What Are Motor Neuron Diseases?

Medically Reviewed by Christopher Melinosky, MD on May 12, 2023
Written by WebMD Editorial Contributors
4 min read

When you take a walk, talk to a friend, or chew a piece of food, motor neurons are behind these movements.

Just like other parts of your body, they can become damaged. You may have heard of ALS, commonly known as Lou Gehrig’s disease. It’s one kind of motor neuron disease, and there are several other lesser-known types.

They are a type of nerve cell, and their job is to send messages around your body so you can move. You have two main kinds:

  • Upper motor neurons are in your brain. They send messages from there to your spinal cord.
  • Lower motor neurons are in your spinal cord. They transmit the messages sent from your brain to your muscles.

As nerve cells die when you have a motor neuron disease, electrical messages can't get from your brain to your muscles. Over time, your muscles waste away. You may hear a doctor or nurse call this “atrophy.”

When this happens, you lose control over movements. It gets harder to walk, talk, swallow, and breathe.

Each kind of motor neuron disease affects different types of nerve cells or has a different cause. ALS is the most common of these diseases in adults.

Here's a look at some of the types of motor neuron diseases.

ALS affects both your upper and lower motor neurons. With ALS, you gradually lose control over the muscles that help you walk, talk, chew, swallow, and breathe. Over time, they weaken and waste away. You may also have stiffness and twitches in your muscles.

Most of the time, ALS is what doctors call “sporadic.” This means anyone can get it. Only about 5% to 10% of cases in the United States run in families.

ALS usually starts between the ages of 40 and 60.  Most people with the disease live for 3 to 5 years after their symptoms start, yet some people can live for 10 years or longer.

PLS is similar to ALS, but it affects only upper motor neurons.

It causes weakness and stiffness in the arms and legs, a slowed walk, and poor coordination and balance. Speech also becomes slow and slurred.

Like ALS, it usually starts in people 40 to 60 years old. The muscles get stiffer and weaker over time. But unlike ALS, people don't die from it.

This is a form of ALS. Many people with this condition will eventually develop ALS.

PBP damages motor neurons in the brain stem, which is at the base of your brain.

The stem has motor neurons that help you chew, swallow, and speak. With PBP, you might slur your words and have trouble chewing and swallowing. It also makes it hard to control emotions. You may laugh or cry without meaning to.

This is similar to progressive bulbar palsy. It affects motor neurons that control the ability to talk, chew, and swallow. Pseudobulbar palsy causes people to laugh or cry with no control.

This form is much less common than ALS or PBP. It can be inherited or sporadic. Progressive muscular atrophy mainly affects your lower motor neurons. Weakness usually starts in your hands and then spreads to other parts of the body. Your muscles get weak and may cramp. This disease can turn into ALS.

This is an inherited condition that affects lower motor neurons. A defect in a gene called SMN1 causes spinal muscular atrophy. This gene makes a protein that protects your motor neurons. Without it, they die. This causes weakness in the upper legs and arms, and in the trunk.

SMA comes in different types that are based on when symptoms first appear:

Type 1 (also called Werdnig-Hoffmann disease). It starts around age 6 months. Children with this type can't sit on their own or hold up their head. They have weak muscle tone, poor reflexes, and trouble swallowing and breathing.

Type 2. It starts between 6 and 12 months. Kids with this form can sit, but they can't stand or walk alone. They may also have trouble breathing.

Type 3 (also called Kugelberg-Welander disease). It starts between ages 2 and 17. It affects how a child can walk, run, stand up, and climb stairs. Kids with this type may also have a curved spine or shortened muscles or tendons around their joints.

Type 4. It usually starts after age 30. People with this type may have muscle weakness, shaking, twitching, or breathing problems. It mainly affects muscles in the upper arms and legs.

It’s also inherited, and it affects only males. Females can be carriers but don’t get ill from it. A woman with the Kennedy’s disease gene has a 50% chance of passing it on to a son.

Males with Kennedy's disease have shaking hands, muscle cramps and twitches, and weakness in their face, arms, and legs. They may have trouble swallowing and speaking. Men can have enlarged breasts and a low sperm count.

The outlook is different for each type of motor neuron disease. Some are milder and progress more slowly than others.

Although there is no cure for motor neuron diseases, medicines and therapy can ease symptoms and improve your quality of life.