Final answer:
Nerve signals from the midthoracic and lower body are carried by the dorsal column system for touch and proprioception, and by the spinothalamic tract for pain and temperature, each crossing the midline at different points in the central nervous system.
Step-by-step explanation:
The ascending tracts that carry nerve signals from the midthoracic and lower parts of the body are parts of the dorsal column system and spinothalamic tract. The dorsal column system is primarily responsible for carrying information about touch and proprioception, and it crosses the midline in the medulla. On the other hand, the spinothalamic tract carries signals regarding pain and temperature sensation and decussates, or crosses over to the other side, at the spinal cord level at which it enters.