Intraoperative applications of the H-reflex and F-response: a tutorial.

Traditional intraoperative monitoring of spinal cord function involves the use of three techniques: 1. Orthodromic ascending somatosensory evoked potentials (SSEPs) and 2. antIDromic descending neurogenic somatosensory evoked potentials (DNSSEPs) monitor long-tract sensory function. SSEPs and DNSSEPs do not monitor interneuronal gray matter function. 3. Transcranial motor evoked potentials (TMEPs) monitor descending long-tract motor function and measure interneuronal gray matter function by activating motor neurons. TMEPs activate from 4-5% of the motor neuron pool. When using TMEPs 95-96% of the motor spinal cord systems activating the motor neurons are not monitored. Our ability to interact with our environment involves not only intact sensation and strength, but also complex coordinated motor behavior. Complex coordinated motor behavior is controlled by groups of electrically-coupled spinal cord central pattern generators (CPGs). The components of CPGs are: descending and propriospinal systems, peripheral input, and segmental interneurons. The point-of-control is the level of excitation of interneurons, which is determined by the integrated activity of the other components. Spinal cord injury (SCI) changes segmental reflex gain by uncoupling these components. Changes in gain are detected by recordings from muscles. SSEPs, DNSSEPs and TMEPs provIDe limited information about the status of CPGs. H-reflexes measure the function of from 20-100% of the motor neuron pool. F-responses measure the function of from 1-5% of the motor neuron pool. H-reflexes and F-responses provIDe information about the degree of coupling between CPG components. Recording H-reflexes and F-responses together with SSEPs and TMEPs not only monitors spinal cord long-tract function, but also provIDes a multiple-systems approach that monitors those spinal cord systems that are responsible for the control of complex coordinated motor behavior. The objective of this paper is to describe how H-reflexes and F-responses can be used to monitor complex coordinated motor behavior.