Effects of activity on axonal excitability: implications for motor control studies

In many motor control studies, such as those involving H reflexes in human subjects, stimuli are delivered to nerves in which axons are or have recently been active. It is assumed that a constant stimulus intensity ensures that the same population of axons has been activated, particularly when there is little change in the M wave. However, the neurophysiological literature is replete with evidence that activity depresses axonal excitability. Indeed, some motor control studies have exploited this phenomenon to stimulate specific afferent populations selectively. This chapter will review studies performed in human subjects demonstrating that a voluntary contraction can have profound effects on the excitability of the active axons and on their ability to transmit impulses. These data need to be considered when interpreting human reflex studies.