Brain Inhibition of Muscle Movement in Normal Volunteers

Overview

This study will use transcranial magnetic stimulation, or TMS (described below), to examine how the brain controls muscle movement to prevent unwanted movements in surrounding muscles. For example, when a person moves a finger, a part of the brain called the cortex prevents unwanted movements in other fingers by a process called cortical inhibition. In people with the muscle disorder dystonia, cortical inhibition does not work properly and patients suffer from uncontrolled and sometimes painful movements. A better understanding of how this process works in normal people may shed light on what goes wrong in dystonia and how the condition can be treated. Healthy normal volunteers 19 years of age and older may be eligible for this study. Candidates will be screened with a medical history and physical and neurological examinations. People with a current medical or surgical condition or neurological or psychiatric illness may not participate, nor may individuals who are taking medication that may influence nervous system function. Participants will undergo TMS to record the electrical activity of muscles in the hand and arm that are activated by magnetic stimulation. For the procedure, subjects are seated in a chair with their hands placed on a pillow in their laps. A wire coil in placed on their scalps. A brief electrical current is passed through the coil, creating a magnetic pulse that stimulates the brain. Subjects will be asked to move their second finger in response to a loud beep or visual cue. In some trials, a brief, mild electrical shock will also be applied to the end of either the second or fifth finger. The shock is not painful. TMS may cause muscle, hand or arm twitching if the coil is near the part of the brain that controls movement, or it may induce twitches or temporary tingling in the forearm, head, or face muscles. The twitching may cause mild discomfort, but the procedure is rarely considered painful.

The purpose of this study is to investigate the effect of homotopic and heterotopic sensory afferent input on surround inhibition in the motor cortex. Surround inhibition is a cortical phenomenon, the function of which may be to suppress unwanted movements in surrounding muscles during voluntary actions. In support of this, a recent study showed that motor output to the little finger was reduced during self-paced, voluntary movements of the index finger, despite an increase in spinal excitability. Work has also shown that in relaxed muscle, homotopic and heterotopic peripheral stimulation results in time dependent modulation of motor cortical excitability. However, no studies have examined the relationship between volitional movement and afferent sensory input on cortical surround inhibition. Therefore, the aim of this study is to examine the changes in surround inhibition when electrical stimulation is applied to either the finger being actively moved (homotopic stimulation) or a separate finger not involved in the movement (heterotopic stimulation) at different time periods prior to the initiation of movements to assess the affect of these stimuli on surround inhibition and excitation in the motor cortex.