An 18 year old male comes to the university clinic suppoed by his roommates because he cannot walk. He describes a rapidly evolving weakness affecting his legs and feet staing 2 days ago. On physical examination he cannot move his feet or ankles and he can barely raise his thighs off the bed. He has symmetrical hyporeflexia of the legs, but his sensorium is completely intact. Scanning his cha, the physician notes that he was treated 10 days previously for an upper respiratory tract infection. The immunological response producing the patient's symptoms is most intense at which of the following locations?