Chapter 1 - Animal and human motor system neurophysiology related to intraoperative monitoring

• Published in: Neurophysiology in Neurosurgery pp. 3 - 15
• Main Author: Amassian, Vahe E.
• Format: Book Chapter
• Language: English
• Published: Elsevier Inc 2020
• Edition: Second Edition
• Subjects: Corticospinal tract; D-wave; I-wave; transcranial electrical stimulation; D-wave; Corticospinal tract; transcranial electrical stimulation; I-wave
• ISBN: 0128150009; 9780128150016; 0128150017; 9780128150009
• DOI: 10.1016/B978-0-12-815000-9.00001-0

This chapter uses data from animal models and human subjects to describe some physiological principles underlying intraoperative spinal cord monitoring of the motor pathways. In the first type of monitoring, conducted impulses in the corticospinal tract (CT) are recorded following transcranial electrical stimulation (TES), or transcranial magnetic stimulation (TMS). Single pulses elicit direct (D), that is, unrelayed CT discharges, which are followed, if the anesthesia is light, by multiple indirect (I) waves that are transynaptically generated in motor cortex. Corticocortical afferent inputs from parietal areas generate the first I wave, subsequent I waves resulting from excitation by a vertically oriented interneuron chain in motor cortex, which functions like a “clock” in quantizing time in periods of 1.3–2.0ms. The D wave increases in amplitude monotonically with TES or TMS intensity; over portions of the stimulus intensity–D response relation, the curve is very approximately linear, so that a percent decrease during an operation approximately reflects a block in conduction proportionately in the number of conducting CT fibers. By contrast, muscle activation by the CT volleys involves a highly nonlinear transfer function from CT to motoneuron, which may render this measure oversensitive to CT damage. Anesthesia not only reduces tonic facilitatory bombardment of the motoneuron, but also I wave components, thereby diminishing temporal facilitation of the motoneuron. Cooling motor cortex reduces especially later I waves and leads to loss of the muscle response. Such diminished excitation of the motoneuron can be counteracted by using a high frequency train of TES, which at the appropriate interstimulus interval can lead to excitatory post synaptic potential (EPSP) summation and motoneuron discharge.