🧵 Nervous System Lesson

DCML, Spinothalamic, Corticospinal: spinal cord tracts

Three tracts carry different information in different directions, crossing sides of the body at different points — explaining a strange and important clinical pattern.

DCML

Fine touch

Pain/temp

Motor

📖 Full Breakdown

Three tracts, and the critical detail of WHERE each one crosses sides

Where a tract crosses (decussates) from one side of the body to the other determines the clinical pattern seen after a specific injury.

Dorsal Column-Medial Lemniscal (DCML)

Fine touch, vibration, proprioception

Travels up the SAME side (ipsilateral) initially, only crossing to the opposite side once it reaches the medulla.

Spinothalamic tract

Pain and temperature

Crosses to the OPPOSITE side (contralateral) almost immediately, right at the spinal cord level — a very different crossing point from DCML.

Corticospinal tract

Voluntary motor control

Descends from the cortex and crosses at the medulla (an event called pyramidal decussation) — this crossing point is exactly why a LEFT brain lesion causes weakness on the RIGHT side of the body.

🩺 Clinical / Exam Application

A patient has a spinal cord injury on one side that causes loss of pain and temperature sensation on the OPPOSITE side of the body, but loss of fine touch and proprioception on the SAME side as the injury. This seemingly contradictory pattern — sensory loss split across two different sides from a single injury — makes perfect sense once you know the spinothalamic tract crosses immediately at the spinal level while the DCML tract doesn't cross until much higher, at the medulla. This is a classic presentation called Brown-Séquard syndrome, and it's entirely explained by these two tracts' different crossing points.

⚠️ Exam Alert

The corticospinal tract's crossing point at the medulla (pyramidal decussation) is a frequently tested fact specifically because it explains a seemingly counterintuitive clinical pattern: damage to the LEFT side of the brain causes weakness on the RIGHT side of the body, not the same side.

🚧 Common Trap

Don't assume all three tracts cross at the same location. DCML crosses at the medulla, spinothalamic crosses at the spinal cord level, and corticospinal also crosses at the medulla — mixing up spinothalamic's early crossing point with the other two is a common source of confusion.

✅ Quick Check

Why can a single spinal cord injury cause pain/temperature loss on one side of the body but fine touch loss on the opposite side?

📝 Exam Prep

Common Exam Questions

❓ What are the three major spinal cord tracts and what does each carry?

✅ The Dorsal Column-Medial Lemniscal (DCML) tract carries fine touch, vibration, and proprioception. The Spinothalamic tract carries pain and temperature. The Corticospinal tract carries voluntary motor signals.

❓ Why does a left-sided brain lesion cause weakness on the right side of the body?

✅ The corticospinal tract, which carries voluntary motor signals, crosses sides at the medulla (pyramidal decussation) — so a lesion in the left brain affects motor control on the right side of the body, since the signal crosses before reaching the muscles.