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This Article Full Text Full Text (PDF) All Versions of this Article: 542/3/939    most recent 2001.013428v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Murphy, P. R. Articles by Stein, R. B. Search for Related Content PubMed PubMed Citation Articles by Murphy, P. R. Articles by Stein, R. B.

Toe flexor muscle spindle discharge and stretch modulation during locomotor activity in the decerebrate cat

P. R. Murphy, K. G. Pearson* and R. B. Stein*

In order to investigate the nature (i.e. static or dynamic) of fusimotor drive to the flexor hallucis longus (FHL) and flexor digitorum longus (FDL) muscles during locomotion we recorded Ia and group II muscle spindle afferent responses to sinusoidal stretch (0.25 and 1 mm amplitude, respectively, 4-5 Hz) in a decerebrate cat preparation. FHL Ia and group II afferents generally had increased discharge rates and decreased modulation to stretch throughout the step cycle, compared to rest, suggesting raised static drive at all locomotor phases. Although the modulation of Ia afferents was reduced during locomotion, most (13 of 18) showed a clear increasing trend during homonymous muscle activity (extension). This was consistent with phasic dynamic drive to FHL spindles linked with drive. In agreement with previous reports, FHL gave a single burst of EMG activity during the step cycle while FDL drive had two components. One was related to extension while the other comprised a brief burst around the end of this phase. Typically FDL Ia and group II afferents also had elevated firing rates and reduced modulation at all locomotor phases, again implicating static drive. Half the afferents (seven Ia, three group II) showed increased discharge during extension, suggesting phasic static drive. There was no drive associated with the late FDL burst. In conclusion, the drives to FHL and FDL differed during locomotion. FHL, which has the drive of a classic extensor, received drive that closely resembled other extensors. The drive of FDL, which exhibits both extensor and flexor synergies, did not match either muscle type. These observations are compatible with the view that fusimotor drive varies in different muscles during locomotion according to the prevailing sensorimotor requirements.

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