Double Discharges in Human Motor Units

Denslow, J. S.

doi:10.1152/jn.1948.11.3.209

Cited by 81 publications

(48 citation statements)

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“…Before training, double motor unit discharges (cf. Denslow, 1948) less than 5 ms apart were observed infrequently during ballistic contractions in the tibialis anterior, and this is consistent with data from the literature indicating that these discharges are typical for motor units in fast muscles and are rarely present in the discharges of slow muscles (Kudina & Churikova, 1990). Our finding that, after ballistic training, double discharges were present in motor units with different recruitment thresholds indicates that this muscle adapted itself and exhibited the properties of faster muscles.…”

Section: Discussionsupporting

confidence: 91%

Changes in single motor unit behaviour contribute to the increase in contraction speed after dynamic training in humans

Cutsem

Duchateau

Hainaut

1998

The Journal of Physiology

617

580

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The adaptations of the ankle dorsiflexor muscles and the behaviour of single motor units in the tibialis anterior in response to 12 weeks of dynamic training were studied in five human subjects. In each training session ten series of ten fast dorsiflexions were performed 5 days a week, against a load of 30–40 % of the maximal muscle strength. Training led to an enhancement of maximal voluntary muscle contraction (MVC) and the speed of voluntary ballistic contraction. This last enhancement was mainly related to neural adaptations since the time course of the muscle twitch induced by electrical stimulation remained unaffected. The motor unit torque, recorded by the spike‐triggered averaging method, increased without any change in its time to peak. The orderly motor unit recruitment (size principle) was preserved during slow ramp contraction after training but the units were activated earlier and had a greater maximal firing frequency during voluntary ballistic contractions. In addition, the high frequency firing rate observed at the onset of the contractions was maintained during the subsequent spikes after training. Dynamic training induced brief (2–5 ms) motor unit interspike intervals, or ‘doublets’. These doublets appeared to be different from the closely spaced (±10 ms) discharges usually observed at the onset of the ballistic contractions. Motor units with different recruitment thresholds showed doublet discharges and the percentage of the sample of units firing doublets was increased by training from 5.2 to 32.7 %. The presence of these discharges was observed not only at the onset of the series of spikes but also later in the electromyographic (EMG) burst. It is likely that earlier motor unit activation, extra doublets and enhanced maximal firing rate contribute to the increase in the speed of voluntary muscle contraction after dynamic training.

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Section: Discussionsupporting

confidence: 91%

Changes in single motor unit behaviour contribute to the increase in contraction speed after dynamic training in humans

Cutsem

Duchateau

Hainaut

1998

The Journal of Physiology

617

580

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“…On the other hand, it is noteworthy that a small percentage of soleus axons generate repetitive potentials after, or in some instances, during prolonged 5 to 20 cPS stimulation (Standaert, 1963). The possibility that repetitive potentials occur in muscles responding to voluntary or reflex drive is suggested by the observations of Hoff and Grant (1944) and Denslow (1948) who noted that electromyograms from cat and human postural muscles frequently contain pairs of potentials interspersed among rhythmic 10 to 20 cPs activity. These double potentials resemble P T R muscle potentials in timing and appearance and in the fact that they rarely occur in the gastrocnemius muscle.…”

Section: Discussionmentioning

confidence: 99%

The Mechanisms of Post-Tetanic Potentiation in Cat Soleus and Gastrocnemius Muscles

Standaert

1964

The Journal of General Physiology

104

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Post-tetanic potentiation of muscle contraction strength (PTP) occurs in cat soleus and gastrocnemius muscles. However, the mechanisms of potentiation are different in these two muscles. Soleus PTP is predominantly a neural event. The application of a high frequency stimulus to the soleus nerve regularly causes each subsequent response to a single stimulus to become repetitive. This post-tetanic repetitive activity (PTR) originates in the motor nerve terminal and is transmitted to the muscle. Consequently each potentiated soleus contraction is a brief tetanus. In gastrocnemius PTR occurs too infrequently to account for PTP. Furthermore, PTP occurs in curarized directly stimulated gastrocnemius muscles to the same extent as in the indirectly stimulated muscle. In this instance PTP is a muscle phenomenon.Post-tetanic potentiation (PTP) of muscle contraction seems to be a general phenomenon of neuromuscular systems. It has been observed in almost every muscle in which it has been sought (for review, see Hughes, 1958). Curiously, this ubiquity seems to be a major factor in the failure to provide a satisfactory explanation for the phenomenon. The fact that m a n y muscles respond similarly to a high frequency stimulus has tostered the belief that they share a common mechanism of potentiation, but to date no single mechanism has been found to reconcile the diverse observations.Neurally stimulated cat soleus and gastrocnemius muscles develop PTP of similar appearance. However, examination of the phenomenon in these two muscles revealed two distinct mechanisms of potentiation: one residing in the motor nerve terminal, the other in the muscle. Furthermore, it was found that both mechanisms operate in both muscles, but in different degree. Soleus PTP is predominantly neural, while gastrocnemius PTP is a l m o s t wholly muscular.

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“…Repetitive double discharges of single motor units during voluntary recruitment have been reported by Denslow (1948) and Kudina (1974) in normal human muscles. Doublets have also been observed during reflexive recruitment (Cordo & Rymer, 1982;Gordon & Holbourii, 1949;Hoff & Grant, 1944), locomotion in decerebrate cats (Zajac & Young, 1980;Hoffer, O'Donovan, Pratt & Loeb, 1981) and intracellular current injections in lumbar motoneurones of the cat (Calvin, 1975).…”

Section: Introductionmentioning

confidence: 94%

Repetitive doublets in human flexor carpi radialis muscle.

Bawa¹,

Calancie²

1983

The Journal of Physiology

133

113

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SUMMARY1. Single-motor-unit activity was recorded from flexor carpi radialis of two human subjects.2. A large number of units showed repetitive doublets at the onset of slow recruitment. A unit starting with doublets would transfer to a normal firing pattern as the force increased.3. At different speeds of ramp contractions, the number of doublets discharging at the onset of contraction decreased as the speed of contraction increased.4. Both low-and high-threshold units discharged repetitive doublets. Motor units which could discharge doublets showed higher maximal firing rates than those units which did not fire doublets.5. Short interspike intervals were also observed at the onset ofballistic movements. From the comparison of these short interspike intervals and the short intradoublet intervals we suggest that the two arise from two distinct phenomena in the spinal cord.6. Linked potentials were observed both with single spikes and doublets. Their origin may lie in the spinal cord or the muscle unit itself or both.

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Double Discharges in Human Motor Units

Cited by 81 publications

References 0 publications

Changes in single motor unit behaviour contribute to the increase in contraction speed after dynamic training in humans

Changes in single motor unit behaviour contribute to the increase in contraction speed after dynamic training in humans

The Mechanisms of Post-Tetanic Potentiation in Cat Soleus and Gastrocnemius Muscles

Repetitive doublets in human flexor carpi radialis muscle.

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