Spinal Cord Tracts & Sensory Pathways
Anatomy · Neuroanatomy · lean revision notes
Spinal Cord Tracts & Sensory Pathways
The spinal cord is the great information highway between the body and the brain. Mastering the three sensory systems (dorsal column–medial lemniscus, spinothalamic, spinocerebellar) and the major motor tracts lets you localise any lesion with confidence — the single most repeated skill in NEET PG neuroanatomy and neurology MCQs.
Orientation: white matter organisation
In cross-section, the spinal cord has a central H-shaped grey matter surrounded by white matter divided into three funiculi (columns):
- Dorsal (posterior) funiculus — carries the dorsal column system (fine touch, proprioception, vibration).
- Lateral funiculus — carries the lateral corticospinal tract, lateral spinothalamic tract, and spinocerebellar tracts.
- Ventral (anterior) funiculus — carries the anterior corticospinal tract and anterior spinothalamic tract.
A tract name usually encodes its origin and destination: spinothalamic = spinal cord → thalamus; corticospinal = cortex → spinal cord; spinocerebellar = spinal cord → cerebellum.
High-yield: Ascending = sensory (carry information up to brain); descending = motor (carry commands down). The exception traps you on dissociated sensory loss — know which modality travels in which column.
The three-neuron sensory plan
Almost every conscious somatic sensory pathway uses three neurons before reaching the cortex:
- First-order neuron — cell body in the dorsal root ganglion (DRG); peripheral process to receptor, central process into cord.
- Second-order neuron — cell body in cord or medulla; its axon decussates (crosses) and ascends.
- Third-order neuron — cell body in the thalamus (VPL nucleus); projects to the postcentral gyrus (primary somatosensory cortex, areas 3, 1, 2).
High-yield: For the body, the relay thalamic nucleus is VPL (ventral posterolateral); for the face it is VPM (ventral posteromedial). Mnemonic: Lateral for Limbs/body, Medial for the Mug (face).
The key examinable difference between the two main conscious pathways is where the second-order axon crosses.
Dorsal column–medial lemniscal (DCML) pathway
Carries fine (discriminative) touch, vibration, conscious proprioception, two-point discrimination, and stereognosis.
Flow: Receptor → DRG (1st order) → ascends ipsilaterally in dorsal column → synapse in nucleus gracilis/cuneatus (lower medulla) → 2nd-order axon = internal arcuate fibres → decussates in the medulla (sensory decussation) → forms the medial lemniscus → VPL of thalamus (3rd order) → internal capsule (posterior limb) → postcentral gyrus.
The dorsal column is somatotopically organised into two fasciculi:
| Fasciculus | Body region carried | Position in dorsal column | Relay nucleus |
|---|---|---|---|
| Fasciculus gracilis | Lower limb & lower trunk (below T6) | Medial | Nucleus gracilis |
| Fasciculus cuneatus | Upper limb & upper trunk (above T6) | Lateral | Nucleus cuneatus |
High-yield: "Gracilis is graceful and central/medial; cuneatus is the cute one lateral." Gracilis appears at all spinal levels; cuneatus appears only above T6 (it carries the upper limb).
High-yield: Decussation of the DCML pathway is in the MEDULLA (internal arcuate fibres). This is the most-tested decussation level. A lesion below the crossover causes ipsilateral loss; a lesion above causes contralateral loss.
Spinothalamic / anterolateral system
Carries pain, temperature, crude (non-discriminative) touch, and pressure.
Flow: Receptor → DRG (1st order) → fibres enter and may travel up/down 1–2 segments in the tract of Lissauer → synapse in dorsal horn (substantia gelatinosa / nucleus proprius) → 2nd-order axon decussates in the anterior white commissure within 1–2 segments → ascends as the spinothalamic tract → VPL of thalamus (3rd order) → postcentral gyrus.
| Feature | Lateral spinothalamic | Anterior spinothalamic |
|---|---|---|
| Modality | Pain & temperature | Crude touch & pressure |
| Funiculus | Lateral | Anterior |
| Clinical importance | Cordotomy target; syringomyelia | Often spared/overlaps DCML |
High-yield: The spinothalamic pathway crosses at or near the level of entry (within 1–2 segments, in the cord itself) — NOT in the medulla. This single fact explains Brown-Séquard and syringomyelia patterns.
High-yield: Because the crossing is within a couple of segments, an anterolateral cordotomy for intractable pain is done a few segments above the desired level of analgesia, and on the opposite side to the pain.
DCML vs spinothalamic — the master comparison
| Parameter | DCML (dorsal column) | Spinothalamic (anterolateral) |
|---|---|---|
| Modalities | Fine touch, vibration, proprioception, 2-point, stereognosis | Pain, temperature, crude touch |
| 1st-order synapse | Medulla (gracile/cuneate nuclei) | Dorsal horn |
| Level of decussation | Medulla (internal arcuate fibres) | Spinal cord (anterior white commissure, 1–2 segments up) |
| Position in cord | Dorsal column | Lateral/anterior funiculus |
| Fibre size/myelination | Large, heavily myelinated (fast) | Small, thinly/un-myelinated (slow) |
| Thalamic relay | VPL | VPL |
| Lesion below crossover | Ipsilateral loss | (crosses immediately, so) contralateral loss starting ~2 segments below |
Spinocerebellar tracts (unconscious proprioception)
These carry proprioceptive information to the cerebellum for coordination — they do not reach consciousness and do not relay in the thalamus.
| Tract | Origin (nucleus) | Crossing | Side reaching cerebellum | Entry to cerebellum |
|---|---|---|---|---|
| Dorsal (posterior) spinocerebellar | Nucleus dorsalis (Clarke's column, C8–L2/3) | Uncrossed | Ipsilateral | Inferior cerebellar peduncle |
| Ventral (anterior) spinocerebellar | Border cells / spinal border neurons | Crosses twice (in cord and in cerebellum) | Effectively ipsilateral | Superior cerebellar peduncle |
| Cuneocerebellar | Accessory cuneate nucleus | Uncrossed | Ipsilateral (upper limb) | Inferior cerebellar peduncle |
High-yield: Dorsal spinocerebellar tract arises from Clarke's column (nucleus dorsalis), which exists only between C8 and L2–L3. For levels below L2/L3, lower-limb proprioception is carried up the dorsal column to relay at L2/3. The ventral spinocerebellar tract crosses twice — so net function is ipsilateral.
Major descending (motor) tracts — quick frame
| Tract | Function | Decussation |
|---|---|---|
| Lateral corticospinal | Voluntary movement, distal limbs | Pyramidal decussation in lower medulla (~90% of fibres) |
| Anterior (ventral) corticospinal | Axial/trunk muscles | Uncrossed in medulla; crosses at segmental level |
| Rubrospinal | Flexor tone (upper limb) | Crosses in midbrain (ventral tegmental decussation) |
| Vestibulospinal | Extensor tone, posture | Mostly uncrossed |
| Reticulospinal | Automatic posture, locomotion | Mixed |
| Tectospinal | Reflex head turning to stimuli | Crosses (dorsal tegmental decussation) |
High-yield: Two decussations in the medulla to never confuse — sensory decussation (DCML, upper/rostral medulla, internal arcuate fibres) and motor/pyramidal decussation (corticospinal, lower/caudal medulla).
Clinical syndromes — the heart of the exam
Brown-Séquard syndrome (hemisection of cord)
Caused by hemisection (trauma, stab, tumour, MS). At and below the lesion you get a characteristic split:
Flow of reasoning: DCML crosses in medulla (above the lesion) → its loss is ipsilateral. Spinothalamic crosses in the cord (below the lesion) → its loss is contralateral. Corticospinal crosses above (medulla) → weakness is ipsilateral.
- Ipsilateral below lesion: loss of fine touch/vibration/proprioception (dorsal column) + UMN weakness (corticospinal) + ipsilateral LMN signs at the segment.
- Contralateral below lesion (starting ~1–2 segments below): loss of pain & temperature (spinothalamic).
- A narrow band of ipsilateral complete anaesthesia at the level of the lesion.
High-yield: Brown-Séquard = ipsilateral motor + ipsilateral proprioception loss, contralateral pain/temperature loss. Best prognosis of all incomplete cord syndromes.
Syringomyelia (dissociated sensory loss)
A central cavity (syrinx), classically in the cervical cord, expands and first destroys the decussating spinothalamic fibres crossing in the anterior white commissure.
- Earliest sign: bilateral, segmental loss of pain & temperature in a cape/shawl distribution (shoulders, upper limbs).
- Dorsal columns preserved → fine touch, vibration, proprioception intact = dissociated sensory loss.
- Progression: anterior horn involvement → LMN wasting of small hand muscles; lateral horn → Horner's syndrome; corticospinal → spastic legs.
- Associations: Arnold-Chiari I malformation, post-traumatic, intramedullary tumour.
High-yield: "Dissociated sensory loss" = pain/temperature lost, touch/proprioception preserved (or vice versa). Cape-like, suspended, bilateral loss of pain/temperature = syringomyelia until proven otherwise.
Tabes dorsalis (neurosyphilis)
Tertiary syphilis degenerates the dorsal columns and dorsal roots.
- Loss of vibration & proprioception → sensory (stamping) ataxia, positive Romberg sign.
- Lightning (lancinating) pains, Argyll Robertson pupil (accommodates but does not react to light), areflexia, Charcot joints, bladder involvement.
High-yield: Romberg positive = proprioceptive (dorsal column) lesion, because vision compensates with eyes open and fails on closing. Cerebellar ataxia is Romberg-negative.
Subacute combined degeneration (SACD)
Vitamin B12 deficiency damages dorsal columns + lateral corticospinal tracts (+ spinocerebellar).
- Loss of vibration/proprioception (dorsal column) + spastic paraparesis with extensor plantars (corticospinal).
- Brisk knee jerk + absent ankle jerk + upgoing toes is the classic mixed UMN/LMN picture (peripheral neuropathy added).
Anterior spinal artery syndrome
Infarct of the anterior two-thirds of the cord (spinothalamic + corticospinal + anterior horn) with dorsal columns spared (posterior spinal arteries intact).
- Bilateral loss of pain & temperature + motor paralysis below lesion; vibration/proprioception preserved. The mirror image of tabes/SACD.
Localising-by-modality summary
| Syndrome | Pain/Temp | Vibration/Proprioception | Motor |
|---|---|---|---|
| Brown-Séquard | Lost contralateral | Lost ipsilateral | Lost ipsilateral |
| Syringomyelia | Lost bilateral, segmental (cape) | Preserved | Late LMN at level |
| Tabes dorsalis | Preserved (mostly) | Lost | Preserved |
| SACD (B12) | Preserved | Lost | Spastic (UMN) |
| Anterior spinal artery | Lost bilateral | Preserved | Lost bilateral |
Investigation of choice
- MRI of the spine is the single best investigation for nearly all cord pathology (syrinx, compression, demyelination, tumour). MRI shows the syrinx cavity, Chiari malformation, and cord signal change.
- Tabes dorsalis: serology — VDRL/RPR + treponemal tests (TPHA/FTA-ABS); CSF VDRL for neurosyphilis.
- SACD: serum vitamin B12, methylmalonic acid and homocysteine (raised); MRI shows "inverted V" / dorsal column T2 hyperintensity.
- Demyelination (MS): MRI brain + cord, CSF oligoclonal bands.
Management / drug of choice
- SACD: parenteral hydroxocobalamin (vitamin B12) — start promptly; neurological recovery depends on early treatment.
- Neurosyphilis (tabes): IV crystalline penicillin G (aqueous) is the drug of choice.
- Syringomyelia: treat the cause — posterior fossa decompression for Chiari; syringo-subarachnoid shunting if progressive.
- Brown-Séquard / compression: surgical decompression, treat underlying cause; steroids if traumatic per protocol.
- Intractable cancer pain: anterolateral cordotomy (lesion contralateral spinothalamic tract, a few segments above the pain).
Complications
- Untreated cord compression → permanent paraplegia, neurogenic bladder/bowel, pressure sores.
- Syringomyelia → painless burns/injuries (loss of pain sensation), neuropathic (Charcot) joints, progressive hand wasting.
- Tabes → Charcot joints, perforating foot ulcers, optic atrophy, visceral crises.
- Autonomic dysreflexia in high cord lesions.
Key differentials
- Sensory ataxia (dorsal column) vs cerebellar ataxia — Romberg distinguishes (positive in dorsal column lesion).
- Syringomyelia vs intramedullary tumour vs anterior spinal artery infarct — MRI + onset (acute vs slowly progressive) + dissociated pattern.
- SACD vs MS vs tabes — all give dorsal column signs; use B12 levels, MRI lesions/oligoclonal bands, and serology.
- Cape sensory loss: syringomyelia vs intramedullary glioma/ependymoma.
Recently asked / exam angle
- Level of decussation questions: "Where do dorsal column fibres cross?" → medulla (internal arcuate fibres); "Where does the spinothalamic tract cross?" → spinal cord, anterior white commissure, 1–2 segments above entry.
- VPL vs VPM: which thalamic nucleus relays body vs face.
- Clarke's column extent (C8–L2/3) and which tract originates there (dorsal spinocerebellar).
- Dissociated sensory loss image/clinical vignette → identify syringomyelia and its cape distribution.
- Brown-Séquard vignette (knife stab) asking which deficit is ipsilateral vs contralateral.
- Argyll Robertson pupil linked to tabes dorsalis; Romberg sign localisation.
- SACD MRI ("inverted V sign") and which tracts are involved.
- Fasciculus gracilis vs cuneatus: medial/lateral position and body part carried.
- Which tract crosses twice → ventral spinocerebellar.
- Anterior spinal artery syndrome: which modality is spared (dorsal column / proprioception).
Rapid revision
- DCML decussates in the medulla; spinothalamic decussates in the spinal cord (anterior white commissure, 1–2 segments up).
- Body sensation relays in VPL, face in VPM of the thalamus; cortex = postcentral gyrus.
- Fasciculus gracilis = medial = lower limb; cuneatus = lateral = upper limb (above T6).
- Spinothalamic carries pain, temperature, crude touch; dorsal column carries vibration, proprioception, fine touch.
- Brown-Séquard: ipsilateral motor + proprioception loss, contralateral pain/temperature loss.
- Syringomyelia: bilateral cape-like loss of pain/temperature with preserved touch = dissociated sensory loss; linked to Chiari I.
- Tabes dorsalis: dorsal column degeneration, Romberg positive, Argyll Robertson pupil, treated with IV penicillin G.
- SACD (B12): dorsal columns + corticospinal tracts; treat with B12; brisk knees, absent ankles, upgoing toes.
- Anterior spinal artery syndrome: pain/temp + motor lost, proprioception spared.
- Dorsal spinocerebellar tract arises from Clarke's column (C8–L2/3); ventral spinocerebellar tract crosses twice (net ipsilateral).
- Pyramidal (motor) decussation = lower medulla; sensory decussation = upper medulla.
- Investigation of choice for cord lesions = MRI spine; cordotomy lesions the contralateral spinothalamic tract for cancer pain.