A 35-year-old man presents with intense nocturnal pruritus affecting his hands, wrists, and genitalia for the past 6 weeks. On examination, thread-like, serpentine gray-white tunnels 3–8 mm in length are noted in the interdigital webs, with a tiny vesicle at one end. The structure marked **A** in the diagram represents this characteristic lesion. Which of the following best describes the pathophysiology of pruritus in this condition?