Good description of how motor neurons degenerate from MDA website

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WHAT HAPPENS TO THE NERVOUS SYSTEM IN ALS?

The muscle-controlling nerve cells, or motor neurons, are divided

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, which are in the brainstem and the spinal cord. (See

illustration)

http://www.mdausa.org/publications/fa-als.html

1. arm muscle

2. tongue

3. UPPER MOTOR NEURONS

4. BRAINSTEM (BULBAR) LOWER MOTOR NEURONS

5. axon bundles (nerves)

6. rib muscles involved in breathing

7. SPINAL LOWER MOTOR NEURONS

8. leg muscle

In ALS, upper and lower motor neurons degenerate. Upper motor

neurons normally send signals to lower motor neurons, which send

signals to muscles.

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, as in

ALS, muscles become progressively weaker and eventually unable to

move (paralyzed).

The lower motor neurons that control most of 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 involved in 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 that allow 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, pick it up,

estimate its weight, use the right amount of force for its weight,

and 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 " (doctors

use the word spastic for this) 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 wasted muscles with tightness (spasticity). Muscle twitches

and cramps are common; they occur because degenerating axons

(nerves) become " irritable. "