Computer feedback reveals quantal aspects of the Hoffmann reflex.
The Hoffmann reflex was studied in 13 normal volunteers, using computer feedback to control stimulus intensity. The intensity of the electrical pulse that evoked the H-reflex was maintained at near threshold values by a computer program, which increased or decreased the intensity on the basis of the amplitude of the recorded response. Stimulus intensity was increased for H-responses less than, and decreased for H-responses greater than, a criterion value of 50 microV peak-to-peak. More than 500 responses were recorded from each subject during a 40-min period. The feedback procedure tended to hold the H-reflexes near the criterion value. Slow drifts in the required current indicated waxing and waning of threshold. Examination of response amplitude distributions showed two different types. For the nonquantal type, all possible values were represented in the range zero to 200 microV. Responses in the upper half of that range were less frequent than those in the lower half. For the quantal type, the majority of the responses were clustered near zero and at an amplitude that was characteristic for a given recording configuration; very few responses of intermediate amplitude were observed. The quantal pattern of response was ascribed to the firing of low-threshold single motor units.
D000328:Adult; D003201:Computers; D005246:Feedback; D006181:H-Reflex; D006801:Humans; D008297:Male; D008875:Middle Aged; D009068:Movement; D012023:Reflex, Monosynaptic