Control properties of single motor units

点击翻译按钮获取中文摘要

Background Motoneurons provide the only link between the central nervous system and muscle and understanding how they transform synaptic input into motor output is fundamental to understanding the control of movement. Motoneuron discharge is controlled by “ionotropic” mechanisms that mediate traditional synaptic excitation and inhibition and “metabotropic” mechanisms that regulate persistent inward currents (PICs) and control the way the cell responds to synaptic inputs. PICs are controlled by monoamines that are released throughout the spinal cord from the brainstem. Monoamines are thought to set a background level of spinal excitability for the task at hand. However, the extent to which monoamines regulate motoneuron discharge in humans is unclear. The proposed experiments will employ a novel low-current electrical stimulation technique to infer the PIC contribution to motoneuron discharge during conditions selected to cover a range of levels of monoamines in the spinal cord.Objectives1. Establish the range over which motoneuronal excitability varies during conditions designed to encompass a wide range of levels of monoaminergic drive to the spinal cord.2. Establish the extent to which inhibitory inputs can be used to control motoneuronal excitability of specific motor pools. ProtocolLow-current stimulation will be applied to the nerves to the muscles that flex and extend the ankle and wrist while motor units are recorded. Surface electromyography will also be recorded. Data will be collected under the following conditions that are presented in order of lowest to highest predicted monoaminergic drive:1. Sleep. Monoamine drive is lowest during rapid eye movement sleep. Previously, we have delivered stimulation successfully in three sleeping subjects at higher intensities than will be used presently. 2. Relax “completely.” Subjects will be asked to relax “completely,” a condition that terminates involuntary motor unit discharge, a hallmark of PIC activation, that outlasts the stimulation.3. Awake relaxed (control). Subjects will be requested to sit normally. This is the “control” condition for these experiments and is intended to approximate the “medium” monoaminergic drive.4. Post-contraction. Low current stimulation will be delivered immediately after subjects perform 5 brief voluntary contractions. This task is intended to “warm up” the PICs in motoneurons and increase their contribution to motoneuron firing.5. Post-maximum voluntary contraction. Subjects will perform a maximum contraction to further warm-up motoneurons and increase monoaminergic drive.6. Post-caffeine. Participants will injest caffeine, an adenosine receptor agonist that increases monoaminergic levels.These experiments will be conducted over the first three years of the funding period. Different amplitudes of contractions of TA will be generated electrically and voluntarily to assess the effect of reciprocal inhibition on PICs in motor pools in the arms and legs. The stimulation will be delivered for 30 s at intensities below motor threshold to that which evokes a contraction of 10% MVC. Voluntary activation will range between relaxed and 10%MVC. These experiments will be completed over the final two years of the funding period. SignificanceThese experiments will provide new information about the extent to which PICs contribute to motoneuron discharge in humans. Currently, much is known about how ionotropic mechanisms control motor unit discharge under different conditions; the present experiments will extend this understanding to provide novel information about task-dependent changes in metabotropic control of motoneurons.