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WHAT HAPPENS TO THE NERVOUS SYSTEM IN MND?
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)
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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 |
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In MND,
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 MND, muscles
become progressively weaker and eventually unable to move (paralysed).
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. People with MND
can have weak and wasted muscles with tightness (spasticity). Muscle
twitches and cramps are common; they occur because degenerating axons
(nerves) become "irritable."
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