Startle responses recorded in the leg of man

The EMG pattern of startle reactions in the leg was studied in man using a 100 msec, 1 kc/sec squre wave tone burst of 114 dB as the auitory stimulus. At rest, 74 responses were recorded in the ankle flexor tibialis anterior (TA) and 23 in the ankle extensor gastrocnemius (G) with a significantly different mean latency of 151 and 123 msecrespectively. During tonic extension or flexion of the ankle, the EMG response in TA or G consisted in a burst of activity superimposed on the voluntary EMG at a latency of approximately 150 msec after the onset of the tone burst and followed, at 200 msec, by a period of EMG silence lasting close to 100 msec. Similar periods of silence also occurred as the sole EMG modulation in response to sound. Although predominant in flexors, startle reactions are significant in extensors during or after tonic ankle extension suggesting that audiospinal influences may be channelled to one or the other group depending on their functional state. The silent period of EMG following an excitatory startle or appearing on its own suggests an important inhibitory component in audiospinal mechanisms particularly related to startle since it had not been disclosed in a previous study of the modulation of the H-reflex by non-startling auditory stimulus. From the distribution and EMG pattern of startle, it is inferred that audiospinal influences may be meaningfully integrated through the reticulospinal system in sound guided behaviour.