Motor unit synchronisation is enhanced during slow lengthening contractions of a hand muscle

The central nervous system employs different strategies to execute specific motor tasks. Because afferent feedback during shortening and lengthening muscle contractions differs, the neural strategy underlying these tasks may be quite distinct. Cortical drive may be adjusted or afferent input regulated. The exact mechanisms are not clear. Here, we examine the control of synaptic transmission across the Ia synapse during shortening and lengthening muscle contractions. Subjects were instructed to maintain isolated activity in a single tibialis anterior (TA) motor unit while muscle length was varied from flexion to extension and back. At a fixed interval after a firing of the active motor unit, a single electrical stimulus was applied to the common peroneal nerve to activate Ia afferents from the TA muscle. We investigated the stimulus-induced change in firing probability of 19 individual low-threshold TA motor units during shortening and lengthening contractions. Any change in firing probability depends on both pre- and postsynaptic mechanisms. In this experiment, motoneuron firing rate was similar during both contraction types. There was no difference in the firing probability between shortening and lengthening contractions (0.23 ± 0.03 and 0.20 ± 0.02, respectively). We suggest that there is no contraction type-specific control of Ia input to the motoneurons during shortening and lengthening muscle contractions. Cortical adjustments may have occurred.

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Dissociated small hand muscle involvement in amyotrophic lateral sclerosis detected by motor unit number estimates

Muscle & Nerve ◽

10.1002/(sici)1097-4598(199907)22:7<870::aid-mus9>3.0.co;2-o ◽

1999 ◽

Vol 22 (7) ◽

pp. 870-873 ◽

Cited By ~ 60

Author(s):

Satoshi Kuwabara ◽

Keiko Mizobuchi ◽

Kazue Ogawara ◽

Takamichi Hattori

Keyword(s):

Amyotrophic Lateral Sclerosis ◽

Motor Unit ◽

Muscle Involvement ◽

Small Hand ◽

Hand Muscle ◽

Lateral Sclerosis

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Motor-unit activation patterns in lengthening and isometric contractions of hindlimb extensor muscles in the decerebrate cat

Journal of Neurophysiology ◽

10.1152/jn.1982.47.5.782 ◽

1982 ◽

Vol 47 (5) ◽

pp. 782-796 ◽

Cited By ~ 18

Author(s):

P. J. Cordo ◽

W. Z. Rymer

Keyword(s):

Firing Rate ◽

Motor Unit ◽

Stretch Reflex ◽

Single Unit ◽

Motor Units ◽

Isometric Contractions ◽

Active Force ◽

Lengthening Contractions ◽

Rate Modulation ◽

Initial Force

1. Multiunit integrated electromyographic (EMG) signals and single-unit EMG potentials were recorded during isometric and lengthening (stretch reflex) contractions of soleus and medial gastrocnemius (MG) muscles in 20 decerebrate cats. Patterns of motor-unit recruitment and rate modulation were examined in isometric muscles and during constant-velocity stretches. 2. Analysis of multiunit EMG activity and its relationship to active force revealed a marked difference between isometric and lengthening contractions. While the force-EMG relationship for isometric contractions was characteristically linear, the relation recorded during stretch-reflex responses showed a disproportionate early EMG increase, which was most obvious at low force levels, suggesting that the efficacy of force production is reduced in lengthening muscle. 3. Single-unit recruitment patterns were found to be qualitatively similar in isometric and lengthening contractions. In each case, motor units were recruited in order of increasing spike voltage. The numbers of newly recruited units declined steeply with each successive increment in active force. For a given unit, the force at which recruitment occurred was found to be greater in lengthening contractions than in isometric contractions, and in lengthening contractions it was also found to depend on the level of initial force. 4. Two patterns of motor-unit rate modulation were observed during muscle stretch, depending on whether a given unit was firing before the beginning of stretch or whether it was recruited during the course of stretch. Motor units that were active prior to stretch were found to increase firing rate at stretch onset and to vary their rate very little thereafter. Motor units recruited in the course of stretch began firing at an initial rate proportional to their force threshold, gradually increased their firing rate with increasing force, and sometimes reached an apparent maximum rate. 5. These results are discussed in terms of the mechanical properties of lengthening muscle and reflex regulation of these properties. Each identified pattern of motor-unit recruitment and rate modulation is evaluated for its potential contribution to the regulation of muscle properties, especially the prevention of muscle yield. We conclude that at low to moderate levels of initial force, recruitment of new motor units is likely to be the most effective compensatory mechanism.

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