halfin
Senior member
- Joined
- Jun 29, 2009
- Messages
- 540
- Reason
- PALS
- Diagnosis
- 08/2009
- Country
- US
- State
- CA
- City
- Santa Barbara
Re: Rhythman Update
One comment on EMG, weakness and atrophy. ALS is a disease of the motor nerves that carry signals to muscles. One of the first things that happens as the nerve cells weaken is that they withdraw connections from the muscle fibers. However initially this does not cause weakness, because other nerve cells sprout new branches at the end (similar to how a tree limb can sprout new branches) and these re-connect to the muscle fibers. This process is called denervation and re-innervation. It actually shows up on EMG even before muscle weakness occurs.
Later as the disease gets worse the re-innervation process cannot keep up with denervation. Too many nerve cells are dying and there is a limit to how many new branches the remaining ones can send out. At this point some of the muscle fibers lose connections and can no longer contract. This is when weakness first begins. However many people do not notice weakness even in this stage, for a few reasons. First, we never use the full strength of our muscles. Always we have some fibers firing while others rest. So losing a few percent of muscle fibers still lets them work at full strength, just using a greater percentage of their total capacity. Also, even when this gets worse and a substantial part of the muscle fibers are not working, even then people may unconsciously switch to using other muscles to compensate. They may change their posture, gait, or patterns of movement without realizing it, to spare their weakened muscles. I have read that often as much as 50% of the muscle fibers will have lost nerve connections before people notice.
Finally atrophy happens when the muscle fibers are not being used. The body automatically sends resources where they are needed. A muscle which is used hard will get more resources and will grow. A muscle which is not being used will be broken down and the components sent elsewhere in the body, or just metabolized for energy. The rate varies from person to person and also with nutritional status. This is one reason people are advised to keep their weight up; if you are losing weight your body will be more aggressive about eliminating unused muscle.
This is why in ALS you typically see first a bad EMG, then weakness, then atrophy. If you have weakness and/or atrophy with a clean EMG then that would not be caused by ALS. (Well actually as Wright says, upper motor neuron failure can also cause some weakness, but that produces other symptoms that are very noticeable.) Note that other things can cause atrophy, including simple disuse of the muscle. If it is sore or feels bad and you are not using it, it is natural to have atrophy, especially if you are not eating well. A body builder at the gym gets big, but if he stops his workouts and supplements his muscles will shrink, and that is atrophy.
One comment on EMG, weakness and atrophy. ALS is a disease of the motor nerves that carry signals to muscles. One of the first things that happens as the nerve cells weaken is that they withdraw connections from the muscle fibers. However initially this does not cause weakness, because other nerve cells sprout new branches at the end (similar to how a tree limb can sprout new branches) and these re-connect to the muscle fibers. This process is called denervation and re-innervation. It actually shows up on EMG even before muscle weakness occurs.
Later as the disease gets worse the re-innervation process cannot keep up with denervation. Too many nerve cells are dying and there is a limit to how many new branches the remaining ones can send out. At this point some of the muscle fibers lose connections and can no longer contract. This is when weakness first begins. However many people do not notice weakness even in this stage, for a few reasons. First, we never use the full strength of our muscles. Always we have some fibers firing while others rest. So losing a few percent of muscle fibers still lets them work at full strength, just using a greater percentage of their total capacity. Also, even when this gets worse and a substantial part of the muscle fibers are not working, even then people may unconsciously switch to using other muscles to compensate. They may change their posture, gait, or patterns of movement without realizing it, to spare their weakened muscles. I have read that often as much as 50% of the muscle fibers will have lost nerve connections before people notice.
Finally atrophy happens when the muscle fibers are not being used. The body automatically sends resources where they are needed. A muscle which is used hard will get more resources and will grow. A muscle which is not being used will be broken down and the components sent elsewhere in the body, or just metabolized for energy. The rate varies from person to person and also with nutritional status. This is one reason people are advised to keep their weight up; if you are losing weight your body will be more aggressive about eliminating unused muscle.
This is why in ALS you typically see first a bad EMG, then weakness, then atrophy. If you have weakness and/or atrophy with a clean EMG then that would not be caused by ALS. (Well actually as Wright says, upper motor neuron failure can also cause some weakness, but that produces other symptoms that are very noticeable.) Note that other things can cause atrophy, including simple disuse of the muscle. If it is sore or feels bad and you are not using it, it is natural to have atrophy, especially if you are not eating well. A body builder at the gym gets big, but if he stops his workouts and supplements his muscles will shrink, and that is atrophy.