concepts: Peripheral somatotopic sensory coverage of the face, trigeminal neuropathies, and the masseter reflex.
Neurologic Exam Highlight:
Peripheral divisions -
- Division 1 — the ophthalmic division, covers the eyes.
- Division 2 — the maxillary division, covers the cheeks.
- Division 3 — the mandibular division, covers the jaw.
The peripheral somatotopic map.
- Division 1 (the ophthalmic division):
- Covers the forehead
- Lies supero-anterior to the corner of the eye
- Division 2 (the maxillary division) lies anterior to:
- The corner of the forehead
- The corner of the mouth
- Division 3 (the mandibular division) lies anterior to:
- The tragus (the anterior aspect of the ear).
- The mentum (the chin).
Key clinical details regarding facial sensation.
- Division 3 covers neither:
- The outer ear
- The angle of the mandible
They will be normal in isolated peripheral trigeminal nerve injury.
- The coverage of the trigeminal nerve extends posteriorly past the superior pole of the head, and thus sensory deficit continues posterior to the superior pole of the head in CN 5 injury.
The coverage of the outer ear is complex: it involves the great auricular nerve (supplied by C2, C3) and cranial nerves 7, 9, and 10.
The angle of the mandible is covered by the great auricular nerve.
key trigeminal neuropathies.
- Trigeminal neuralgia causes frequent paroxysms of pain in one or more trigeminal nerve distribution that are worse with activity (like talking or eating), and characteristically have periods of remission and relapse. For the pathology of trigeminal neuralgia, like hemifacial spasm, think demyelination either from compression or more rarely, multiple sclerosis. Compressive causes include blood vessel compression, which is why microvascular decompression of the nerve is an important treatment.
- Shingles often presents in a trigeminal nerve distribution because herpes zoster can lie dormant in the trigeminal ganglion. Pain and vesicle outbreak occur in the peripheral distributions. Antivirals are key to protecting the eye against herpes zoster ophthalmicus (zoster along the 1st division).
The masseter reflex:
Relevant Anatomy
- Midbrain, pons (include the medulla and cervical spinal cord for Part 2 of the tutorial).
- The trifurcated trigeminal ganglion, which means the "three twins." It lies at the apex of the petrous temporal bone (in a low depression known as the trigeminal impression), and is enveloped in Meckel's cave (a dural-based, cerebrospinal fluid-filled cavern that lies adjacent to the posterolateral aspect of the cavernous sinus).
- From rostral to caudal, trigeminal ganglion receives divisions 1 through 3:
- The ophthalmic nerve.
- The maxillary nerve.
- The mandibular nerve.
The trigeminal ganglion comprises pseudounipolar neurons, akin to the dorsal nerve roots of the spinal cord.
- The motor trigeminal nucleus in the mid-pons.
- The motor root to the trigeminal ganglion along the mandibular division; it exits the brainstem separately from the sensory divisions but joins the mandibular division as the nerve passes peripherally.
- The motor division of the trigeminal nerve exits as the motor root and passes peripherally to innervate the muscles of mastication.
- The mesencephalic nucleus comprises primary sensory neurons for proprioceptive afferents from the muscles of mastication.
- A branch of the mandibular nerve joins the motor root and pass through the mesencephalic nucleus as the nerve fiber of the sensory neuron and innervate the motor trigeminal nucleus.
The mesencephalic n. is unique in that it comprises primary sensory neurons w/in the CNS. The cells are homologous to dorsal root ganglion cells of the PNS. It is the only central nervous system nucleus to house primary sensory neurons.
- Indicate that this synapsis produces the masseter reflex (ie, the jaw jerk). When the muscles of mastication are stretched, they activate the mesencephalic nucleus, which triggers the jaw jerk.
A brisk jaw jerk suggests upper motor neuron distribution pathology.