As noted above, PLS is caused by degeneration of the upper motor neurons in the brain and spinal cord. Upper motor neurons control voluntary movement.
The cell bodies of these neurons are located in the motor cortex area of the brain. They have long, hair-like processes called axons that travel to the brainstem and down the spinal cord. Axons relay the messages to move to lower motor neurons that are located all along the brainstem and spinal cord. Lower motor neurons then carry the messages out to the muscles. Click on the diagram at right to see how upper motor neurons connect to lower motor neurons that innervate leg muscles.
When upper motor neurons degenerate, impulses cannot adequately reach the lower motor neurons and the lower motor neurons cannot deliver the proper message to the corresponding muscle. The result is muscle weakness and spasticity. As the degeneration progresses, symptoms increase.
In PLS, the degenerative process impacts the length of the spinal cord, thus affecting the legs, arms and speech and swallowing muscles. For most patients, the process begins in the legs and then travels up the spinal cord. In some people, the process begins in the brainstem, affecting the speech and swallowing muscles first, and then descends down the spinal cord.