I normally explain the spine like a motorway with two carriageways (north and south) with exits at each vertebrae. All the traffic (i.e. signals) heading north is heading for the same destination: the brain. The traffic heading south is going to different places so is taking different exits (the spinal nerves, at each vertebrae). A lesion is like an accident on one carriageway or, if it's a bad accident, both carriageways. Bad accidents tend to cause permanent paralysis, so let's assume MS lesions are a one-carriageway accident. When there's an accident on one carriageway, all the traffic travelling in that direction is affected. If it's going north, then everything is affected because they are all heading for the end of the motorway and they can't get off and find another route - so traffic gets stuck and/or goes more slowly through the jam (in other words, all the signals coming from below the lesion can be affected). If it's going south, then only the traffic that is still on the motorway at that point is affected, because the other cars (signals) have already reached their exits. Of course, you may get rubberneckers too - signals on the other side of the motorway may go slow too because of the chaos on the side with the accident.
MS "accidents" can happen anywhere in the central nervous system. They are more common in the cervical spinal cord and the brain, but they can occur just in the thoracic spinal cord and can also occur in the lumbar spinal cord. My first lesion was in the thoracic cord - my brain and the rest of my spine were clear, although MRI was pretty basic in those days.