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We can divide the muscle-controlling nerve cells, or motor neurons, into two types. The upper motor neurons are located in the upper part of the brain and exert some control over the lower motor neurons in the brainstem and the spinal cord. The lower motor neurons are attached to muscles through wires called axons. Bundles of these axons leave the spinal cord and extend out to the muscles all over the body. (These are the bundles that doctors are referring to when they talk about the nerves.) The function of lower motor neurons is straightforward; they send “go” signals to muscles. When these cells gradually die, as is the case with ALS, muscles become progressively weaker and eventually unable to move. This causes paralysis in patients.
The lower motor neurons that control most of the body are in the spinal cord, and those that control the muscles of speaking, swallowing, and facial expression are in the brainstem. Another name for these is bulbar motor neurons, because the part of the brainstem that houses them has a bulblike shape. The term “bulbar involvement” means that the muscles of the face, mouth and throat are involved in the ALS disease.
The upper motor neurons have more complex functions. It is harder to study them, and there is much, we don’t understand about them, although some new techniques are gradually changing that. These cells seem to exert complex control over the lower motor neurons that allow movements to be smooth. When upper motor neurons are lost and lower motor neurons remain, movements are still possible. However, these movements can become spastic. Beside this symptom, affected muscles became less precise. In ALS, a combination of these effects is usually present, because both upper and lower motor neurons are dying. People with ALS can have weak and wasted muscles combined with tightness. Muscle twitches and cramps are common, and they occur because degenerating axons of the nerves become irritable.