SPINAL CORD CROSS-SECTION

Three tracts destroyed in SACD — shade posterior + lateral on exam day
Gracilis (legs) Cuneatus (arms) DORSAL COLUMNS vibration · proprioception · fine touch LATERAL CORTICOSPINAL voluntary motor (UMN) ANTERIOR / SPINOTHALAMIC pain · temperature — SPARED in SACD SACD = shaded areas POSTERIOR ANTERIOR grey matter PERIPHERAL NERVE sensory (LMN) → absent ankle jerk
Dorsal Columns — vibration, proprioception, fine touch. Loss → positive Romberg, sensory ataxia, stamping gait. Gracilis (medial) = legs. Cuneatus (lateral) = arms.
Lateral Corticospinal — voluntary motor (UMN descending). Loss → spasticity, hyperreflexia, upgoing plantars (Babinski +).
Peripheral Nerve — outside cord. B12 also causes axonal neuropathy → absent ankle jerks (LMN). This creates the SACD paradox.
Anterior/Spinothalamic — pain + temperature. SPARED in SACD. This is why pain sensation is intact.
THE SACD PARADOX: Absent ankle jerks (LMN — peripheral nerve destroyed) + upgoing plantars (UMN — corticospinal tract destroyed) in the same patient. Almost pathognomonic. Write this line = 2 marks.