Time dependent selective recurrent discharge of motor units in F-response1

The interaction among the recurrent discharge of motor units is studied in surface recorded composite F-response. In the first experiment, 200 serially elicited polyphasic F-responses from foot muscles of patients with different lower motor neurone (LMN) disorders were rearranged to understand the behaviour of individual negative and positive peaks. These peaks were considered on the basis of simultaneous recording with a single fibre EMG needle, to be the partial expression of either a single motor unit potential (MUP) or more than one MUP generated simultaneously. In another experiment, two serially averaged F-responses (50 each) were superimposed over each other to look for their similarities at 15 min intervals 6 times. Result analysis indicated interaction of MUPs by in-phase summation and out-of-phase subtraction. Less affected MUPs recurred as negative or positive peaks in morphologically different F-responses. Certain peaks were observed more frequently than the others. Two serially averaged F-responses were nearly identical on superimposition over each other but their morphologies differed at different time intervals. The study suggests more frequent generation of F-response in a specific group of alpha motoneurones at one point of time, which is replaced by another group at another point of time. The characteristic variation in F parameters is, however, mainly due to the interaction of these frequently generated F-response MUPs with other sporadically generated ones. This interaction can be appreciated on visual screening of F-responses in patients with lower motor neurone disorders, perhaps because of a reduction in the number of participating MUPs.