Hi Nancy,
Scientists have made two very broad hypotheses about the location where the ALS starts. To explain it simply: One supposes ALS starts in the motor area in the brain (“dying forward”) and the other tells it starts at the junction between a muscle and the nerves that act on this muscle.
Both hypotheses provide good evidence, there is a problem however as there are several feet between the two locations so one of these hypotheses should be wrong.
The “dying forward” hypothesis is the current mainstream opinion, but it is so only since a few decades. From the publications I infer that roughly 3 out of 4 scientists agree with this hypothesis. A good reference article by the founder of this hypothesis was published recently.
We can see the same kind of debate in Alzheimer and Parkinson diseases. One of my favorite scientists is Heiko Braak. He tells that Parkinson starts in the guts. Other scientists say the same for Alzheimer.
My personal take on this is that the problem is ill defined, we still rely on century ideas old concepts. It was only recently that scientists understood that neurons are not the only kind of cells when new microscopes and imaging techniques revealed new information. Now we begin to understand that cells do not live in isolation, they compete, cooperate and negotiate at the same time. it’s a very fluid situation and today’s conceptual tools in biology are too simplistic to deal with the complexity of biology.
Probably both hypotheses are true … and false!
(A caveat: I am not a scientist)