Your suspicion is well-founded, though less-well understood and investigated. Both MS and ALS primarily affect the nervous system, the former through demyelination of the nerve fibers, the latter degenerartes the motor neurons that control muscle movements. Since proper signalling of muscle cells is no longer possible in the affected neuronal pathways, the muscle atrophy resulting from disuse may well promote and exacerbate muscle cell senescence. Not only there, though, but also in the nerve cells themselves. That's why research is increasingly investigating how senolytics may help stop or even reverse these processes. I'm not aware of studies that particulalry focus on the cognitive effects, though.