How ALS affects the body

ALS affects the body’s motor neurons (nerve cells) and causes voluntary muscles to atrophy (deteriorate and weaken). Voluntary muscles are muscles that are under an individual’s conscious control. These muscles include the ones in the arms (e.g. biceps, triceps) and the legs (e.g. quadriceps, calves). The muscles of the face, neck and tongue are also voluntary muscles that allow you to eat, support and move your head, and make facial expressions.

Muscles that are not under a person’s concious control, such as the heart muscles, are called involuntary muscles.

ALS does not typically affect the five senses: sight, hearing, taste, smell and touch. It also does not usually affect the heart. Recent studies have shown that some people with ALS may experience a reduced ability to smell or detect odors.

The biology of ALS

When you have ALS/MND, your motor neurons start to die. As they die, your brain is no longer able to send the message to your voluntary muscles telling them to move. This means your muscles atrophy. Over time, ALS spreads throughout the body and atrophies all voluntary muscles, eventually weakening muscles required for breathing – which leads to respiratory problems.

The two main groups of motor neurons are the upper and lower motor neurons.

  • The upper motor neurons are located in the brain. They tell your lower motor neurons when and how to move your mucles
  • The lower motor neurons are located in the spinal cord. They send signals directly to your muscles to tell them to move, according to the directions of the upper motor neurons.

Life expectancy

ALS is a disease that decreases life expectancy. There are some treatments and no known cures. As a result, 80 per cent of perople with ALS have a life expectancy of two to five years after diagnosis. How fast a person loses the use of their muscles can vary significantly. Some people deteriorate rapidly, while other people have long periods of very slow degeneration.

Content adapted from the ALS Guide, which was published in 2019 as a joint project of ALS Societies across Canada.