ALS results in muscles that are reduced in size (atrophic), weak, and soft, or muscles that are stiff, tight, and spastic. Muscle twitches and cramps are common; they occur because degenerating axons (long fibers extending from nerve-cell bodies) become “irritable.” Symptoms may be limited to a single body region, or mild symptoms may affect more than one region. When ALS begins in the bulbar motor neurons, localized in the brainstem, the muscles used for swallowing and speaking are affected first. Rarely, symptoms begin in the respiratory muscles. As ALS progresses, symptoms become more widespread, and some muscles become paralyzed while others are weakened or unaffected. In late-stage ALS, most voluntary muscles are paralyzed. The involuntary muscles, such as those that control the heartbeat, gastrointestinal tract and bowel, bladder, and sexual functions are not directly affected in ALS. Sensations, such as vision, hearing, and touch, are also unaffected. In many cases, ALS does not affect a person’s thinking ability. However, as many as 50 percent of people with ALS develop some degree of cognitive (thinking) or behavioral abnormalities. For more information on ALS symptoms, see Signs and Symptoms and Medical Management.