Moving a muscle usually involves communication between the muscle and the brain through nerves. The impetus to move a muscle may originate with the senses. For example, special nerve endings in the skin (sensory receptors) enable people to determine what something feels like, as when they feel the texture of the fabric or reach in their pocket to find a nickel in an assortment of coins. This information is sent to the brain, and the brain may send a message to the muscle about how to respond.
Also, the amount of muscle tissue must be normal, and the tissue must be able to contract in response to the signal from the nerves. Therefore, true weakness results only when one or more part of this pathway―brain, spinal cord, nerves, muscles, or the connections between them―is damaged or diseased.
Weakness may develop suddenly or gradually. Weakness may affect all of the muscles in the body (called generalized weakness) or only one part of the body. For example, depending on where the spinal cord is damaged, spinal cord disorders may cause weakness only of the legs.
Symptoms depend on which muscles are affected. For example, when weakness affects the muscles of the chest, people may have difficulty breathing. When weakness affects muscles that control the eyes, people may have double vision.
