A 60-year-old man develops flaccid paralysis of the right upper limb after a poliomyelitis infection. Weeks later, spinal cord sections show enlarged, eccentrically placed neuronal nuclei, dispersed Nissl substance toward the periphery of the soma, and central pallor of the cytoplasm. This change, called chromatolysis, begins at which specific region?
- A Axon terminal, where synaptic vesicles accumulate
- B Dendritic origin, where Nissl bodies are densest
- C Axon hillock, where Nissl substance is normally absent ✓
- D Node of Ranvier, where sodium channels cluster
Correct answer: C. Axon hillock, where Nissl substance is normally absent
Explanation
Chromatolysis is the response of the neuronal soma to axonal injury: the nucleus moves eccentrically, ribosomes detach from rough ER and disperse, and cytoplasmic basophilia is lost, starting at the axon hillock, the cone-shaped origin of the axon where Nissl substance is physiologically absent even in health. Dendrites do carry Nissl substance, unlike the axon and hillock. The finding indicates failure of the protein-synthesising machinery after loss of target-derived trophic support.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.