Block 6 · CNS · CNS04 · Rev 1.2 · Updated Oct 23, 2026
CNS04 · Neuroscience · Study Guide
Spinal Cord & Brainstem Anatomy
Learning objectives
- LO 1Explain how the adult spinal cord, its roots and its meningeal coverings sit inside the bony vertebral canal, predict how that fit shifts as the body grows, and pick the vertebral level for a lumbar puncture.
- LO 2Sketch and label a cord cross-section at cervical, thoracic, lumbar and sacral levels, and name the level of an unlabeled section from its shape, horns and white matter.
- LO 3Give the number of cord segments in each region (including coccygeal) and explain how one cord segment maps onto one strip of skin (a dermatome).
- LO 4State the job of the α motor neuron, γ motor neuron, motor unit, motor neuron pool and Renshaw cell, and explain how tetanus works through the Renshaw cell.
- LO 5Recognize brainstem cross-sections through the closed medulla, open medulla, pons and midbrain.
- LO 6Explain how the plates of the developing neural tube create the layout of the spinal cord and brainstem.
- LO 7Name cranial nerves III–XII, sort them into midbrain, pons and medulla, and describe where each one leaves the brainstem relative to nearby landmarks.
★ Step 1 HY
⚠ Step 1 trap
✎ Memory tool
“ Buzzwords
MAPBig picture
- Short cord in a long canal → cord ends L1–L2 → LP at L3–L4 hits only roots (cauda equina)
- 31 segments → each dorsal root → one dermatome
- Cross-section → gray H (cell bodies) inside white matter (tracts) → proportions change by level → name the level on sight
- Gray matter → α / γ motor neurons · autonomic neurons · interneurons (Renshaw cell ← tetanus toxin)
- Brainstem = same tube opened up → motor medial, sensory lateral → CN exits 2 · 2 · 4 · 4
Memory tool — three cases to solve by the end of the series
- Ladder fall: one leg weak, hyperreflexic and numb to light touch; the other leg can't feel pin or heat → half the cord is cut (Brown-Séquard, LO 2).
- Burnt hands: both hands lose pain and temperature but keep touch, spreading over months to a cape → a fluid cavity at the center of the cord (syringomyelia, LO 2).
- "Drunk" driver with zero alcohol: slow-onset ataxia, dysmetria and nystagmus → a cerebellar problem (later in the series).
LO 1The cord inside the vertebral canalCoverings, the conus, the cauda equina, and where to put the needle
By the end: you can name the layers around the cord, say where the cord and dural sac end in an adult and a newborn, explain why, and choose the safe interspace for a lumbar puncture.
Q1. What wraps the spinal cord inside the bony canal?
The cord wears the same three meninges as the brain, from outside in:
| Layer / space | What it is | Spinal detail worth knowing |
|---|---|---|
| Epidural space | Between bone and dura | A real space in the spine (fat + internal vertebral venous plexus); only a potential space in the skull → target of epidural anesthesia |
| Dura mater | Tough outer sac | Forms a dural root sleeve that follows the dorsal and ventral roots into the intervertebral foramen, then merges with the epineurium of the spinal nerve |
| Subdural space | Potential space | Normally closed |
| Arachnoid mater | Thin web lining the dura | Holds CSF beneath it |
| Subarachnoid space | CSF-filled | Widens below the cord as the lumbar cistern → where CSF is sampled |
| Pia mater | Stuck to the cord surface | Sends out denticulate ligaments (lateral teeth anchoring cord to dura) and continues as the filum terminale |
Quick vocabulary you will use all lecture: sulcus = shallow groove · fissure = deep groove · commissure = a bridge where fibers cross to the other side.
CheckWhich meningeal layer makes the denticulate ligaments, and which layer makes the root sleeve?
Pia → denticulate ligaments (and filum terminale). Dura → dural root sleeve, which blends into the spinal nerve's epineurium.
Step 1 trap
Epidural ≠ spinal anesthesia. An epidural stops in the fat outside the dura; a spinal (and an LP) goes through dura and arachnoid into the subarachnoid space. The pia is never pierced.
Q2. Where does the cord end, and what is below it?
Top to bottom
- Conus medullaris = tapered end of the cord, at L1–L2 in adults
- Cauda equina ("horse's tail") = lumbar, sacral and coccygeal roots drifting down the canal to reach their own foramina
- Lumbar cistern = CSF-filled subarachnoid space from the conus to S2, where the dural sac ends
- Filum terminale (pia): internum inside the sac → externum (coccygeal ligament) anchors to the coccyx
- Dorsal root ganglia sit in the intervertebral foramina; dorsal root = sensory, ventral root = motor; they join into a mixed spinal nerve
CheckWhy do lumbar and sacral roots have to run so far down inside the canal?
Each root must still leave through its own intervertebral foramen. The cord ends high (L1–L2), so the lower roots travel a long way down before they can exit → the cauda equina.
Q3. Why does the cord end higher as we grow?
Early fetusCord fills the whole canal; roots exit straight out
→
NewbornCord tip ≈ L3
→
AdultCord tip ≈ L1–L2
- The vertebral column grows faster and longer than the cord, so the cord tip appears to "rise".
- The roots stay attached to their own foramina → they become longer and more slanted the lower you go.
- Practical result: in an infant the cord sits lower, so the needle goes one space lower (L4–L5).
CheckPredict: a 6-week-old needs an LP. Is L2–L3 safe?
No. A newborn's cord can reach about L3, so L2–L3 risks hitting the cord. Use L4–L5.
Q4. How do I choose the level for a spinal tap, and what does the needle pass through?
- Adult: L3–L4 (L4–L5 also acceptable). Child: L4–L5 → a child's cord and column are closer in length, so the cord ends lower.
- Know the names again: cord ends near L2 → end = conus medullaris; tether = filum terminale; roots = cauda equina.
- Landmark: a line joining the tops of the iliac crests crosses the L4 spinous process (supracristal line).
- Why it's safe: below the conus only mobile roots float in CSF; they drift away from the needle.
Skin+ subcutaneous fat
→
Ligamentssupraspinous → interspinous
→
Lig. flavumfirst "pop"
→
Epidural spacefat, veins
→
Dura + arachnoidsecond "pop"
→
SubarachnoidCSF (lumbar cistern)
CheckAn anesthesiologist feels the first loss of resistance and stops. Where is the needle tip?
In the epidural space, just past the ligamentum flavum. That is where epidural catheters go; an LP continues through the dura and arachnoid.
Step 1 HY — LP numbers
- Cord ends L1–L2 (conus); subarachnoid space/dural sac ends S2.
- LP between L3–L4 or L4–L5, at the level of the iliac crests.
- Layers: skin → supraspinous lig → interspinous lig → ligamentum flavum → epidural space → dura → arachnoid → subarachnoid space. Pia not pierced.
Step 1 trap
Spinal cord ends at L1–L2, but the dural sac / CSF continues to S2. A stem asking "where does the subarachnoid space end?" wants S2, not L2.
Memory tool
"Keep the cord alive, tap at L4–5." And the cord is a short man in a tall coat: the coat (column) kept growing, the man (cord) didn't.
Buzzwords
"horse's tail" → cauda equina"lumbar cistern" → CSF below the conus"line between iliac crests" → L4 spinous process"two pops" → ligamentum flavum, then dura
Q5. Conus medullaris vs cauda equina: what changes when the lesion moves a few centimeters?
The conus is cord (CNS, can give UMN signs); the cauda equina is roots (peripheral = LMN).
| Feature | Conus medullaris syndrome | Cauda equina syndrome |
|---|---|---|
| What's hit | Cord tip (≈ L1–L2 vertebra; S2–Co segments) | Roots below the conus (L2–Co roots) |
| Onset / symmetry | Sudden, bilateral, symmetric | Gradual, asymmetric |
| Pain | Less radicular pain | Prominent radicular (sciatica-like) pain |
| Motor / reflexes | Mixed UMN + LMN; knee jerk may be spared, ankle jerk lost | Pure LMN: flaccid weakness, ↓ reflexes |
| Bladder / bowel | Early dysfunction | Later dysfunction |
| Shared | Low back pain, saddle anesthesia, bladder/bowel/sexual dysfunction → neurosurgical emergency (MRI, decompress) | |
CheckA large central L4–L5 disc compresses the roots in the canal. UMN or LMN picture?
LMN. The cauda equina is peripheral nerve roots → flaccid, areflexic weakness, saddle anesthesia, urinary retention.
Step 1 HY — cauda equina
Saddle anesthesia + urinary retention/overflow + ↓ anal tone + bilateral sciatica = cauda equina syndrome → emergent MRI and surgical decompression. Common causes: large central disc herniation, tumor, epidural abscess or hematoma.
Step 1 trap
A lesion below L2 vertebra cannot give a Babinski sign or spasticity: there is no cord there. UMN signs point to the conus or higher.
ApplyLO 1 application questions
A1A resident wants to do an LP on a 45-year-old at the L1–L2 interspace "to get more CSF." What is the risk?
Puncturing the conus medullaris. The adult cord ends at L1–L2; go to L3–L4 or L4–L5, where only cauda equina roots float in the lumbar cistern.
A2Why is the dural root sleeve clinically relevant when a dye is injected into the subarachnoid space (myelogram)?
Dye outlines each root sleeve into the foramen. The sleeve carries the subarachnoid space a short way along the root before the dura blends with the epineurium, so a compressed root shows as a "cut-off" sleeve.
A3A 62-year-old has 2 weeks of worsening asymmetric leg pain, flaccid weakness of both legs, absent ankle reflexes and numbness over the buttocks and perineum. No Babinski sign. Localize.
Cauda equina. Gradual, asymmetric, radicular, pure LMN with saddle anesthesia = roots below the conus.
A4A 3-month fetus and an adult each have a needle placed at L3. Which one is in danger of hitting cord?
The fetus. Early in development the cord fills the whole canal; only later does the column outgrow it.
LO 2Spinal cord cross-sections, level by levelDraw it, label it, and name the level of an unlabeled section
By the end: you can draw a cord section with its sulci, horns, funiculi and main tracts, and name a section as cervical, thoracic, lumbar or sacral from three clues: white-matter amount, ventral-horn size, lateral horn.
Q1. What are the landmarks on the outside of a cord section?
- Posterior (dorsal) median sulcus — shallow midline groove at the back.
- Posterior intermediate sulcus — splits each dorsal column into gracilis (medial) and cuneatus (lateral). Present only above T6.
- Posterolateral sulcus — where dorsal rootlets enter (dorsal root entry zone).
- Anterior (ventral) median fissure — deep front groove (the anterior spinal artery lies here).
- Anterior white commissure — the bridge in front of the central canal where fibers cross (pain/temperature 2nd-order axons).
CheckYou see two dorsal columns on each side, separated by a little groove. What does that tell you about the level?
The posterior intermediate sulcus is present → you are at or above T6 (cuneatus only exists where upper-limb fibers have joined).
Step 1 trap — staining
On a myelin stain the "white" matter is dark and the "gray" matter is pale. On a Nissl stain it flips (cell bodies dark). Read the image before you read the shape.
Memory tool
Fissure = front (deep, like a fault line); sulcus = back (shallow). Both start with the letter that matches their size: Fissure is Fat and deep.
Q2. What is inside: horns, funiculi and the main tracts?
Dorsal horn — sensory
- Receives dorsal-root input
- Substantia gelatinosa (lamina II) and nucleus proprius: pain/temp relay
- Built from the alar plate
Lateral horn — visceral
- Intermediolateral cell column: sympathetic preganglionics, T1–L2/3
- Sacral S2–S4: parasympathetic preganglionics in lateral gray
Ventral horn — motor
- α and γ lower motor neurons
- Medial = axial, lateral = limbs; flexors dorsal, extensors ventral
- Built from the basal plate
White matter = three funiculi
- Dorsal, lateral and ventral funiculi (columns), each holding parallel bundles of myelinated and unmyelinated axons.
- Axons both ascend and descend; white matter also holds root fibers entering/leaving, glia and blood vessels.
- Lissauer's tract (= dorsolateral funiculus/fasciculus) caps the dorsal horn: small pain/temp fibers run up or down 1–2 segments here before synapsing.
Tracts to find on a section
- Dorsal funiculus (DCML) → gracilis carries information from below T6; cuneatus from above T6; the posterior intermediate sulcus separates them.
- Anterolateral system runs anterolaterally → 2nd-order fibers cross in the anterior white commissure.
- Clarke's nucleus (dorsal nucleus, medial base of the dorsal horn) exists only C8 to L2 → feeds the dorsal spinocerebellar tract
- Leg information below L2 rides up in the gracilis until it reaches Clarke's
- Corticospinal: UMN starts in layer 5 of motor cortex → LMNs = α and γ motor neurons in the ventral horn.
CheckWhich horn would you expect to be absent in a C6 section, and why?
The lateral horn. Sympathetic preganglionic cells live only from T1 to L2/3; C6 has no intermediolateral column.
Step 1 HY — what each tract does and where it crosses
| Tract | Carries | Crosses | Lesion of one side of cord → |
|---|---|---|---|
| Dorsal columns (DCML) | Fine touch, vibration, conscious proprioception | Medulla (internal arcuate fibers) | Ipsilateral loss below lesion |
| Lateral corticospinal | Voluntary movement (UMN) | Pyramidal decussation (caudal medulla) | Ipsilateral spastic weakness below lesion |
| Anterolateral (spinothalamic) | Pain, temperature, crude touch | Anterior white commissure, within 1–2 segments of entry | Contralateral loss starting 1–2 segments below lesion |
| Spinocerebellar | Unconscious proprioception | Dorsal: none (ipsilateral); ventral: crosses twice | Ipsilateral ataxia |
Buzzwords
"intermediolateral cell column" → sympathetic preganglionics, T1–L2/3"substantia gelatinosa" → lamina II, pain"Clarke's column" → dorsal spinocerebellar, C8–L2"dorsolateral fasciculus" → Lissauer's tract
Q3. How is the gray matter organized into layers (laminae)?
Under the microscope the gray matter forms 10 layers (Rexed laminae I–X), numbered from the dorsal-horn tip forward. You only need the landmarks:
| Lamina | Named nucleus | Role |
|---|---|---|
| I | Marginal zone (posteromarginal) | Pain/temperature relay |
| II | Substantia gelatinosa | Pain modulation (interneurons) |
| III–IV | Nucleus proprius | Touch/pain relay |
| VII | Clarke's column (C8–L2) · intermediolateral nucleus (T1–L2/3) | Spinocerebellar relay · sympathetic preganglionics |
| IX | Medial and lateral motor nuclei | α and γ LMNs |
| X | Around the central canal | Commissural gray |
Neurons in the ventral horn form elongated nuclei (columns running several segments) or layers; their proportions change from segment to segment.
Q4. How do cervical, thoracic, lumbar and sacral sections differ?
Two simple rules explain everything:
- White matter shrinks as you go down. High levels carry ascending axons from every segment below plus descending axons to every segment below.
- Ventral horns bulge where limbs are. More muscle → more LMNs → cervical (C5–T1) and lumbar (L2–S2) enlargements.
| Level | Overall shape | White matter | Ventral horn | Lateral horn | Extra clue |
|---|---|---|---|---|---|
| Cervical | Wide oval (wider than deep) | Most | Large (C5–T1) | Absent | Gracilis + cuneatus (posterior intermediate sulcus) |
| Thoracic | Small, round | Lots (relative to thin gray) | Small, slender horns | Present (T1–L2/3) | Clarke's nucleus at base of dorsal horn; cuneatus above T6 only |
| Lumbar | Round | Less | Large (L2–S2) | Absent below L2/3 | Only gracilis in the dorsal column |
| Sacral | Small | Least | Stubby but bulky relative to size | No horn (parasympathetic S2–S4 in lateral gray) | Mostly gray matter |
- Cervical: large ventral horns + the most white matter · Thoracic: small ventral horns + a lateral horn · Lumbar: large ventral horns · Sacral: mostly gray matter.
- White matter amount reduces caudally → sacral is a small segment with proportionally more gray than white.
CheckAn unknown section is round with big ventral horns and a single dorsal column on each side. Cervical or lumbar?
Lumbar. Big ventral horns mean an enlargement; one dorsal column (no cuneatus) and less white matter mean below T6 → lumbar enlargement.
Memory tool
"Cervical is Chunky, Thoracic is Thin, Lumbar is Loaded (horns), Sacral is Small." A lateral horn = the "T" level has a Tab sticking out sideways.
Step 1 trap
Cervical and lumbar both have big ventral horns. Break the tie with white matter (cervical ≫ lumbar) and shape (cervical is a wide oval; lumbar is round) and the cuneatus (cervical only).
Q5. Image ID drill: name each level.
Myelin-stained sections (white matter dark). Decide, then open each card.
Section 1 →
Lumbar. Round, big bulky ventral horns, moderate white matter, one dorsal column per side.
Section 2 →
Cervical. Wide oval, lots of dark white matter, large ventral horns.
Section 3 →
Sacral. Small, mostly pale gray matter with a thin rim of white matter.
Section 4 →
Thoracic. Small round section, thin slender horns, small lateral horn, white matter dominates.
Q6. Who feeds the cord, and what does a vascular lesion look like?
One anterior spinal artery (ASA)
- Runs in the anterior median fissure; from the vertebral arteries, reinforced by segmental arteries incl. the artery of Adamkiewicz (≈ T9–L2, usually left)
- Supplies the anterior 2/3: ventral horns, corticospinal, anterolateral system
- Single midline vessel → occlusion gives bilateral deficits
Two posterior spinal arteries (PSA)
- One per side, near the dorsal root entry
- Supply the posterior 1/3: dorsal columns
- One-sided occlusion → ipsilateral dorsal-column loss
- Loss of the posterior spinal arteries → loss of DCML (PSA problem → ipsilateral).
- Loss of the anterior spinal artery → loss of ALS and corticospinal (ASA problem → bilateral).
Step 1 HY — anterior spinal artery syndrome
Classic setting: aortic aneurysm repair or dissection (Adamkiewicz compromised). Picture: bilateral spastic paralysis below the lesion (flaccid at the level), bilateral loss of pain/temp, bladder dysfunction, with vibration and proprioception spared (dorsal columns live on the PSAs).
Q7. How do cord sections read a lesion? (and the opening cases)
After a cord lesion, cut axons degenerate away from their cell bodies and lose their myelin (pale on a myelin stain):
Sensory (ascending) tracts
Cell bodies are below → tract looks pale ABOVE the lesion, normal below. E.g., T9 lesion: gracilis pale at C5, cuneatus normal (arm fibers entered above T9).
Motor (descending) tracts
Cell bodies are in the cortex → corticospinal looks pale BELOW the lesion, normal above. Clinically: spastic paralysis, positive Babinski below.
Spinocerebellar fibers entering below the lesion are also lost (seen above L2, since Clarke's nucleus only starts there).
Brown-Séquard (hemisection) ladder fall
Same side, below lesion: UMN weakness + hyperreflexia (lateral corticospinal), loss of fine touch/vibration/proprioception (dorsal column).
Opposite side, 1–2 segments below: loss of pain + temperature (spinothalamic already crossed).
At the level: ipsilateral LMN signs + band of total sensory loss.
Syringomyelia burnt hands
Fluid cavity (syrinx) expands the central canal, usually C8–T1 → first destroys crossing fibers in the anterior white commissure.
Bilateral "cape-like" loss of pain + temperature in arms/shoulders, touch spared. Enlarging → ventral horns (LMN atrophy of hands), lateral horn (Horner).
Associated with Chiari I malformation and trauma.
CheckLadder-fall case: the LEFT leg is paralyzed, hyperreflexic and can't feel a Q-tip; the RIGHT leg can't feel pinprick or heat. Which half of the cord is cut?
Left hemisection (thoracic, since the arms are fine). Motor and dorsal-column losses are on the lesion side; pain/temp loss is on the opposite side.
Step 1 HY — classic cord syndromes
- Brown-Séquard: ipsilateral UMN + DCML loss, contralateral pain/temp loss (stab wound, trauma).
- Syringomyelia: cape-like bilateral pain/temp loss; Chiari I.
- Tabes dorsalis (tertiary syphilis): dorsal columns + roots → sensory ataxia, positive Romberg, Argyll Robertson pupils.
- Subacute combined degeneration (B12, copper, nitrous oxide): dorsal columns + lateral corticospinal + spinocerebellar.
- ALS: combined UMN + LMN, no sensory loss.
- Polio / SMA: ventral horn only → LMN signs.
Step 1 trap
In Brown-Séquard, pain/temp loss starts 1–2 segments below the lesion on the opposite side (fibers ride Lissauer's tract before crossing). Don't put the pain/temp loss on the same side as the weakness.
Buzzwords
"cape-like"/"shawl" loss of pain and temp → syringomyelia"painless hand injuries from heat" → syrinx"stab wound to the back" → Brown-Séquard"after aortic aneurysm repair" → anterior spinal artery
Image drill · Michigan SecondLook
How to use: name what you see, then tap the card to check. Images credited to the Michigan SecondLook Anki Deck (University of Michigan Medical School).
1 · Name the yellow, blue and red funiculi. (tap for answer)
Funiculi (singular: funiculus).
Yellow = dorsal funiculus (= dorsal column) · Blue = ventral funiculus · Red = lateral funiculus.
Yellow = dorsal funiculus (= dorsal column) · Blue = ventral funiculus · Red = lateral funiculus.
Image: Michigan SecondLook Anki Deck · SecondLook Neuroanatomy (Michigan Virtual Microscopy Slide Collection), University of Michigan Medical School · CC BY-NC-SA 4.0
2 · Which level? Name the feature that gives it away. (tap for answer)
Thoracic cord: (1) more white than gray matter; (2) small, slender ventral horns; (3) a lateral horn (intermediolateral cell column).
Thoracic LMNs supply the trunk/chest and abdominal-wall muscles.
Thoracic LMNs supply the trunk/chest and abdominal-wall muscles.
Image: Michigan SecondLook Anki Deck · SecondLook Neuroanatomy (Michigan Virtual Microscopy Slide Collection), University of Michigan Medical School · CC BY-NC-SA 4.0
3 · Name the green, yellow and red circled regions. (tap for answer)
Green = ventral horn (α and γ LMNs) → motor.
Yellow = intermediolateral cell column (lateral horn) → autonomic (sympathetic preganglionic).
Red = dorsal nucleus of Clarke (nucleus dorsalis) → non-conscious proprioception.
Yellow = intermediolateral cell column (lateral horn) → autonomic (sympathetic preganglionic).
Red = dorsal nucleus of Clarke (nucleus dorsalis) → non-conscious proprioception.
Image: Michigan SecondLook Anki Deck · SecondLook Neuroanatomy (Michigan Virtual Microscopy Slide Collection), University of Michigan Medical School · CC BY-NC-SA 4.0
4 · Which level? What is the gray-to-white ratio here? (tap for answer)
Sacral cord: more gray matter than white matter and a small overall size (few ascending fibers have joined yet, and most descending fibers have already ended).
The large cells in the ventral horn (boxed in the answer image) are lower motor neurons.
The large cells in the ventral horn (boxed in the answer image) are lower motor neurons.
Image: Michigan SecondLook Anki Deck · SecondLook Neuroanatomy (Michigan Virtual Microscopy Slide Collection), University of Michigan Medical School · CC BY-NC-SA 4.0
ApplyLO 2 application questions
A1A myelin-stained section has thin horns, a small pointed lateral horn, a rounded cluster of large cells at the medial base of the dorsal horn, and only one dorsal column per side. Level?
Lower thoracic (T7–T12). Lateral horn + Clarke's nucleus = thoracic; no cuneatus = below T6.
A2A patient had a complete T9 transection years ago. Predict the appearance of the fasciculus gracilis and the lateral corticospinal tract at C5.
Gracilis pale (degenerated); corticospinal normal. Ascending leg fibers have lost their cell bodies below; descending corticospinal axons at C5 still connect to cortex.
A3After a thoracoabdominal aneurysm repair, a man cannot move his legs and cannot feel pinprick below the umbilicus, yet feels a tuning fork on his toes. Which artery?
Anterior spinal artery (via artery of Adamkiewicz). Anterior 2/3 infarct with dorsal columns spared.
A4Why are white-matter tracts bigger at C2 than at L2?
C2 carries all ascending axons from the whole body and all descending axons to the whole cord; at L2 the arm, trunk and upper-body fibers have already joined or left.
LO 3Segments and dermatomesHow many segments, and how one dorsal root maps to one strip of skin
By the end: you can give the segment count for each region, explain how spinal nerves are numbered relative to vertebrae, define segment and dermatome, and use dermatome landmarks to localize a root or a shingles rash.
Q1. How many segments are in each region?
8 CervicalC1–C8
+
12 ThoracicT1–T12
+
5 LumbarL1–L5
+
5 SacralS1–S5
+
1 CoccygealCo1
=
31 pairsof spinal nerves
- Cervical is the odd one out: 7 vertebrae but 8 nerves, because C1 exits above C1 vertebra (between skull and atlas).
- C1–C7 exit above their same-numbered vertebra; C8 exits below C7; from T1 down, each nerve exits below its vertebra.
- Write it as 8 · 12 · 5 · 5 (+1 coccygeal); each segment's dorsal root supplies one sensory dermatome.
CheckWhich nerve exits between the C6 and C7 vertebrae? Which exits between T6 and T7?
C7 (cervical nerves exit above their own vertebra). T6 (thoracic nerves exit below their own vertebra).
Memory tool
Breakfast at 8, lunch at 12, dinner at 5, bed at 5, one snack at night → 8-12-5-5-1.
Q2. What exactly is a segment, and what is a dermatome?
Spinal segment
The length of cord that gives rise to one pair of spinal nerves (one set of dorsal + ventral rootlets on each side).
Sensory dermatome
The area of skin supplied by the sensory fibers of a single dorsal root.
Skin receptorin one dermatome
→
Peripheral axonin spinal nerve
→
DRG cell bodypseudounipolar, in foramen
→
Dorsal rootenters posterolateral sulcus
→
Dorsal hornof that segment
- Adjacent dermatomes overlap → cutting a single root rarely gives a fully numb strip; you need two or three.
- The spinal nerve splits into a dorsal ramus (back muscles/skin) and a ventral ramus (limbs, front of trunk). Root ≠ ramus: roots are proximal to the DRG junction, rami distal.
Q3. Which dermatome landmarks and reflexes do I need?
| Landmark | Dermatome |
|---|---|
| Back of head | C2 |
| Thumb | C6 |
| Middle finger | C7 |
| Little finger | C8 |
| Nipple | T4 |
| Xiphoid | T6 |
| Umbilicus | T10 |
| Inguinal ligament | L1 |
| Knee / medial leg | L4 |
| Dorsum of foot / big toe | L5 |
| Lateral foot / sole | S1 |
| Perianal (saddle) | S3–S5 |
| Reflex | Root |
|---|---|
| Biceps | C5–C6 |
| Brachioradialis | C5–C6 |
| Triceps | C7–C8 |
| Cremasteric | L1–L2 |
| Patellar | L2–L4 |
| Achilles | S1–S2 |
| Anal wink | S2–S4 |
CheckA posterolateral L4–L5 disc herniation: which root, and what deficit?
L5 root (lumbar roots exit below their vertebra; a posterolateral disc hits the root heading to the next foramen down) → weak dorsiflexion / great-toe extension, numb dorsum of foot.
Memory tool
- T4 at the teat pore; T10 at the belly butt-ten.
- Reflexes: 1, 2 buckle my shoe (S1–2 Achilles) · 3, 4 kick the door (L3–4 patellar) · 5, 6 pick up sticks (C5–6 biceps) · 7, 8 lay them straight (C7–8 triceps).
Step 1 HY — disc and root rules
- Cervical: disc C5–C6 → C6 root (thumb numb, weak biceps/brachioradialis).
- Lumbar posterolateral disc compresses the root one below: L4–L5 → L5; L5–S1 (most common) → S1 (↓ Achilles, lateral foot numb).
- Far-lateral herniation hits the root exiting at that level.
Step 1 trap — referred pain
Diaphragm irritation refers to the shoulder (C3–C5); early appendicitis to the umbilicus (T10); cardiac pain to the left arm (T1–T4). Use dermatomes, not the organ's location.
Q4. Why does shingles draw a dermatome on the skin?
ChickenpoxVZV infects sensory neurons
→
Latencyin a dorsal root ganglion (or cranial sensory ganglion)
→
Reactivationage, immunosuppression
→
Travels out one rootpain first, then vesicles
- Rash is unilateral and stops at the midline, following one dermatome.
- Neuralgic pain often precedes the rash; vesicle eruption spreads virus; postherpetic neuralgia can last months.
- Most common: thoracic dermatomes; V1 (herpes zoster ophthalmicus — tip-of-nose vesicles = Hutchinson sign).
Step 1 HY — shingles
VZV latent in DRG / trigeminal ganglion. Treat with acyclovir/valacyclovir; prevent with the recombinant zoster vaccine (age ≥50, or immunocompromised adults). Complications: postherpetic neuralgia, zoster ophthalmicus, Ramsay Hunt (geniculate ganglion, CN VII).
Buzzwords
"band-like rash that stops at the midline" → zoster in one dermatome"burning pain before the rash" → prodromal zoster neuralgia"vesicles at the tip of the nose" → V1, eye at risk
ApplyLO 3 application questions
A1A 70-year-old has painful vesicles in a strip around the right side of his trunk at the level of the umbilicus. Where is the virus?
Right T10 dorsal root ganglion. Umbilicus = T10; rash on one side follows one dorsal root.
A2Why does cutting only the T5 dorsal root not leave a numb band on the chest?
Dermatomes overlap: T4 and T6 also cover most of T5's skin.
A3A patient has a decreased Achilles reflex and numbness of the lateral foot after lifting. Which disc level is most likely?
L5–S1 herniation compressing the S1 root.
A4Why are there 8 cervical nerves but only 7 cervical vertebrae?
C1 nerve exits above the C1 vertebra (between occiput and atlas), so cervical nerves are numbered by the vertebra below them, and one extra (C8) exits below C7.
LO 4Motor neurons, motor units and the Renshaw cellThe cells of the gray matter, and how tetanus hijacks one of them
By the end: you can define UMN vs LMN, α vs γ motor neuron, motor unit vs motor pool, explain small vs large units, and draw the Renshaw feedback loop and where tetanus toxin breaks it.
Q1. What kinds of neurons live in the spinal gray?
Visceral motor neurons
Sympathetic preganglionics: T1–L2/3 intermediolateral cell column → sympathetic chain or (splanchnics) prevertebral ganglia → glands, cardiac and smooth muscle.
Parasympathetic preganglionics: S2–S4 → long fibers to ganglia near/in pelvic organs.
Somatic motor neurons (LMNs)
α and γ motor neurons in the ventral horn, grouped in topographic columns.
Interneurons
The majority of gray-matter neurons; fully contained in the cord; link segments; relay sensory → motor; excitatory or inhibitory.
Sensory processing neurons
Dorsal-horn projection neurons that start ascending pathways (next lectures).
Q2. Upper vs lower motor neuron: what is the difference?
UMNCell body in brain (cortex layer 5, brainstem); never touches muscle
→
LMNCell body in ventral horn or brainstem motor nucleus
→
MuscleLMN axon synapses directly at the NMJ
Every motor pathway in this series (corticospinal, corticobulbar, rubrospinal, vestibulospinal, reticulospinal) = UMN → LMN.
| Sign | UMN lesion | LMN lesion |
|---|---|---|
| Weakness | Yes (groups of muscles) | Yes (individual muscles / myotome) |
| Tone | ↑ spasticity (clasp-knife) | ↓ flaccid |
| Reflexes | ↑ hyperreflexia, clonus | ↓ / absent |
| Babinski | Present (upgoing toe) | Absent |
| Atrophy | Mild (disuse) | Marked |
| Fasciculations | No | Yes |
Step 1 HY
Mixed UMN + LMN signs with no sensory loss = ALS (riluzole, edaravone). Pure LMN = polio, SMA (Werdnig-Hoffmann: "floppy baby", tongue fasciculations). Acute spinal shock gives flaccid areflexia first, then UMN signs appear over days–weeks.
Q3. α vs γ motor neurons: who does what?
| Feature | α motor neuron | γ motor neuron |
|---|---|---|
| Size | Large | Small |
| Innervates | Extrafusal fibers (ordinary striated muscle) | Intrafusal fibers (inside the muscle spindle) |
| Result | Contraction / force | Keeps the spindle taut and sensitive to stretch → sets reflex gain and tone |
| Location | Ventral horn (lamina IX), side by side | |
- α–γ coactivation: brain fires both together so the spindle doesn't go slack as the muscle shortens.
- Topography in the ventral horn: medial neurons → axial/trunk; lateral neurons → limb/distal; flexor neurons dorsal to extensor neurons.
CheckPredict: γ motor neurons are selectively silenced. What happens to the knee-jerk?
Reflex decreases. The spindle goes slack and is less sensitive to stretch, so the tap evokes less Ia firing.
Memory tool
α = Action (the muscle you see moving). γ = Gain (tunes the spindle's sensitivity). Flexors are "up" (dorsal), extensors "down" (ventral); limbs go out to the side (lateral).
Q4. Motor unit vs motor pool: why do size and spread matter?
Motor unit
One α motor neuron + all the muscle fibers it innervates.
- Small units (≈ 1:10) → fine control: extraocular, finger muscles
- Large units (≈ 1:1000) → high force: gastrocnemius, postural
Motor pool
All the motor neurons that innervate one muscle.
- Forms a longitudinal column that can span several segments
- Biceps: musculocutaneous n., ≈ C5–C7 → losing one root = weakness, not paralysis
CheckWhy is a single root lesion rarely paralyzing for a limb muscle?
The muscle's motor pool spans several segments; one root removes only part of the pool → weakness.
Step 1 HY — size principle
Henneman size principle: small motor units are recruited first, large ones later as force rises. EMG in denervation shows fibrillations; reinnervation produces giant motor unit potentials.
Step 1 trap
Motor unit = one neuron (many fibers). Motor pool = many neurons (one muscle). Don't swap them in a "which term" stem.
Q5. What is the Renshaw cell, and how does tetanus break it?
- Renshaw cell = an inhibitory (glycinergic) interneuron in the ventral horn.
- Input: an excitatory collateral of the α motor neuron. Output: glycine back onto that same α motor neuron (and its neighbors).
- Job: a feedback brake that prevents continuous firing.
- Targets of Clostridium tetani toxin and of strychnine.
- Context: tetanus vaccine developed 1924; about 1 million deaths/year in 1980 → about 20,000 today
| Agent | Mechanism | Result |
|---|---|---|
| Tetanospasmin (C. tetani) | Retrograde axonal transport to cord → protease cleaves synaptobrevin (SNARE) in inhibitory interneurons → blocks glycine & GABA release | Spastic paralysis: trismus (lockjaw), risus sardonicus, opisthotonos |
| Strychnine | Glycine-receptor antagonist | Similar spasms, convulsions |
| Botulinum toxin (contrast) | Cleaves SNAREs at the NMJ → blocks ACh release | Flaccid paralysis, descending |
CheckBoth tetanus and botulinum toxins cleave SNARE proteins. Why opposite paralysis types?
Different target cells. Tetanus silences inhibitory interneurons (Renshaw) → too much motor output (spastic). Botulinum silences the NMJ → no ACh (flaccid).
Step 1 HY — tetanus
Deep, dirty puncture wound + incomplete vaccination → trismus, risus sardonicus, opisthotonos, autonomic instability. Treat: tetanus immune globulin + tetanus toxoid vaccine, wound debridement, metronidazole, benzodiazepines (boost GABA-A) for spasms. Neonatal tetanus from unclean cord care.
Step 1 trap
Tetanus toxin does not act at the NMJ, and it does not destroy motor neurons. It blocks release of inhibitory transmitters (glycine, GABA) in the CNS.
Buzzwords
"lockjaw" / "fixed grin" / "arched back" → tetanus"rusty nail, no booster" → C. tetani"rat poison with convulsions" → strychnine, glycine receptor"recurrent inhibition" → Renshaw cell
Image drill · Michigan SecondLook
How to use: name what you see, then tap the card to check. Images credited to the Michigan SecondLook Anki Deck (University of Michigan Medical School).
1 · Ventral horn. Name the large arrowed cells. (tap for answer)
Lower motor neurons (α motor neurons): large multipolar cell bodies with abundant Nissl substance and a central nucleolus.
One α motor neuron + all the muscle fibers it innervates = a motor unit.
One α motor neuron + all the muscle fibers it innervates = a motor unit.
Image: Michigan SecondLook Anki Deck · SecondLook Neuroanatomy (Michigan Virtual Microscopy Slide Collection), University of Michigan Medical School · CC BY-NC-SA 4.0
ApplyLO 4 application questions
A1A farmer with a deep puncture wound and no boosters develops jaw stiffness, a fixed grin and back arching. Which cell has lost its output, and which transmitter?
Renshaw (and other inhibitory) interneurons; glycine (and GABA). Tetanospasmin cleaves synaptobrevin.
A2Why do extraocular muscles have motor units of about 1:10 and the gastrocnemius about 1:1000?
Eye needs precision, calf needs force. Small units give fine gradation; large units give big force per neuron.
A3A C6 root is compressed. The biceps is weak, not paralyzed. Explain.
The biceps motor pool spans C5–C7; the C5 and C7 portions still drive it.
A4A patient has a weak, wasted hand with fasciculations, and brisk reflexes with a Babinski sign in the legs; sensation is normal. Which neurons are affected?
Both UMNs and LMNs (ALS). Wasting + fasciculations = LMN; hyperreflexia + Babinski = UMN; spared sensation.
LO 5Brainstem levels in cross-sectionClosed medulla, open medulla, pons, midbrain
By the end: you can name the three parts of the brainstem with their embryologic names, point out the ventral and dorsal surface landmarks, and identify a section as closed medulla, open medulla, pons or midbrain from its ventricle and its "signature" structure.
Q1. What are the parts and surface landmarks of the brainstem?
| Part | Embryo name | Ventral | Dorsal |
|---|---|---|---|
| Midbrain | Mesencephalon | Cerebral peduncles | Superior colliculi (vision) · inferior colliculi (hearing) |
| Pons | Metencephalon | Basilar pons (transverse fibers) | Floor of 4th ventricle; middle cerebellar peduncle |
| Medulla | Myelencephalon | Pyramids · olives | Gracile/cuneate tubercles; inferior cerebellar peduncle |
- Cerebellar peduncles: superior (midbrain, mostly output), middle (pons, input from pontine nuclei), inferior (medulla, spinocerebellar and olive input).
- Pyramids = corticospinal fibers; they decussate at the cord–medulla junction → left brain moves right body.
Memory tool
M-M-M top to bottom: Mesencephalon → Metencephalon → Myelencephalon (alphabetical: e-s, e-t, y). Colliculi: Superior = Sight, Inferior = "In-ear".
Q2. What is the signature of each level in cross-section?
| Level | Ventricle | Signature landmarks | Cranial nerve nuclei |
|---|---|---|---|
| Rostral midbrain | Cerebral aqueduct | Superior colliculi, red nucleus, substantia nigra, cerebral peduncles | III (+ Edinger-Westphal); III fibers pass through red nucleus → interpeduncular fossa |
| Caudal midbrain | Cerebral aqueduct | Inferior colliculi, decussation of superior cerebellar peduncles | IV |
| Rostral (mid) pons | 4th ventricle | Big basilar pons; middle cerebellar peduncles | V (main sensory + motor) |
| Caudal pons | 4th ventricle | Basilar pons; facial colliculus in the floor | VI, VII, vestibular (VIII), spinal V |
| Open (rostral) medulla | 4th ventricle, open dorsally | Inferior olivary nucleus, pyramids, ICP | XII, dorsal motor X, solitary, n. ambiguus (IX, X) |
| Closed (caudal) medulla | Central canal (no 4th ventricle) | N. gracilis + cuneatus, internal arcuate fibers, pyramidal decussation | Spinal V nucleus; caudal XII |
The obex marks the boundary: where the 4th ventricle narrows into the central canal. Above = open medulla; below = closed medulla.
- Four levels to know: closed medulla, open medulla, pons, mesencephalon.
- Corticospinal: cerebral peduncle (midbrain) → broken into bundles among the transverse fibers of the basilar pons → pyramids (medulla).
- DCML: nucleus gracilis + cuneatus, internal arcuate fibers and the start of the medial lemniscus are all in the closed medulla.
- Anterolateral system: anterolateral in the cord → pushed behind the olive in the medulla → forms a "C/L" with the now horizontal medial lemniscus in the pons.
CheckA section has a wide 4th ventricle above and a wavy "purse-like" nucleus on each side ventrally. Level?
Open (rostral) medulla; the wavy nucleus is the inferior olivary nucleus.
Step 1 trap
The "X" of crossing fibers in the closed medulla can be the pyramidal (motor) decussation (ventral, caudal-most) or the sensory decussation of internal arcuate fibers (a bit higher, arching from the dorsal nuclei). Ventral X near the cord = motor; arching fibers from gracile/cuneate = sensory.
Q3. How do I localize a brainstem lesion? (rule of 4s, intro)
4 Medial "M"s
- Motor pathway (corticospinal) → contralateral weakness
- Medial lemniscus → contralateral touch/vibration loss
- MLF → internuclear ophthalmoplegia
- Motor nuclei: III, IV, VI, XII (the ones that divide evenly into 12) → ipsilateral
4 Side "S"s
- Spinocerebellar → ipsilateral ataxia
- Spinothalamic → contralateral body pain/temp
- Sensory nucleus of V → ipsilateral face pain/temp
- Sympathetic pathway → ipsilateral Horner
4 levels × 4 nerves: above pons I–IV (midbrain III, IV) · pons V–VIII · medulla IX–XII. A crossed finding (ipsilateral cranial nerve + contralateral body) = brainstem.
| Syndrome | Artery | Key findings |
|---|---|---|
| Medial medullary | Anterior spinal / vertebral | Contralateral hemiparesis + contralateral ↓ proprioception; tongue deviates toward lesion (XII) |
| Lateral medullary (Wallenberg) | PICA / vertebral | Dysphagia, hoarseness (n. ambiguus), ipsilateral face + contralateral body pain/temp loss, Horner, vertigo, ataxia |
| Medial pontine | Paramedian basilar branches | Contralateral hemiparesis + ipsilateral VI palsy |
| Lateral pontine | AICA | Ipsilateral facial paralysis, ↓ hearing, vertigo, face/body pain-temp split, ataxia |
| Weber (medial midbrain) | PCA branches | Ipsilateral III palsy (down and out, dilated pupil) + contralateral hemiparesis |
Step 1 HY — rule of 4s
Medial lesion = M structures (motor tract, medial lemniscus, MLF, motor nuclei III/IV/VI/XII). Lateral lesion = S structures. Then the cranial nerve tells you the level: XII → medulla, VI → pons, III → midbrain. Lateral lesions: IX/X → medulla, VII/VIII → pons.
Memory tool
Medial medullary: "lick your wounds" — the tongue points toward the side of the lesion. Lateral medullary: "don't PICK (PICA) a horse (hoarseness) that can't eat (dysphagia)."
Buzzwords
"wavy, purse-shaped nucleus" → inferior olive, open medulla"facial colliculus" → caudal pons, VI nucleus + VII fibers"obex" → open/closed medulla boundary"crossed signs" → brainstem lesion
ApplyLO 5 application questions
A1A myelin-stained section shows a central canal, dorsal nucleus gracilis and cuneatus, and fibers crossing in the ventral midline. Level?
Closed (caudal) medulla at the pyramidal (motor) decussation.
A2A section has a narrow aqueduct, two dorsal bumps, large ventral peduncles, and CN III fibers coursing through a round nucleus. Level?
Rostral midbrain (superior colliculus level); the fibers pass through the red nucleus.
A3Right hemiparesis, right loss of vibration, and the tongue deviates to the left when protruded. Where is the lesion?
Left medial medulla (medial medullary syndrome): left XII, left pyramid, left medial lemniscus.
A4A section shows a 4th ventricle, a giant ventral mass with horizontal fibers and scattered bundles, and a bulge in the floor of the ventricle. Which nerves have nuclei here?
Caudal pons: VI and VII (the bulge is the facial colliculus), plus vestibular nuclei and spinal V.
LO 6The neural tube blueprintWhy sensory is dorsal in the cord and lateral in the brainstem
By the end: you can name the alar and basal plates and the sulcus limitans, trace each into the adult cord, explain the "open book" shift in the brainstem, and place the seven functional columns from medial to lateral.
Q1. How does the neural tube become the spinal cord?
- The neural tube wall thickens into a dorsal alar plate and a ventral basal plate, separated by a groove in the lumen: the sulcus limitans.
- Alar → dorsal horn (sensory processing). Basal → ventral horn (motor).
- The intermediate zone between them → lateral horn (visceral motor).
- Dorsal root ganglia come from neural crest, not from the tube. Somites (muscles the LMNs drive) come from paraxial mesoderm.
- Motor = medial, from the basal plate (GSE, GVE, SVE). Sensory = lateral, from the alar plate (GVA, SVA, GSA, SSA); the sulcus limitans separates them.
CheckWhich germ-line structure makes the dorsal root ganglion cells: alar plate or neural crest?
Neural crest. The alar plate makes the dorsal horn (inside the cord), not the ganglion outside it.
Step 1 trap
"Alar" sounds like "alarm" → a sensory alarm. Don't flip it: the basal plate is the base of movement. And remember: dorsal in the cord = lateral in the brainstem.
Q2. Why does the brainstem put motor medial and sensory lateral?
Cordcentral canal closed; alar dorsal, basal ventral
→
Canal widensinto the 4th ventricle
→
Alar plates swing outlike the covers of a book
→
Resultbasal (motor) medial, alar (sensory) lateral
Picture slicing the cord down its dorsal midline and laying it open flat: what was dorsal is now lateral.
| Column (medial → lateral) | Plate | Supplies | Example nuclei |
|---|---|---|---|
| GSE | Basal · most medial | Skeletal muscle from somites (eye, tongue) | III, IV, VI, XII |
| GVE | Basal | Parasympathetic preganglionics | Edinger-Westphal (III), salivatory (VII, IX), dorsal motor nucleus of X |
| SVE | Basal (migrates ventrolateral) | Branchial (pharyngeal arch) muscle | Motor V, facial (VII), nucleus ambiguus (IX, X) |
| GVA / SVA | Alar | Visceral sensation / taste | Solitary nucleus (VII, IX, X) |
| GSA | Alar | Touch, pain, temp of face | Trigeminal sensory nuclei |
| SSA | Alar · most lateral | Hearing, balance | Vestibular + cochlear nuclei (VIII) |
CheckThe hypoglossal nucleus hugs the midline; the vestibular nuclei are far lateral. Explain with development.
XII = GSE from the basal plate → most medial. Vestibular = SSA from the alar plate → most lateral.
Memory tool
- Motor = Medial (both M). Sensory = Side.
- Read the floor from the midline out: GSE → GVE → (SVE, deeper) | sulcus limitans | GVA/SVA → GSA → SSA. The two "special" ends anchor it: somatic motor in the middle, special senses at the side.
Step 1 HY — neural tube
- Neural crest → DRG, autonomic ganglia, Schwann cells, adrenal medulla, melanocytes.
- Neural tube defects: failed closure of the posterior neuropore (week 4) → spina bifida occulta (tuft of hair, normal AFP) → meningocele → myelomeningocele (↑ AFP, ↑ AChE). Prevent with folate.
- Tethered cord: a low conus pulled by a thick filum terminale.
ApplyLO 6 application questions
A1The solitary nucleus receives taste. Predict its plate and its position relative to the dorsal motor nucleus of X.
Alar plate (SVA/GVA) → lateral to the dorsal motor nucleus of X (GVE, basal).
A2A disorder destroys neural crest cells early. Which spinal structure is missing: dorsal horn or dorsal root ganglion?
Dorsal root ganglion. The dorsal horn is alar plate (neural tube).
A3Nucleus ambiguus moves muscles of the larynx and pharynx. Which column, and why does it sit deep, away from the ventricle floor?
SVE (branchial motor), basal plate. SVE nuclei migrate ventrolaterally during development.
LO 7Cranial nerves III–XII: level and exitWhich part of the brainstem, and where each nerve leaves it
By the end: you can list CN III–XII in order, sort them as 2 · 2 · 4 · 4 by level, describe each exit point using neighbors (peduncles, pons, pyramid, olive), and predict which nerves a lesion at a given spot will hit.
Q1. Which nerves belong to which part of the brainstem?
CerebrumI olfactory · II optic
↓
MidbrainIII oculomotor · IV trochlear
↓
PonsV trigeminal · VI abducens · VII facial · VIII vestibulocochlear
↓
MedullaIX glossopharyngeal · X vagus · XI accessory · XII hypoglossal
- List III–XII by name → midbrain (III, IV), pons (V–VIII), medulla (IX–XII).
- VIII is the only purely sensory cranial nerve attached to the brainstem (I and II attach to the cerebrum).
CN XI is the exception: its nucleus is in the upper cervical cord (C1–C5); the rootlets ascend through the foramen magnum and leave with IX and X via the jugular foramen.
Memory tool
2 · 2 · 4 · 4: two above the brainstem, two in the midbrain, four in the pons, four in the medulla. Names in order: "Oh Oh Oh To Touch And Feel Very Good Velvet... AH!" (Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal).
Q2. Exactly where does each nerve leave the brainstem?
| CN | Level | Exit point (neighbors) | Main job | Skull exit |
|---|---|---|---|---|
| III Oculomotor | Midbrain | Interpeduncular fossa, between cerebral peduncles (passes between PCA and SCA) | Most eye muscles, levator, pupil constriction | Superior orbital fissure |
| IV Trochlear | Midbrain | Dorsal, just below inferior colliculus; wraps around peduncle | Superior oblique | Superior orbital fissure |
| V Trigeminal | Pons | Lateral mid-pons, at the middle cerebellar peduncle | Face sensation, chewing | V1 SOF · V2 rotundum · V3 ovale |
| VI Abducens | Pons | Pontomedullary junction, medially (above the pyramid) | Lateral rectus | Superior orbital fissure |
| VII Facial | Pons | Pontomedullary junction, laterally (cerebellopontine angle) | Facial expression, taste ant. 2/3, glands | Internal acoustic meatus |
| VIII Vestibulocochlear | Pons (–medulla junction) | Cerebellopontine angle, lateral to VII | Hearing, balance | Internal acoustic meatus |
| IX Glossopharyngeal | Medulla | Postolivary sulcus (behind the olive), top | Gag (sensory), taste post. 1/3, parotid, stylopharyngeus | Jugular foramen |
| X Vagus | Medulla | Postolivary sulcus, below IX | Palate, pharynx, larynx, viscera | Jugular foramen |
| XI Accessory | Cervical cord | Rootlets from C1–C5, ascend through foramen magnum | SCM, trapezius | Jugular foramen |
| XII Hypoglossal | Medulla | Preolivary sulcus (between pyramid and olive) | Tongue muscles | Hypoglossal canal |
- Cranial nerves exit in columns: pure motor (III, IV, VI, XII) exit medially → mixed (V, VII, IX, X) exit laterally → pure sensory VIII is the most lateral.
- Same medial-motor / lateral-sensory rule as the nuclei.
CheckWhich nerve is the only one to leave the dorsal brainstem, and what else is odd about it?
CN IV (trochlear). It also decussates before exiting, so each trochlear nucleus supplies the contralateral superior oblique.
Step 1 HY — lesions by exit point
- PCom aneurysm or uncal herniation → CN III compression: "down and out" eye, ptosis, dilated pupil (parasympathetic fibers are on the outside). Diabetic (ischemic) III palsy spares the pupil.
- CN IV palsy → vertical diplopia worse looking down (stairs, reading); head tilts away from the lesion.
- CN VI has a long course → first to stretch with ↑ ICP; eye can't abduct.
- Cerebellopontine angle: vestibular schwannoma → hearing loss, tinnitus (VIII), then facial weakness (VII), then ↓ corneal reflex (V). Bilateral = NF2.
- Jugular foramen lesion → IX, X, XI (Vernet): ↓ gag, hoarseness, uvula deviates away, weak SCM/trapezius.
- Cavernous sinus: III, IV, V1, V2 in the wall; VI free beside the internal carotid → VI is hit first.
Step 1 trap
- XII exits in front of the olive (preolivary); IX/X exit behind it (postolivary). Swapping them is the classic error.
- Tongue deviates toward a XII lesion; uvula deviates away from a X lesion.
- A UMN facial lesion spares the forehead; an LMN (facial nucleus in pons or nerve) lesion paralyzes the whole half-face.
Buzzwords
"down and out, blown pupil" → CN III compression"trouble walking down stairs, head tilt" → CN IV"unilateral hearing loss + tinnitus" → CP-angle schwannoma"bull's-eye rash, whole face droops" → Lyme facial (VII) palsy
ApplyLO 7 application questions
A1A slow-growing tumor at the right cerebellopontine angle. Which nerves are compressed first, and which next?
VIII first (hearing loss, tinnitus), then VII (facial weakness); a large tumor reaches V (↓ corneal reflex).
A2A small medial lesion at the pontomedullary junction damages the nerve that exits just above the pyramid. Deficit?
CN VI palsy: the eye can't abduct → horizontal diplopia; eye rests medially.
A3A posterior communicating artery aneurysm presses on the nerve that has just left the interpeduncular fossa. Which nerve is at risk and what is the first sign?
CN III; a dilated, poorly reactive pupil often comes first because the pupillary fibers run on the nerve's surface.
A4A fracture through the right jugular foramen. Predict the deficits.
Right IX, X, XI: ↓ gag sensation, hoarseness/dysphagia, uvula deviates to the left, weak right SCM and trapezius (shoulder droop).
A5Which four cranial nerves leave the brainstem in the medial (motor) column?
III, IV, VI, XII, the pure somatic motor nerves (GSE) to eye muscles and tongue. (IV is medial in its nucleus but exits dorsally.)
MCQPractice questionsClick an answer to see the explanation.
Score: 0 / 0
1A 34-year-old woman with fever, headache and neck stiffness needs a lumbar puncture. The resident palpates the tops of the iliac crests to choose the site. Which statement best explains why this interspace is chosen?
C. The iliac-crest line marks L4; below the conus (L1–L2) the needle meets only floating roots. A: the dural sac ends at S2. B: pia continues as the filum terminale. D: the epidural space is a real space in the spine. E: denticulate ligaments anchor the cord laterally along its length; they don't explain the safe level.
2A 58-year-old man has 10 days of worsening low back pain radiating asymmetrically down both legs, numbness of the perineum and buttocks, urinary retention, flaccid leg weakness and absent ankle reflexes. Plantar responses are flexor. MRI is most likely to show compression of which structure?
A — cauda equina syndrome. Gradual, asymmetric, radicular pain with pure LMN signs and saddle anesthesia = roots. B: conus lesions are more sudden, symmetric, with early sphincter loss and possible UMN signs. C: corticospinal lesions give spasticity and Babinski. D: dorsal columns cause sensory ataxia, not flaccid weakness. E: wrong level (arms).
3A 29-year-old cook notices painless blisters from hot pans on both hands. Exam shows loss of pain and temperature over both hands, forearms and shoulders, with normal light touch and vibration. Two years earlier MRI showed a Chiari I malformation. Damage to which structure explains the sensory pattern?
E. A syrinx expands the central canal and first destroys the crossing 2nd-order pain/temp fibers in the anterior white commissure → bilateral cape-like loss with touch spared. A: cuneatus carries touch/vibration (spared). B: DRG loss would remove all modalities. C: tract damage would cause loss below the level, not a cape. D: Lissauer's tract injury wouldn't spare segments below.
4A myelin-stained spinal cord section is small and round. White matter is abundant relative to slender gray horns, and a small pointed projection juts from the side of the gray matter between the dorsal and ventral horns. Which neurons are found in that projection?
B. Small round section + lateral horn = thoracic; the intermediolateral cell column holds sympathetic preganglionics (T1–L2/3). A, D: ventral horn. C: parasympathetic preganglionics are at S2–S4, in a section that is mostly gray. E: those are in the medulla (n. gracilis/cuneatus).
5Eight days after stepping on a nail, an unvaccinated 40-year-old farmer develops jaw stiffness, a fixed grin and painful arching of the back. The toxin responsible reaches its target by retrograde axonal transport. Which effect directly causes the motor findings?
D. Tetanospasmin cleaves synaptobrevin in inhibitory interneurons, so glycine and GABA aren't released → α motor neurons fire unchecked (spastic paralysis). A: botulinum toxin (flaccid). B: curare-like. C: polio/ALS. E: not the mechanism.
6A 74-year-old woman on chemotherapy has burning pain followed by a band of vesicles that wraps from her left lower back to her umbilicus and stops at the midline. Where has the virus been latent?
A. Umbilicus = T10; VZV is latent in sensory (dorsal root) ganglia and reactivates along one dermatome. B: motor neurons don't harbor VZV. C: T4 = nipple. D: autonomic ganglia aren't the classic site. E: unilateral rash, ganglion not horn.
7A brainstem section shows a wide 4th ventricle opening dorsally, convoluted "purse-shaped" nuclei ventrolaterally, and pyramids ventrally. A nucleus lying immediately next to the midline in the floor of the ventricle at this level innervates which muscles?
C. Open medulla (inferior olives); the most medial floor nucleus is the hypoglossal (GSE). A: VI nucleus, caudal pons. B: facial nucleus, pons. D: IV, caudal midbrain. E: motor V, mid-pons.
8A 66-year-old has a small infarct in the ventral pons near the midline at the pontomedullary junction. His left eye cannot move laterally, and he has right-sided arm and leg weakness. The cranial nerve involved exits the brainstem at which site?
E. Left CN VI + right hemiparesis = left medial pontine lesion; VI exits medially at the pontomedullary junction. A: III. B: V. C: XII. D: IV.
9A 47-year-old has 2 years of progressive right-sided hearing loss and tinnitus, and now mild right facial weakness. MRI shows an enhancing mass where the pons, medulla and cerebellum meet. As it enlarges, which reflex is most likely to be lost next?
B. A cerebellopontine-angle vestibular schwannoma hits VIII, then VII, then V (corneal afferent = V1). A: IX/X lie lower at the jugular foramen. C: III, midbrain. D: wrong side. E: C5–C6.
10During development, which brainstem nucleus arises from the alar plate?
D. Alar plate = sensory; the solitary nucleus handles taste and visceral sensation (SVA/GVA) and sits lateral to the sulcus limitans. A, E: GSE (basal). B: SVE (basal). C: GVE (basal).
11In an experiment, the small motor neurons that innervate intrafusal fibers in the quadriceps are selectively silenced while the large motor neurons remain intact. Which change is expected?
C. γ motor neurons keep spindles taut; without them the spindle goes slack and the stretch reflex weakens. A: α neurons still produce force. B: fasciculations come from α/LMN disease. D: UMN signs. E: Renshaw cells are driven by α collaterals.
12A 71-year-old has sudden weakness of the left arm and leg and loss of vibration and position sense on the left. When he sticks out his tongue it deviates to the right. Occlusion of which artery is most likely?
A — right medial medullary syndrome. Right pyramid + right medial lemniscus (both cross below) → left body findings; right XII → tongue deviates right (toward lesion). B: lateral medullary (dysphagia, Horner, crossed pain/temp). C: lateral pons (VII, VIII). D: Weber (III palsy). E: cortical, no tongue-plus-crossed pattern of this kind.
13A 25-year-old is stabbed in the back. Exam shows weakness and hyperreflexia of the right leg, with loss of vibration and position sense in the right leg. Pain and temperature are lost in the left leg beginning a little below the umbilicus. Arm function is normal. Where is the lesion?
D — Brown-Séquard. Ipsilateral UMN weakness and dorsal-column loss (right), contralateral pain/temp loss starting 1–2 segments below (left). A: wrong side. B: ASA syndrome is bilateral and spares vibration. C: no motor or pain loss. E: a peduncle lesion would also hit the arm and wouldn't split sensations this way.
14A man died years after a complete spinal cord transection at T9. A myelin-stained section is taken at C5. Which pattern is expected?
B. Ascending fibers degenerate above a lesion: leg fibers in the gracilis lost their DRG cell bodies below T9. Cuneatus fibers entered above T9 (normal), and descending corticospinal axons at C5 still connect to cortex (they degenerate below the lesion). A: reversed. C: wrong column. D: gracilis must change. E: ventral horns are gray matter, unaffected at C5.