The progression of weakness and muscle wasting can follow several different patterns. It may begin, for example, with difficulty of fine finger movements, such as handling keys or buttons, and progress to affect muscles of the hand, arm, shoulder, and legs. Later, muscles that control swallowing and respiration become involved. Regardless of how it begins, ALS is relentless in its progression. The symptoms of ALS reflect the death of motor neurons, nerve cells in the brain and spinal cord that innervate muscles and cause them to contract, making movement possible.
Motor neurons are remarkable machines.
Their cell bodies, which hold the genetic blueprints and manufacture most cell components, lie in the brain and spinal cord, but each one extends a long thin fiber called an axon that projects far to connect precisely to muscle cells. Although the axons are microscopic in diameter, the total volume of a motor neuron can be 5000 times that of a typical cell because the axons are so long. The large size of motor neurons, their high energy requirements, and the extent of axons out of the protected environment of the brain and spinal cord contribute to the vulnerability of these cells. These are nerve cells in the cerebral cortex that activate the motor neurons. The cerebral cortex helps coordinate the planning and initiation of voluntary movement.