Muscle-controlling nerve cells, or motor neurons, are divided into two types: upper and lower. The upper motor neurons are located on the surface of the brain and exert control over the lower motor neurons, which are in the brainstem and the spinal cord.
The lower motor neurons are directly attached to muscles through “wires” called axons. Bundles of these axons leave the spinal cord and extend out to the muscles. It’s these 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 in ALS, muscles atrophy (shrink) and become progressively weaker and eventually unable to contract, resulting in paralysis.
The lower motor neurons that control most of the muscles in the body are in the spinal cord. Those that control the muscles of speaking, swallowing and facial expression are in the brainstem. They’re sometimes called 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 affected by the disease.
The upper motor neurons have more complex functions. It’s harder to study them, and not as much is understood about them, although new techniques are changing that.
These cells seem to exert complex control over the lower motor neurons. This control allows movements to be smooth, directed and varied in intensity. (For instance, they’re part of an elaborate system that allows a person to aim a hand at a glass of water, estimate its weight, pick it up, and use the right amount of force to lift it to his or her mouth, all while thinking about something else.) When upper motor neurons are lost
and lower motor neurons remain, movements are still possible but can become tight (spastic) and less precise.
In ALS, a combination of these effects is usually seen because both upper and lower motor neurons are dying. People with ALS can have weak and atrophied muscles with tightness (spasticity). Muscle twitches (called “fasciculations”) and cramps are common; they occur because degenerating nerves become irritable.
My ALS started in my arms and is now present in all of my muscles. I have obvious signs of atrophy, have extreme spasticity and struggle every day with the effects of the loss of my upper and lower motor neurons.
NOTE: 2017 DATES