Abstract：
:Successive motor unit (MU) twitches often do not sum linearly. Also, muscle spindle (MS) afferents may react nonlinearly to MU contractions occurring at short intervals. Little data is presently available on the interactions between two (or more) MUs regarding their effects on tension output and MS responses. We have studied these effects in cat Mm. gastrocnemius medialis (MG), soleus and semitendinosus. In adult anaesthetized cats, MUs of the muscle under study were electrically stimulated via their ventral root axons with random sequences of brief pulses having mean rates between 6 and 12 pulses per second. Isometric tension fluctuations were recorded from the muscle under study, and discharge patterns of MS afferents (Ia and group II) were recorded from dorsal root filaments. A cross-correlation analysis was performed to display linear and nonlinear effects evoked by selected time constellations of MU activations. 18 (67%) of 27 MG MUs showed marked potentiation of the second of two twitches in response to pairs of stimuli separated by 5 to about 25 ms. The remainder of these and 16 of the soleus MUs did not exhibit conspicuous nonlinearities. MS responses to such pairs of MU activations usually showed a prolonged spindle pause. About 28% of 36 pairs of MG MUs produced twitch tension less than expected for linear summation if activated nearly simultaneously. If two MUs both produced a spindle pause and possibly a relaxation discharge in an MS afferent, the near-synchronous activation of the units produced respective discharge variations that were less than expected for linear summation. If one MU produced an early discharge, contraction of another MU would often prevent it. These results are discussed in regard to mechanisms of tremor suppression.
journal_name
Exp Brain Resjournal_title
Experimental brain researchauthors
Niemann U,Windhorst U,Meyer-Lohmann Jdoi
10.1007/BF00237486subject
Has Abstractpub_date
1986-01-01 00:00:00pages
639-49issue
3eissn
0014-4819issn
1432-1106journal_volume
63pub_type
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