Cranial nerve nuclei: a brainstem map by level and position
Ten pairs of cranial nerves have nuclei in the brainstem, and they look like a list to memorise. Two rules sort them, level and position from the midline, with one exception: the trigeminal nuclei run the whole length.
Chapters (8)
Locate the cranial nerve nuclei by brainstem level (midbrain, pons, medulla) and by medial–lateral position (motor medial, sensory lateral, across the sulcus limitans) so that a brainstem lesion can be localised.
The short version
- Ten of the twelve cranial nerves arise from the brainstem; the olfactory and optic nerves belong to the forebrain.
- Rule one is level: two in the midbrain (III, IV), four in the pons (V, VI, VII, VIII) and four in the medulla (IX, X, XII and the cranial part of XI).
- The trigeminal nuclei are the exception: they run through the midbrain, the pons and the medulla.
- Rule two is position: the sulcus limitans separates medial motor nuclei from lateral sensory nuclei, and III, IV, VI and XII lie nearest the midline.
- A nuclear lesion usually gives same-side lower motor neuron signs, and a hypoglossal lesion points the tongue toward the damaged side.
Cranial nerve nuclei: one lesion or two?
A woman cannot turn her eyes to the right, and the right side of her face droops. Two different cranial nerve systems have failed together. Is that two lesions, or one?
The cranial nerve nuclei answer that question once you place them by brainstem level and by distance from the midline. This lesson builds that map, then returns to her.
Twelve pairs, ten from the brainstem
There are twelve pairs of cranial nerves. The first two, olfactory and optic, belong to the forebrain. The other ten arise from the brainstem.[1]
Their motor cell bodies, and the nuclei where their sensory fibres relay, sit in the tegmentum, the dorsal part of the brainstem under the fourth ventricle and the cerebral aqueduct.[10]
A cranial nerve motor nucleus holds the lower motor neurons of the head, as in the motor neuron lesson. Damage there gives lower motor neuron signs, usually on the same side.[1][2]
Rule one: cranial nerve nuclei by brainstem level
Each brainstem level holds a fixed set of nuclei. The count is two, four, four.[2]
| Level | Nuclei | Count |
|---|---|---|
| Midbrain | III and IV | 2 |
| Pons | V, VI, VII and VIII (VIII at the junction with the medulla) | 4 |
| Medulla | IX, X, XII and the cranial part of XI | 4 |
The spinal part of XI comes from the upper cervical cord, not the brainstem.[2][10]
The trigeminal nerve breaks the level rule, because its nuclei run through the midbrain, the pons and the medulla.[1]
- The mesencephalic nucleus in the midbrain carries proprioception from the jaw muscles.[1]
- The principal sensory nucleus in the pons carries touch, like the dorsal column.[1]
- The spinal nucleus runs down the medulla and carries pain and temperature from the face, like the spinothalamic tract.[1]
- The motor nucleus lies in the pons.[1]
Rule two: position from the midline and the sulcus limitans
The brainstem is built like the spinal cord opened out. A groove, the sulcus limitans, in the floor of the pons and medulla separates motor nuclei, which lie medially, from sensory nuclei, which lie laterally.[3][5]
| Position | Nuclei |
|---|---|
| Nearest the midline (somatic motor) | III, IV and VI (eye movement) and XII (hypoglossal) |
| Deeper and further out (second motor group) | Motor trigeminal, facial and ambiguus |
| Lateral to the groove (sensory) | Solitary (taste), spinal trigeminal, vestibular and cochlear |
The nucleus ambiguus drives the palate, pharynx and larynx for IX and X, and sits in the lateral medulla.[3][7]
So the groove sorts motor from sensory, and the somatic motor nuclei hug the midline.
The facial colliculus: one lesion, two deficits
Back to the opening patient. The abducens nucleus, which drives looking to the same side, lies in the caudal pons, near the midline. The facial nerve fibres loop around it, forming the facial colliculus.[2][4]
One small lesion there causes both the gaze palsy and the facial droop. It is one lesion, not two.[4]
Hypoglossal lesions: the tongue points toward the damage
If the hypoglossal nerve or nucleus is damaged, the tongue points toward the damaged side, often with wasting and fasciculations.[1][10]
Three things to keep from the cranial nerve nuclei map
- LevelMidbrain, III and IV. Pons, V to VIII. Medulla, IX, X, XII and the cranial part of XI. The trigeminal nuclei run through all three levels.[2]
- PositionThe groove separates medial motor nuclei from lateral sensory nuclei, and III, IV, VI and XII lie nearest the midline.[3]
- SideA nuclear lesion usually gives same-side lower motor neuron signs, and the tongue points toward a hypoglossal lesion.[1][2]
Next in the series: brainstem crossed findings, the localisation rule.
Practice question
Original · USMLE styleA 53-year-old man has a sudden neurological deficit. MRI shows a small infarct confined to the left medulla, immediately beside the midline.
Common questions
Which cranial nerve nuclei are in the midbrain, pons and medulla?
The midbrain holds III and IV. The pons holds V, VI, VII and VIII, with VIII at the junction with the medulla. The medulla holds IX, X, XII and the cranial part of XI. That makes two, four, four.
Which cranial nerves do not arise from the brainstem?
The olfactory and optic nerves belong to the forebrain. The spinal part of XI comes from the upper cervical cord.
What does the sulcus limitans separate?
It separates motor nuclei, which lie medially, from sensory nuclei, which lie laterally, in the floor of the pons and medulla.
Why can one small lesion cause a gaze palsy and a facial droop together?
At the facial colliculus in the caudal pons, the facial nerve fibres loop around the abducens nucleus, so one lesion there affects both.
Which way does the tongue point in a hypoglossal lesion?
Toward the damaged side, often with wasting and fasciculations.
Transcript
Read the full narration, by chapter713 words
0:00The puzzle: a gaze palsy and a facial droop, one lesion or two?
A woman cannot turn her eyes to the right, and the right side of her face droops. Two different cranial nerve systems have failed together. Is that two lesions, or one? Today: place the cranial nerve nuclei by brainstem level and by distance from the midline, so you can answer that.
0:18Twelve pairs of cranial nerves, ten from the brainstem
There are 12 pairs of cranial nerves. The first two, olfactory and optic, belong to the forebrain. The other 10 arise from the brainstem. Their motor cell bodies, and the nuclei where their sensory fibres relay, sit in the tegmentum, the floor of the brainstem. From our motor neuron lesson, a motor nucleus holds the lower motor neurons of the head. So damage there gives lower motor neuron signs, on the same side.
0:47Rule 1: level (two, four, four)
Rule one is level. The midbrain holds the nuclei of three and four. The pons holds five, six, seven and eight, with eight sitting at the junction with the medulla. The medulla holds nine, 10 and 12, and the cranial part of 11. The spinal part of 11 comes from the upper cervical cord. So: two in the midbrain, four in the pons, four in the medulla.
1:13The exception: trigeminal nuclei run through all three levels
The trigeminal nerve is the exception, because its nuclei run through the midbrain, the pons and the medulla. The principal sensory nucleus in the pons carries touch, like the dorsal column. The spinal nucleus runs down the medulla, and carries pain and temperature from the face, like the spinothalamic tract.
1:33Rule 2: position from the midline and the sulcus limitans
Rule two is position from the midline. The brainstem is built like the spinal cord opened out. A groove, the sulcus limitans, in the floor of the pons and medulla separates motor nuclei, which lie medially, from sensory nuclei, which lie laterally. Nearest the midline are the somatic motor nuclei: the eye movement nuclei, three, four and six, and the hypoglossal nucleus, 12. A second motor group lies deeper and further out: the motor trigeminal, facial and ambiguus nuclei. The nucleus ambiguus drives the palate, pharynx and larynx for nine and 10, and sits in the lateral medulla. The sensory nuclei lie lateral to the groove: the solitary nucleus for taste, the spinal trigeminal nucleus, and the vestibular and cochlear nuclei. So the groove sorts motor from sensory, and the somatic motor nuclei hug the midline.
2:32The facial colliculus: one lesion, two deficits (and the hypoglossal tongue sign)
Now our patient. The abducens nucleus, which drives looking to the same side, lies in the caudal pons, near the midline. The facial nerve fibres loop around it, forming the facial colliculus. So one small lesion there takes out both, and it is one lesion, not two. Because motor nuclei hold lower motor neurons, the signs usually appear on the same side as the lesion. The trochlear nucleus is the exception, because its fibres cross, and the oculomotor nucleus has crossed and two-sided parts, so details there need care. If the hypoglossal nerve or nucleus is damaged, the tongue points toward the damaged side, often with wasting and fasciculations.
3:19Practice question
Your turn. A 53-year-old man has a sudden neurological deficit. MRI shows a small infarct confined to the left medulla, immediately beside the midline. Which of the following findings is most likely? A, inability to look to the left. B, drooping of the left face. C, tongue deviation to the left. D, loss of pain over the left face. E, a dilated left pupil with ptosis. Pause here and decide. A is out: the abducens nucleus is in the pons, not the medulla. B is out: the facial nucleus is also in the pons. D is out: pain from the face reaches the spinal trigeminal nucleus, which lies laterally in the medulla, not beside the midline. E is out: the oculomotor nerve and its nucleus are in the midbrain. So the answer is C. The hypoglossal nucleus is a motor nucleus in the medulla beside the midline, and the tongue points toward the damaged side.
4:30High-yield recap and next lesson
Three things to keep. One: midbrain, three and four; pons, five to eight; medulla, nine, 10 and 12. The trigeminal nuclei run through all three levels. Two: the groove separates medial motor nuclei from lateral sensory nuclei, and three, four, six and 12 lie nearest the midline. Three: a nuclear lesion usually gives same-side lower motor neuron signs, and the tongue points toward the lesion. Next, brainstem crossed findings: the localisation rule.
Sources
Numbers in the text, e.g. [1], point here- OpenStax. Anatomy and Physiology 2e: 13.2 The Central Nervous System; 13.4 The Peripheral Nervous System; 14.3 Motor Responses; 16.3 The Cranial Nerve Exam. openstax.org/books/anatomy-and-physiology-2e/pages/13-2-the-central-nervous-system · openstax.org/books/anatomy-and-physiology-2e/pages/13-4-the-peripheral-nervous-system · openstax.org/books/anatomy-and-physiology-2e/pages/14-3-motor-responses · openstax.org/books/anatomy-and-physiology-2e/pages/16-3-the-cranial-nerve-exam
- Cranial nerve nuclei by brainstem level (review table). PMC6945639. pmc.ncbi.nlm.nih.gov/articles/PMC6945639/
- Columns of cranial nerve nuclei in the brainstem. PMC3125522. pmc.ncbi.nlm.nih.gov/articles/PMC3125522/
- Facial colliculus, abducens nucleus and facial nerve fibres. PMC3676972. pmc.ncbi.nlm.nih.gov/articles/PMC3676972/
- Molecular organization and patterning of the medulla oblongata. PMC9409237. pmc.ncbi.nlm.nih.gov/articles/PMC9409237/
- Human vagal trigone. PMC8085322. pmc.ncbi.nlm.nih.gov/articles/PMC8085322/
- Anatomical subtypes of medullary infarction. PMC13053248. pmc.ncbi.nlm.nih.gov/articles/PMC13053248/
- Oculomotor and trochlear nucleus exceptions (cited for the same-side rule). PMC12328912. pmc.ncbi.nlm.nih.gov/articles/PMC12328912/
- Oculomotor and trochlear nucleus exceptions (cited for the same-side rule). PMC9795710. pmc.ncbi.nlm.nih.gov/articles/PMC9795710/
- Wikipedia (secondary, used to cross-check): Cranial nerves; Cranial nerve nucleus; Sulcus limitans; Spinal trigeminal nucleus; Hypoglossal nucleus; Hypoglossal nerve; Accessory nerve; Facial colliculus; Abducens nucleus. en.wikipedia.org/wiki/Cranial_nerve_nucleus · en.wikipedia.org/wiki/Sulcus_limitans · en.wikipedia.org/wiki/Facial_colliculus
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Brainstem illustration: Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); adapted (recoloured, outlined, resized). Gray's Anatomy plates 694, 697 and 698 (Henry Vandyke Carter), public domain, via Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Gray694.png · File:Gray697.png · File:Gray698.png); adapted (recoloured to paper, outlined, resized). All other diagrams are original.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-NEURO-009.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Upper vs lower motor neuron lesions: signs and localisation5:08
- EP 02Dorsal column vs spinothalamic tract: where each pathway crosses5:01
- EP 03Spinal cord syndromes compared: rebuild each from three tracts5:06
- EP 04Cranial nerve nuclei: a brainstem map by level and position5:10
- EP 05Brainstem crossed findings: the localisation rule4:50
- EP 06Medial vs lateral medullary syndrome: artery, structures and signs5:30
- EP 07Facial nerve palsy: why the forehead tells you the lesion4:59
- EP 08Stroke syndromes by artery: ACA, MCA and PCA4:41
- EP 09Lacunar stroke syndromes: pure motor, pure sensory and more4:54
- EP 10Visual pathway lesions and field defects4:42
- EP 11Basal ganglia direct and indirect pathways: D1, D2 and movement5:19
- EP 12Cerebellar lesion localisation: vermis vs hemisphere4:42
- EP 13Brain herniation syndromes: what is squeezed, and where4:56
- EP 14CSF flow and hydrocephalus: where the flow fails5:12
- EP 15Corticospinal tract: from cortex to anterior horn4:59
- EP 16Hypothalamic nuclei and their functions5:24