This study will use transcranial magnetic stimulation (TMS) to investigate how the brain controls movement by sending messages to the spinal cord and muscles.

Healthy normal volunteers 21 years of age or older may participate in this study.

They must have no medical, neurological or psychiatric illnesses nor have been taking medications that affect nervous system function.

Participants will undergo transcranial magnetic stimulation. For this procedure, an insulated wire coil is placed on the scalp or skin. A brief electrical current is passed through the coil, creating a magnetic pulse that stimulates the brain. If the coil is placed over a nerve or area of the brain that controls muscles, it may cause a muscle twitch, possibly strong enough to move the limb. In other cases, the subject may have a feeling of movement or feel a tingling sensation in a limb. Stimulation over the muscles on the side of the head may cause some discomfort in that area or twitching of the jaw.

During the stimulation, the subject may be asked to tense certain muscles slightly or perform other simple actions. The electrical activity of the muscles activated by the stimulation will be recorded using metal electrodes taped to the skin over the muscle. In most cases, the study takes less than 3 hours.