Pain-related modulation of the human motor cortex.

Pain is a complex multi-dimensional phenomenon that influences a wide variety of nervous system functions, including sensory--discriminative, affective--motivational and cognitive--evaluative components. So far, these components have been studied in both patients with chronic pain and in normal subjects in whom pain was induced experimentally. The interaction between pain and motor function is not fully understood, although from everyday life it is known that pain affects movements. The effects of pain on motor control are typically seen as a limited or impaired ability to perform movements. Most studies have dealt with the effects of pain on the spinal cord reflexes, but in recent years, several lines of evidence suggest that the interaction between motor and pain systems in conditions of pain induced experimentally, rather than a simple spinal reflex, is a more complex process that involves also supraspinal brain areas. Although pain-motor interaction shows different features and time course depending on different pain variables, such as duration (tonic versus phasic pain), submodalities (deep versus superficial pain) and location (distal versus proximal pain), a common finding is that pain is able to inhibit the motor cortex. This motor cortex inhibition may act as a sort of motor 'decerebration' so as to allow the spinal motor system to freely develop protective responses to noxious stimulation. Further studies are required to assess the effects of pain on the motor system in patients suffering from chronic pain, in order to develop innovative rational therapeutic strategies to reduce both pain and motor disability.