Spinal and long-latency reflexes

Abstract

This chapter describes the spinal and long-latency reflexes (LLRs). Spinal and long-latency reflexes (LLRs) provide information in neurological conditions characterized by overt and sub-clinical upper motor neuron (UMN) involvement. The techniques also give insights into the pathophysiology of the processes including involvement of segmental and suprasegmental mechanisms. In amyotrophic lateral sclerosis (ALS), both H-reflex and LLRs can be used to indirectly confirm involvement of the central nervous system (CNS). The H-reflex is a monosynaptic reflex that results from the activation of alpha motoneurons by electrically induced Ia afferent excitatory volleys. The peripheral silent period (SP) is a temporary, relative, or absolute decrease in the electromyographic activity of a voluntarily contracted muscle, occurring in response to stimulation of a cutaneous or mixed nerve by electrical or mechanical stimuli. Peripheral SP can be obtained using stimulation on a cutaneous nerve, either on a mixed nerve or even on muscle tendons. Long-latency hand muscle reflexes are also useful to study sensorimotor functions of the CNS. Significant correlation has been revealed between the low threshold and high clinical score of UMN involvement, and increased latency corresponds to lower motor neuron (LMN) impairment. This approach provides a rationale for future application in trials aiming to assess the therapeutic potential of different pharmacological agents.