Chapter 44 Indices of Cortical Motor Function Following Severe Brain Injury In man

Abstract

Publisher Summary This chapter assesses the degree to which the function of the corticospinal tract and the motor cortex in man is affected by severe brain injury of a severity that results in a lack of purposeful movements and no apparent awareness of the self or surroundings. The subjects mentioned in the chapter had suffered either anoxic or traumatic (possibly including anoxia) brain injury that resulted initially in an indefinite period of coma. On emergence from coma, a group of the subjects assumed sleep/wake cycles, with eyes open during the awake periods, but they exhibited no purposeful movements and showed no signs of awareness to a wide range of sensory stimuli. Such a condition is usually referred to as “persistent vegetative state” or “apallic syndrome.” Certain attributes of transcranial magnetic stimulation (TMS), as a method for eliciting a corticospinal output from the motor cortex, suggest that the technique may allow the level of excitability of the motor cortex to be assessed. TMS excites corticospinal neurons transynaptically, or at least at the initial segment. Only at higher strengths, well above the threshold and at specific orientations of the magnetic coil, does direct stimulation of corticospinal axons occur. TMS also activates cortical inhibitory circuits. Thus, the stimulation of corticospinal neurons by TMS is likely to be influenced by concurrently active pre-synaptic inputs.