Long Latency Reflexes and the Silent Period

Long latency reflexes (LLRs) and the silent period are normal electromyographic (EMG) phenomena recorded from muscles following a stimulus. LLRs are defined as involuntary responses, which follow the short latency response and precede the voluntary response. They can be elicited irrespective of whether the stimulus is muscle stretch, electrical stimulation of nerve (mixed nerve or cutaneous), or a more complex stimulus. LLRs help compensate for mechanical perturbations in different body parts in order to ensure goal success. They may be absent, delayed or enhanced in different disease states. The silent period can be evoked by strong shocks to either a mixed nerve or cutaneous nerve and reflects transient suppression of EMG activity. This is considered a protective reflex and may be shortened or absent in disease states. Together, LLRs and the silent period reflect the complex interplay of spinal cord, brainstem, and cortical influences in motor control.