Q. You are working in the emergency room. A 36 year-old-man presents with numbness from the chest downward, including the hands, legs, and feet. You perform a neurological examination on the patient and discover that on sensory examination of the hands and feet, he has a loss of proprioception and joint position sensation (when you ask him to detect joint movement he cannot feel even when his big toe is pulled upward several centimeters), and a loss of vibration sensation (he cannot feel the tuning fork vibration) but he has normal pin-prick sensation and cold temperature sensation. In addition, his muscle stretch reflexes are hyperactive (brisk) and he has evidence of pathologic reflexes (the presence of Babinski signs in both feet). How can you explain the distinction in the patient's sensory loss?