This study will provide information about changes that occur in the motor neurons of the spinal cord (the nerve cells that control the muscles) when the motor cortex (the region of the brain that controls movement) is unable to send messages to the spinal cord and muscles in the normal way. This information will help elucidate how the nervous system adapts after injury or disease of the motor cortex.

Healthy adult volunteers and adults with a spasticity disorder and moderate weakness may be eligible for this study. Patients will be screened with a medical history, physical examination and diagnostic studies as needed. Healthy volunteers will have a neurological examination. Muscle weakness and spasticity will be evaluated in both groups of subjects.

All participants will have electromyography (measurement of electrical activity in muscles) during nerve stimulation and transcranial magnetic stimulation, described below. (Some patients, such as those with a pacemaker or implanted medication pumps, metal objects in the eye, history of epilepsy and others, will not have magnetic stimulation.)

Electromyography - The electrical activity of muscles will be measured either by 1) using metal electrodes taped to the skin overlying a muscle, or 2) using thin wires inserted into the muscle through a needle.

Nerve stimulation - The nerves will be stimulated by applying a small electrical pulse through metal disks on the skin of the arm or leg.

Transcranial magnetic stimulation - A brief electrical current is passed through a wire coil placed on the scalp. This creates a magnetic pulse, which stimulates the brain. During the test, the participant may be asked to tense certain muscles slightly or perform other simple actions.

Nerve block - Some patients will have a nerve block of one of the nerves in the arm. For this procedure, a local anesthetic is injected under the skin to produce numbness and weakness in some arm muscles.