Long Latency Reflexes and the Silent Period

Abstract

Long latency reflexes (LLRs) are EMG activity occurring during the transition from reflex to voluntary motor activity, which probably arise from a transcortical loop, including afferents within the dorsal column/medial lemniscal system to the sensorimotor cortex and corticospinal tract efferents. Depending upon the site of a lesion and its pathophysiology, LLRs may be absent, delayed, or enhanced. In disorders of cortical hyperexcitability, including cortical myoclonus, an LLR occurring 40–60 ms after stimulation of the median nerve at rest may be present (“C-reflex.”) In response to noxious stimuli to the lower extremities, a polysynaptic network of spinal neurons, flexor reflex afferents, induce a patterned withdrawal response, including hip and knee flexion. These flexor reflexes may aid in the diagnosis of disorders of spinal cord hyperexcitability. Normally, following high stimulation of a peripheral nerve innervating a muscle that is being strongly contracted, no electrical activity occurs for approximately 100 ms (“silent period.”_ In disorders of distal peripheral nerve or muscle hyperexcitability, the silent period may be absent.