Relationships between characteristics of electrical stimulation, muscle pain and blink responses in man.

The present research was undertaken in 11 healthy volunteers with the aim of investigating the relationship between blink response, a feature of the startle reaction, and muscular pain evoked by muscle electrical stimulation (train of stimuli). At a repetition rate of 0.1 Hz, a close relationship was found between the threshold for muscle pain and that for the blink response, without apparent habituation. At the same rate and with stimulation intensity maintained at pain threshold, the subject could voluntarily inhibit the blink response, but not pain. Under the same conditions both pain and blink response were inhibited during a mental task. By increasing the current intensity pain became more intense and the blink response tended to increase in amplitude and to decrease in latency. Similar effects were achieved by increasing the repetition rate with the current intensity maintained at pain threshold. Habituation of the blink response, but not of pain, was found at repetition rates of 1 Hz or more. Present results indicate that nociceptive inputs from muscle have a facilitatory effect on central mechanisms subserving the blink response. It is suggested that the comparative analysis of pain sensation and the blink response may be a useful tool for pain research in man.