Somatic Sensations & Ascending Pathways
Learning objectives
- LO 1Match each skin receptor to the sensation it picks up, and say which route (body vs face) carries that signal to the sensory cortex.
- LO 2For the dorsal column and trigeminal touch pathways, explain what they carry, where the 1st-, 2nd- and 3rd-order cell bodies sit, where the first synapse happens, which CNS tract carries the signal upward, and where it crosses the midline.
- LO 3Sketch cross-sections of the cervical cord and the brainstem, and mark where the DCML and trigeminal touch tracts and their nuclei lie.
- LO 4Know which arteries feed which territories of the brainstem and spinal cord, and predict the sensory and motor losses when one of them is blocked.
- LO 5Trace taste from the tongue to the cortex.
MAPBig picture
- Skin receptors → pressure · vibration · stretch → Aβ fibers (pain/temp → Aδ, C)
- Body → ipsilateral dorsal column → caudal medulla nuclei → cross (internal arcuate) → medial lemniscus → VPL → postcentral gyrus
- Face → CN V → principal sensory nucleus (pons) → VTTT (crossed) + DTTT (uncrossed) → VPM → cortex
- Lesion before vs after the crossing → which side loses sensation
- Which artery feeds the lesion → what else is lost
- Taste → CN VII, IX, X → rostral solitary nucleus → VPM → gustatory cortex
LO 1Skin receptors and the route each signal takesWhich receptor feels what, how fast it adapts, which fiber carries it, and which pathway it joins.
Q1. How do we classify sensory receptors in the first place?
By location
- Exteroceptors → skin / body surface → touch, pressure, pain, temperature
- Proprioceptors → muscles, tendons, joints → stretch, joint position, movement
- Enteroceptors (visceroceptors) → internal organs → gut distension, blood pH, other internal changes
By stimulus
- Mechanoreceptors → pressure, vibration, stretch (skin, ear, muscle)
- Thermoreceptors → temperature (skin, hypothalamus)
- Nociceptors → pain
- Chemoreceptors → O₂, CO₂, pH (carotid and aortic bodies)
- Photoreceptors → light (rods, cones)
CheckA muscle spindle in the quadriceps: classify it by location and by stimulus.
Q2. Which skin receptor detects which sensation?
The peripheral endings of DRG neurons either stay bare (free nerve endings) or are wrapped in a specialized structure (encapsulated receptors, plus the Merkel cell complex).
| Receptor | Sensation | Adapts | Where / look | Fiber → pathway |
|---|---|---|---|---|
| Meissner corpuscle | Light (fine) touch, low-frequency vibration / flutter | Fast | Dermal papillae of hairless skin (fingertips, lips); small receptive field | Aβ → DCML |
| Pacinian corpuscle | Vibration (high-frequency) and deep pressure | Fast | Deep dermis, joints, ligaments; "onion-skin" lamellar capsule; large field | Aβ → DCML |
| Merkel disc (cell) | Pressure and texture, sustained static touch, shape | Slow | Basal epidermis, fingertips; small field | Aβ → DCML |
| Ruffini ending | Skin stretch, joint angle, slippage | Slow | Dermis, joint capsules (encapsulated); large field | Aβ → DCML |
| Free nerve ending | Pain and temperature (and itch) | Slow / none | All skin, most tissues | Aδ, C → anterolateral (spinothalamic) |
- Labeled skin drawing:
- Merkel's disc → pressure & texture
- Free nerve ending → pain & temp
- Meissner → light touch
- Pacinian → vibration & pressure
- Ruffini → stretch of skin
- Simplest grouping: Merkel = touch, slow-adapting; Meissner/Pacinian = pressure, fast-adapting.
CheckYou feel a phone buzz in your pocket. Which receptor, and why that one?
Step 1 HY — receptors
- Pacinian = vibration + deep pressure, lamellated, fast.
- Meissner = light discriminatory touch in glabrous skin, fast.
- Merkel = static pressure, texture, slow.
- Ruffini = stretch, joint angle, slow.
- Free nerve endings = pain/temp via Aδ and C.
Memory tool
"Pacinian = Pulsing (vibration), Meissner = Mild touch, Merkel = Maintained pressure, Ruffini = Rubber-band stretch." The two that end in -cian/-sner (corpuscles) adapt fast; the disc and the ending (Merkel, Ruffini) adapt slow.
Q3. What does "adaptation" mean, and how do Merkel cells differ from corpuscles at the molecular level?
Adaptation = how a receptor's firing changes during a constant stimulus.
Merkel cell → slow adapting
- Pressure opens Piezo2 mechanosensitive channels in the Merkel cell
- Merkel cell releases serotonin onto the Aβ terminal
- Acts on ionotropic and metabotropic receptors → sustained firing → slow adapting
Meissner / Pacinian → fast adapting
- Physical deformation opens pressure-sensitive Na⁺ channels in the axon itself
- Na⁺ influx → generator potential → action potential
- Capsule layers soak up steady pressure → fast adapting
- Everyday example: you stop noticing the chair once seated (deep pressure adapts rapidly), but a pinprick keeps hurting (nociceptors = no adaptation).
- Contrast to know: Merkel = serotonin release, ionotropic + metabotropic receptors, slow adapting vs Meissner/Pacinian = deformation opens pressure-sensitive Na⁺ channels, Na⁺ in → AP, fast adapting.
CheckWhich transmitter does a Merkel cell release, and what kind of adaptation results?
Step 1 trap
"Slow adapting" does not mean slow conduction. Merkel and Meissner both use the same fast Aβ fiber; adaptation is a property of the receptor ending, conduction speed is a property of the axon.
Q4. Which fiber carries which sensation, and how fast?
Rule: bigger + more myelin = faster. Touch and proprioception get the express lane; pain and temperature take the local.
| Fiber | Diameter (µm) | Myelin | Speed (m/s) | Carries | Pathway |
|---|---|---|---|---|---|
| Aα (group Ia, Ib) | 13–20 | Heavy | 80–120 | Muscle spindle (Ia), Golgi tendon organ (Ib) → proprioception | DCML (conscious) / spinocerebellar (non-conscious) |
| Aβ (group II) | 6–12 | Heavy | 35–75 | Mechanoreceptors of skin: touch, pressure, vibration | DCML |
| Aδ (group III) | 1–5 | Thin / light | 5–30 | Pain, temperature: "fast", sharp, well-localized pain; cold | Anterolateral |
| C (group IV) | 0.2–1.5 | None | 0.5–2 | Pain, temperature, itch: "slow", dull, aching pain; warmth | Anterolateral |
- Three skin fibers: Aβ (6–12 µm, 35–75 m/s) → touch, pressure; Aδ (1–5 µm, 5–30 m/s) → pain, temperature; C (0.2–1.5 µm, 0.5–2 m/s) → temperature, pain, itch.
- Discriminative touch and pressure use fast, myelinated Aβ fibers.
CheckYou touch a hot pan. Which fiber gives the instant sharp "ouch," and which gives the lingering ache?
Step 1 HY — fibers
Local anesthetics block small, myelinated fibers first (Aδ pain), then C, then Aβ touch, then Aα motor/proprioception. Boards like "patient feels pressure but no pain after lidocaine."
Memory tool
A-β = "Big touch", A-δ = "Darn, that's hot!", C = "Crawling slow". Alphabet order = shrinking size = slowing speed: α > β > δ > C.
Q5. Why can your lips tell two points apart but your back cannot? (two-point discrimination and the homunculus)
- Two-point discrimination = the smallest gap at which two touching points feel like two.
- It depends on receptor density and receptive-field size:
- Fingertips and lips: dense receptors, tiny fields → only a few mm (lips ~7 mm, fingertips ~2–3 mm)
- Back, calf, thigh: sparse receptors, large fields → ~42 mm
- The primary somatosensory cortex (S1, postcentral gyrus) is somatotopic: neighboring body parts map to neighboring cortex.
- The sensory homunculus shows cortical area per body part, scaled to receptor density / acuity, not to body size → huge hands, lips, tongue.
CheckA test for two-point discrimination is abnormal on the right fingertips. Which ascending pathway is in trouble?
Step 1 HY — homunculus and strokes
ACA stroke → contralateral leg/foot sensory + motor loss (medial surface). MCA stroke → contralateral face and arm > leg. Loss of two-point discrimination, vibration and position = dorsal column problem.
Buzzwords
Q6. Once inside the cord, which pathway does each signal take — and what replaces it for the face?
DCML
Touch, pressure, vibration, conscious proprioception (fine, discriminative)
Aβ + Aα · crosses in the medulla
Anterolateral (spinothalamic)
Pain and temperature (+ crude touch)
Aδ + C · crosses in the cord
Spinocerebellar
Non-conscious proprioception → cerebellum
Spindles, tendon organs
- Body touch → DCML → VPL → cortex.
- Face touch → CN V → principal (main) sensory trigeminal nucleus → VTTT/DTTT → VPM → cortex (the "DCML of the face").
- The ascending pathways sit next to the descending corticospinal tract (motor) — "start thinking in 3D": one lesion often hits several tracts.
CheckVibration on the cheek vs vibration on the shin: which thalamic nucleus receives each?
Step 1 trap
Crude touch travels with pain/temp in the anterior spinothalamic part of the anterolateral system. So a patient with dorsal column loss still feels a cotton swab, but cannot localize it or tell two points apart. Do not answer "total loss of touch" for a dorsal column lesion.
Image drill · Michigan SecondLook
1 · Name the green region, the white line and the blue tract. (tap for answer)
White line = ventral (anterior) white commissure: ALS axons cross here within 1–2 segments.
Blue = anterolateral system (spinothalamic tract) in the contralateral lateral/ventral white matter.
ApplyLO 1 application questions
A1Put on a wristwatch: you feel it for a few seconds and then forget it is there. Which receptors made that happen?
A2A drug blocks Piezo2 in the skin. Which receptor loses most of its function, and what sensation suffers?
A3On nerve conduction testing, a patient's fastest sensory fibers conduct at 50 m/s. Which fiber class and modality is that?
A4Why is the hand on the homunculus bigger than the entire trunk?
LO 2The DCML and the trigeminal touch pathwayCell bodies, first synapse, ascending tract, crossing point — then the lesions.
Q1. How do large and small fibers sort themselves out as the dorsal root enters the cord?
- Axons enter through the dorsal root entry zone (DREZ), a wide zone spanning white and gray matter; sensory axons can enter anywhere in it, sorted by size:
- Medial to the dorsal horn = myelinated (Aβ)
- Lateral to the dorsal horn = lightly/unmyelinated (Aδ, C)
- Medial division = large, myelinated (Aβ) → turn up into the dorsal column (DCML).
- Lateral division = small, lightly or unmyelinated (Aδ, C) → Lissauer's tract / dorsal horn → anterolateral system.
CheckA surgeon cuts only the lateral part of a dorsal rootlet (DREZ lesioning for pain). What is spared?
Memory tool
"Big guys walk down the middle aisle." Large Aβ fibers go medial into the dorsal column; small fibers stay lateral.
Q2. What are fasciculus gracilis and cuneatus, and why are they arranged S-L-T-C?
- The dorsal column on each side has two bundles, split by the posterior intermediate sulcus:
- Fasciculus gracilis → medial → lower body (legs; below T6, i.e. T7 and down)
- Fasciculus cuneatus → lateral → upper body (arms; T6 and above)
- Axons line up in the order they arrive: each new level is added on the lateral side, pushing older fibers medially → medial to lateral = S → L → T → C.
- Consequence: below T6 there is no cuneatus — a lumbar section shows only gracilis.
CheckOnly the lateral part of the right dorsal column is damaged at C4. What is lost?
Step 1 HY
Boards phrase it as: gracilis = T7 and below (legs), cuneatus = T6 and above (arms). Same line, different wording.
Memory tool
"Graceful legs walk down the middle; the cuneiform arms reach out." Gracilis = legs = medial; cuneatus = arms = lateral. "Legs in, arms out."
Q3. Walk the DCML neuron by neuron. Where are the cell bodies, synapses and crossing?
| Neuron | Cell body | Axon travels in | Synapses in |
|---|---|---|---|
| 1st order | Dorsal root ganglion | Ipsilateral dorsal column (gracilis / cuneatus) | Nucleus gracilis / cuneatus, caudal (closed) medulla |
| 2nd order | Nucleus gracilis / cuneatus | Internal arcuate fibers (cross) → contralateral medial lemniscus | VPL of thalamus |
| 3rd order | VPL | Posterior limb of the internal capsule | Primary somatosensory cortex (postcentral gyrus; "sensorimotor cortex") |
The DCML is a 3-neuron relay that transmits touch, pressure and conscious proprioception. Body chain: DRG → fasciculus gracilis/cuneatus → synapse in nucleus gracilis/cuneatus (closed medulla) → decussate via internal arcuate fibers → medial lemniscus → synapse in VPL → sensory cortex.
Somatotopy is kept the whole way (S-L-T-C in the cord → leg/arm bands in the lemniscus → homunculus).
Conscious proprioception rides the same route: large myelinated fibers from muscle spindles (Ia, II) and Golgi tendon organs (Ib). The non-conscious copy of that data goes to the cerebellum (spinocerebellar tracts).
CheckName the structure where the DCML crosses and the tract the crossed fibers form.
Step 1 HY
DCML: up first, cross in the medulla. Spinothalamic: cross first (anterior white commissure, within 1–2 segments), then up. This one difference generates every Brown-Séquard question.
Step 1 trap
The DCML does not cross at the pyramidal decussation — that is the corticospinal (motor) tract. Sensory decussation (internal arcuate fibers) sits slightly rostral and dorsal to it in the caudal medulla.
Q4. What is the thalamus doing in the middle of every sensory pathway?
Every sensory system except olfaction relays through the thalamus before reaching cortex.
| Nucleus | Relays | Input pathway |
|---|---|---|
| VPL (ventral posterolateral) | Body sensation | Medial lemniscus (DCML) + spinothalamic |
| VPM (ventral posteromedial) | Face sensation + taste | Trigeminothalamic tracts; solitary nucleus |
| LGN (lateral geniculate) | Vision | Optic tract |
| MGN (medial geniculate) | Hearing | Inferior colliculus |
CheckWhich sense skips the thalamus entirely?
Memory tool
VPM = "Makeup goes on the face (and in the Mouth)" → face + taste. VPL = Limbs. LGN = Light, MGN = Music.
Step 1 HY
A small VPL/VPM lacunar infarct → pure contralateral hemisensory loss (face + body). Weeks later: thalamic pain syndrome (Dejerine-Roussy) — burning contralateral pain and allodynia.
Q5. How does the face do the same job? (the principal sensory trigeminal pathway)
- CN V is the main sensory nerve of the face: V1 ophthalmic (forehead, upper lid, cornea, nose bridge), V2 maxillary (cheek, upper lip, upper teeth, palate), V3 mandibular (lower lip, chin, jaw, lower teeth, general sensation of anterior tongue). It also runs the muscles of mastication.
- CN V has several brainstem nuclei; sensory nuclei sit lateral to the sulcus limitans, motor nuclei medial to it.
| Sensory nucleus of V | Location | Modality |
|---|---|---|
| Mesencephalic | Midbrain → upper pons | Jaw proprioception (jaw-jerk reflex) |
| Principal (main, chief) sensory | Mid-pons, lateral to the motor nucleus of V | Discriminative touch, pressure, vibration (DCML equivalent) |
| Spinal trigeminal | Pons → caudal medulla → upper cervical cord | Pain and temperature (crude touch) |
| Neuron | Cell body | Axon / tract | Synapses in |
|---|---|---|---|
| 1st order | Trigeminal (semilunar) ganglion | CN V root, enters at mid-pons | Principal sensory nucleus of V (pons, ipsilateral) |
| 2nd order | Principal sensory nucleus | Most cross → VTTT (contralateral); some stay → DTTT (ipsilateral) | VPM of thalamus |
| 3rd order | VPM | Posterior limb of internal capsule | Face area of postcentral gyrus (lateral) |
- Face chain: trigeminal ganglion → synapse in principal (main) sensory trigeminal nucleus → VTTT/DTTT → synapse in VPM → sensory cortex.
- Damage to the nerve or principal nucleus → ipsilateral face loses touch/pressure.
- Cutting the VTTT alone usually does not remove contralateral facial touch → the DTTT is a redundant backup.
CheckWhere is the first synapse for vibration from the chin?
Step 1 trap
- Principal nucleus = touch (pons). Spinal nucleus = pain/temp (medulla, down to C2). A lateral medullary stroke spares facial touch but kills facial pain/temp.
- Angle of the jaw is C2/C3 (great auricular), not V3.
Memory tool
Trigeminal exits: "Standing Room Only" → V1 Superior orbital fissure, V2 foramen Rotundum, V3 foramen Ovale. VTTT = Via the other side (crossed); DTTT = Doesn't cross.
Q6. A lesion is ipsilateral or contralateral — how do I decide every time?
One rule: before the crossing → ipsilateral deficit; after the crossing → contralateral deficit. In the cord, a lesion hits fibers entering at that level and all those coming up from below; higher segments have not entered yet.
| Lesion site | Side of deficit | Region affected |
|---|---|---|
| Peripheral nerve / dorsal root | Ipsilateral | That nerve's / dermatome's territory (all modalities) |
| Dorsal column (cord) | Ipsilateral | Dermatomes at and below the lesion |
| Nucleus gracilis / cuneatus | Ipsilateral | Leg (gracilis) or arm (cuneatus) |
| Medial lemniscus (rostral medulla, pons, midbrain) | Contralateral (already crossed) | Whole body below the face |
| VPL / internal capsule / S1 | Contralateral | Body (VPL) ± face (VPM); cortex follows homunculus |
| CN V or principal sensory nucleus | Ipsilateral face | Face touch |
| VTTT alone | Little / no loss | DTTT backup |
Dorsal column damage — symptoms
- Loss of tactile sense: vibration, deep touch, two-point discrimination, position sense; symptoms ipsilateral, in dermatomes at or below the lesion
- Astereognosis: cannot describe an object in the hand with eyes closed
- Sensory ataxic ("stomping") gait: slams the feet down to get feedback
- Positive Romberg: steady with eyes open, sways/falls with eyes closed
Why Romberg works
- Balance needs 2 of 3: proprioception, vision, vestibular
- Dorsal column loss + eyes closed → only vestibular left → falls
- Cerebellar ataxia is unsteady with eyes open and closed → Romberg separates sensory from cerebellar ataxia
CheckRight dorsal column destroyed at T8. Vibration lost in the right hand? In the right foot?
Step 1 HY — Brown-Séquard (hemisection)
Right hemisection at T10: right loss of vibration/position below T10 (DCML), right UMN weakness below (corticospinal), left loss of pain/temp starting 1–2 levels below (spinothalamic already crossed), right LMN signs and all-modality loss at the level. Above T1 → ipsilateral Horner.
Step 1 trap
Romberg tests proprioception, not the cerebellum. A patient who falls only with eyes closed has a sensory ataxia (dorsal columns or large-fiber neuropathy); a cerebellar patient is wobbly regardless.
Q7. Which diseases target the dorsal columns? (Step 1 tie-ins)
Subacute combined degeneration B12
Dorsal columns + lateral corticospinal + spinocerebellar (demyelination; ↑ methylmalonic acid).
Bilateral vibration/position loss, positive Romberg, spastic paresis, Babinski, ± macrocytic anemia. Pernicious anemia, vegans, metformin, gastrectomy, N₂O abuse.
Tabes dorsalis syphilis
Tertiary neurosyphilis → degeneration of dorsal roots and dorsal columns.
Positive Romberg, stomping gait, lightning (shooting) pains, Argyll Robertson pupils (accommodate, don't react), absent DTRs, Charcot joints.
Friedreich ataxia AR · GAA
GAA repeat in frataxin (FXN, chr 9) → mitochondrial iron overload.
Teen with ataxia, dorsal column + spinocerebellar + corticospinal loss, pes cavus, kyphoscoliosis, diabetes; death from hypertrophic cardiomyopathy.
Copper deficiency
Mimics B12 SCD (myelopathy with dorsal column signs). Think after bariatric surgery or excess zinc intake.
Posterior spinal artery infarct
Rare. Ipsilateral vibration/position loss below the lesion; motor largely spared (see LO 4).
Step 1 HY — classic vignettes
- Older adult, glossitis, macrocytosis, falls in the dark, Babinski → B12 / SCD.
- Man with shooting leg pains, small irregular pupils that accommodate but don't react to light → tabes dorsalis.
- 14-year-old, worsening gait, high arches, scoliosis, abnormal ECG → Friedreich ataxia.
Buzzwords
Step 1 trap
Folate deficiency gives the same megaloblastic anemia as B12 but no dorsal column disease and normal methylmalonic acid. Giving folate alone to a B12-deficient patient fixes the blood and lets the cord keep degenerating.
ApplyLO 2 application questions
A1A lesion destroys the left nucleus cuneatus. What deficit, and which side?
A2A small infarct hits the left medial lemniscus in the pons. Which side loses vibration and position sense?
A3A small lesion cuts only the right VTTT in the upper pons. Does the patient lose touch on the left face?
A4A man with a left L1 dorsal column lesion: what is lost and where?
A5A strict vegan has bilateral vibration loss in the feet, a positive Romberg and up-going toes. Which tracts are damaged?
LO 3Drawing the cord and brainstem cross-sectionsWhere the DCML and trigeminal touch tracts and nuclei sit at each level.
Q1. What does a cervical cord cross-section look like, and where is the DCML?
- DCML lives in the dorsal funiculus, between the dorsal horns: gracilis medial, cuneatus lateral, split by the posterior intermediate sulcus (only present at T6 and above).
- Cervical cord: large, oval, lots of white matter (all ascending fibers have joined), big ventral horns (arm motor neurons), both fasciculi present.
- Lumbar cord: rounder, more gray than white, gracilis only.
- Know how to draw and label: the cervical profile, the dorsal columns (gracilis medial = lower body, cuneatus lateral = upper body), the posterior intermediate sulcus, and the S-L-T-C order (medial to lateral).
CheckWhy is there more white matter at C5 than at L3?
Step 1 HY
Boards show a cord section with one region shaded and ask the deficit. Dorsal = DCML (ipsilateral vibration/position), lateral = corticospinal (ipsilateral UMN) + spinocerebellar, anterolateral = spinothalamic (contralateral pain/temp), ventral horn = LMN.
Q2. How do I recognize each brainstem level, and where is the DCML at each?
| Level | Landmarks to identify it | DCML structures | Trigeminal touch |
|---|---|---|---|
| Closed (caudal) medulla | No 4th ventricle (central canal); pyramids ventral | Nucleus gracilis + cuneatus dorsally (capped by their fasciculi); internal arcuate fibers sweeping ventromedially; start of ML | — (spinal V nucleus here, for pain) |
| Open (rostral) medulla | 4th ventricle on top; inferior olives; pyramids | Medial lemniscus = vertical band beside the midline, just dorsal to the pyramid. Arm dorsal, leg ventral. | — |
| Upper / mid pons | Bulky basilar pons with transverse pontine fibers; middle cerebellar peduncles; 4th ventricle | ML turns horizontal at the tegmentum–basilar border. Arm medial, leg lateral. | Principal (chief) sensory nucleus dorsolateral, lateral to motor V; crossed VTTT joins the ML |
| Midbrain (superior colliculus) | Cerebral aqueduct; superior colliculi; cerebral peduncles | ML moved dorsolaterally in the tegmentum, heading for VPL | VTTT alongside, heading for VPM |
- Closed medulla: no 4th ventricle; GN/CN (nucleus gracilis/cuneatus) beginning to appear; internal arcuate fibers cross.
- Open medulla: olives + pyramids + 4th ventricle.
- Pons: horizontal pontine fibers.
- Midbrain: superior colliculi + cerebral peduncles.
- The DCML moves as it ascends → expect to locate it on each section.
CheckIn which brainstem region do the gracile/cuneate nuclei, the internal arcuate fibers and the start of the medial lemniscus all appear together?
Memory tool
The medial lemniscus is a person standing beside the midline in the medulla (head = arm dorsal, feet = leg ventral), who lies down in the pons (arm medial, leg lateral) and then walks out to the side in the midbrain.
Buzzwords
Step 1 trap
Don't confuse the medial lemniscus (sensory, beside midline, dorsal to pyramid) with the MLF (medial longitudinal fasciculus — eye movements, just under the 4th ventricle floor) or with the pyramid (motor, most ventral).
Q3. Image drill — name the level and the shaded structures.
Open each card after you commit to an answer.
Image 1 · level? green & orange?
Image 2 · level? shaded tract?
Image 3 · level? shaded tract? nucleus?
Image 4 · level? shaded tract?
Image drill · Michigan SecondLook
1 · Locate fasciculus gracilis and cuneatus. (tap for answer)
Upper body (T6 and above) → fasciculus cuneatus, lateral (red).
Both carry uncrossed 1st-order axons that synapse in the nucleus gracilis/cuneatus of the caudal medulla.
ApplyLO 3 application questions
A1A section shows a dorsal column with only one fasciculus and no posterior intermediate sulcus. Which level could it be?
A2On an upper pons section, which pathway leaves the chief sensory nucleus and joins the medial lemniscus?
A3A tiny lesion hits only the lateral tip of the left medial lemniscus in the mid-pons. Predict the deficit.
LO 4Blood supply of the cord and brainstemWhich artery feeds which tracts — and what a blockage takes away.
Q1. How is the spinal cord supplied, and what does each infarct spare?
| Artery | Supplies | Infarct → loses | Spares |
|---|---|---|---|
| Anterior spinal (1, midline) | Anterior 2/3: ventral horns, lateral corticospinal, spinothalamic | Bilateral weakness (LMN at level, UMN below) + bilateral pain/temp loss below; bladder dysfunction | Dorsal columns (vibration, position) |
| Posterior spinal (2) | Posterior 1/3: dorsal columns, dorsal horns | Ipsilateral vibration / position loss below the lesion | Motor largely spared; pain/temp spared |
CheckAfter a thoracoabdominal aneurysm repair, a man can't move his legs or feel pinprick below the umbilicus, but feels a tuning fork on his toes. Which artery?
Step 1 HY — anterior spinal artery syndrome
Sudden bilateral paralysis + loss of pain/temperature, with preserved vibration and proprioception. Classic triggers: aortic aneurysm repair/dissection, aortic cross-clamping, hypotension. Watershed: upper-mid thoracic cord.
Step 1 trap
"Preserved vibration" points away from the dorsal columns. Don't pick B12 deficiency or tabes when the tuning fork is still felt; pick anterior spinal artery / anterior cord syndrome.
Q2. How is the brainstem supplied? (medial vs lateral, level by level)
General pattern: paramedian branches → medial structures (motor tracts, medial lemniscus, motor CN nuclei); circumferential branches → lateral structures (spinothalamic, spinal V, cerebellar peduncles, sympathetics).
| Level | Medial (paramedian) | Lateral |
|---|---|---|
| Midbrain | PCA paramedian branches (+ top of basilar) | PCA circumferential, superior cerebellar |
| Pons | Basilar paramedian branches | AICA (caudal), superior cerebellar (rostral) |
| Rostral (open) medulla | Anterior spinal + vertebral | PICA + vertebral (+ posterior spinal, dorsal) |
| Caudal (closed) medulla | Anterior spinal | Vertebral + posterior spinal (dorsal: gracile/cuneate nuclei) |
- Territory map: rostral medulla = posterior spinal, PICA, vertebral, anterior spinal; caudal medulla = posterior spinal, vertebral, anterior spinal.
- For each territory, predict what is lost and on which side:
- Ascending sensory (medial lemniscus, spinothalamic)
- Descending motor (pyramid)
CheckWhich artery feeds the nucleus gracilis and cuneatus in the closed medulla?
Memory tool — "rule of 4s" (handy shortcut)
- 4 medial structures start with M: Motor pathway (CST), Medial lemniscus, MLF, Motor nucleus of CN (III, IV, VI, XII).
- 4 lateral structures start with S: Spinocerebellar, Spinothalamic, Sensory nucleus of V, Sympathetic pathway.
- Medulla holds CN IX–XII, pons V–VIII, midbrain III–IV.
Q3. What are the classic brainstem stroke syndromes, and which of them touch the DCML?
Medial medullary ASA
Pyramid → contralateral hemiparesis. ML → contralateral vibration/position loss. CN XII → tongue deviates toward lesion.
DCML hit ✔
Lateral medullary (Wallenberg) PICA
Ipsilateral face pain/temp loss + contralateral body pain/temp loss, dysphagia, hoarseness (n. ambiguus), ipsi Horner, vertigo/nystagmus, ipsi limb ataxia.
DCML spared ✘ (ML is medial)
Medial pontine basilar paramedian
Corticospinal → contralateral hemiparesis; ML → contralateral vibration/position loss; ± ipsilateral CN VI (eye can't abduct).
Bilateral basilar → locked-in syndrome.
Lateral pontine AICA
Like Wallenberg plus facial paralysis (CN VII nucleus), hearing loss (VIII), ↓ lacrimation/salivation.
DCML spared.
Weber (medial midbrain) PCA paramedian
Ipsilateral CN III palsy (down and out, ptosis, dilated pupil) + contralateral hemiparesis (cerebral peduncle).
CheckLeft ASA occlusion in the rostral medulla: which way does the tongue point, and which side is weak?
Step 1 HY — medial vs lateral
Medial brainstem stroke = motor + medial lemniscus + medial CN motor nucleus (crossed hemiparesis, "alternating" syndrome). Lateral = pain/temp crossed pattern + Horner + cerebellar + the level's CN, with power and vibration preserved.
Memory tool
"Don't PICK a horse (hoarse) that can't eat (dysphagia)" — nucleus ambiguus signs = PICA. "Facial droop means AICA's pooped." Medial medullary: "lick your wounds" — tongue points toward the lesion.
Step 1 trap
Wallenberg gives sensory loss but the DCML is spared — vibration and position remain normal on both sides. If the vignette has contralateral vibration loss + hemiparesis, it is medial, not lateral.
Buzzwords
ApplyLO 4 application questions
A1Right PICA occlusion. Is the DCML affected? List four expected findings.
A2Paramedian basilar branches occlude in the left pons. Which motor and sensory pathways are hit?
A3A posterior spinal artery infarct on the left at T4. Predict the deficit.
A4Left PCA paramedian occlusion damages the cerebral peduncle and CN III fascicles. Predict the deficits.
LO 5Taste: tongue to cortexThree cranial nerves, one nucleus, one thalamic relay.
Q1. Which nerve carries taste from which part of the mouth?
| Nerve | Taste region | 1st-order cell body (ganglion) |
|---|---|---|
| Facial (VII) — chorda tympani | Anterior 2/3 of tongue | Geniculate |
| Glossopharyngeal (IX) | Posterior 1/3 of tongue | Inferior (petrosal) |
| Vagus (X) | Minor role: soft palate / epiglottis region | Inferior (nodose) |
- Summary: VII → anterior 2/3; IX → posterior 1/3; X → minor role, any taste from soft palate and epiglottis. All gustatory fibers go to the solitary nucleus.
- Taste is carried by special visceral afferents; general sensation (touch, pain) of the anterior 2/3 is V3 (lingual nerve), of the posterior 1/3 is IX.
- Taste buds sit on papillae (fungiform anteriorly, circumvallate at the back); all five tastes are detected across the whole tongue.
CheckAfter middle-ear surgery, a patient loses taste on the front of the right tongue but still feels touch there. Which nerve?
Step 1 trap
- Taste ≠ general sensation: anterior 2/3 taste = VII, touch/pain = V3. Posterior 1/3 = IX for both. Motor to the tongue = XII (palatoglossus = X).
- Boards usually credit X with the epiglottis/pharynx; palate taste is mostly VII (greater petrosal). Pick X when the stem says epiglottis.
- The old "tongue map" (sweet at the tip, bitter at the back) is a myth.
Q2. What is the solitary nucleus, and where does taste go from there?
- The nucleus of the solitary tract (solitary nucleus) is a column in the dorsal medulla, wrapped around the solitary tract — the brainstem's visceral sensory nucleus.
- Organized rostral → caudal:
- Rostral → taste (gustatory nucleus)
- Middle (medial) → respiratory and baroreceptor input
- Caudal → visceral (gut, cardiorespiratory) sensation
- Taste chain: taste receptors → CN VII/IX/X → synapse in solitary nucleus → VPM → sensory cortex (the tract from solitary nucleus to thalamus is not named).
- From the rostral solitary nucleus → VPM of the thalamus (mainly ipsilateral; sometimes written "VP nucleus", the specific relay is VPM) → gustatory cortex (insula / frontal operculum, next to the face area of S1).
CheckWhich part of the solitary nucleus is the "gustatory nucleus," and which thalamic nucleus does it project to?
Step 1 HY
Nucleus solitarius = visceral sensory: taste, baroreceptors (IX carotid sinus, X aortic arch), gut distension. Nucleus ambiguus = motor to pharynx/larynx (IX, X). Dorsal motor nucleus of X = parasympathetic to the viscera. VPM handles face sensation and taste.
Memory tool
"Solitary = Sensory; Ambiguus = Action (motor)." Taste is the "head" of the solitary nucleus (rostral); the gut is the "tail" (caudal).
Buzzwords
ApplyLO 5 application questions
A1A Bell's palsy patient (lesion in the facial canal above the chorda tympani branch) reports food tastes "metallic and dull" on one side. Which region of the tongue is affected, and is general sensation lost?
A2A lesion of the left VPM. Predict two sensory deficits.
A3A small infarct destroys only the caudal part of the right solitary nucleus. Is taste affected?
MCQPractice questionsClick an answer to see the explanation.
1A 68-year-old woman has 6 months of tingling feet and unsteadiness that is worse in the dark. Vibration and joint-position sense are absent at the toes bilaterally. She stands steadily with eyes open but sways and nearly falls with eyes closed. Knee reflexes are brisk and plantar responses are extensor. Hemoglobin is 9.8 g/dL with MCV 112 fL. Which is the most likely diagnosis?
2A 60-year-old man has sudden loss of vibration and position sense in the right arm and right leg. Pain and temperature are normal. Strength is normal, and the face is unaffected. Where is the single lesion most likely located?
3A researcher records from a cutaneous receptor in the deep dermis of the palm. The receptor responds briskly to a 250-Hz vibration and at the onset and offset of steady pressure, but is silent while the pressure is held. Histology shows concentric lamellae around a single nerve ending. Which receptor is this?
4Nerve conduction studies of the sural nerve show a sensory conduction velocity of 48 m/s in a healthy volunteer. The fibers generating this response most likely carry which information, by which axon class?
5A 25-year-old man is stabbed in the back. He has weakness of the right leg with a right Babinski sign. Which additional sensory findings are expected if the lesion is a right hemisection of the cord at T10?
6A 72-year-old man undergoes open repair of a thoracoabdominal aortic aneurysm. On waking he cannot move either leg and has lost pinprick and temperature sensation below the umbilicus bilaterally. Which modality is most likely preserved in his legs?
7A 66-year-old woman suddenly develops right arm and leg weakness and loss of vibration and position sense on the right side. On protrusion, her tongue deviates to the left. Facial sensation and pain/temperature sensation are normal. Occlusion of which vessel best explains this?
8Second-order neurons in the principal sensory nucleus of the trigeminal nerve send most of their axons across the midline in the ventral trigeminothalamic tract. In which thalamic nucleus do these axons terminate?
9After surgery for chronic otitis media on the left, a 40-year-old woman notes that food tastes bland on the front of the left side of her tongue. Light touch and pinprick on the same area are normal, and facial movements are symmetric. Which structure was most likely injured?
10The central processes of taste fibers from the posterior third of the tongue make their first synapse in which structure?
11A 55-year-old man has episodes of sudden stabbing leg pain, a wide-based stomping gait and a positive Romberg sign. Knee and ankle reflexes are absent. His pupils are small and irregular; they constrict on near focus but not to light. Degeneration of which structures explains his gait?
12A 14-year-old girl has 2 years of progressive clumsiness and falls. Exam shows absent ankle reflexes, loss of vibration sense in the feet, bilateral Babinski signs, high-arched feet and kyphoscoliosis. ECG shows left ventricular hypertrophy. Which genetic mechanism is most likely?
13A 70-year-old man has sudden numbness and weakness of the left foot and leg. His left arm and face are normal. Two-point discrimination on the left sole is impaired. Infarction in the territory of which artery best explains this?