Group Ia synaptic input to fast and slow twitch motor units of cat triceps surae

1. We tested the hypothesis that reflex inhibition of soleus motor units reflects selective inhibition of slow-twitch (type S) motor units throughout the triceps surae. Physiological properties including type, together with firing behavior, were measured from single motor units in the medial gastrocnemius (MG) muscle of decerebrate cats with the use of intra-axonal recording and stimulation. MG unit firing was contrasted during net inhibition or excitation of the slow-twitch soleus muscle produced by ramp-hold-release stretches of MG. 2. Stretch of the MG muscle increased the firing of type S motor units in the MG regardless of the reflex response of the soleus muscle. When stretch inhibited soleus, each of the 14 type S units sampled from MG either was newly recruited or exhibited increases in the rate of ongoing firing. Increased firing was observed in 320 of 321 stretch trials. For 8 of these 14 units, a total of 155 stretch trials evoked reflex excitation of soleus, and unit firing increased in all trials. 3. For the eight MG type S motor units studied during both reflex inhibition and excitation of soleus, firing rate tended to be higher during inhibition. The higher rates were also associated with the higher MG forces required to elicit soleus inhibition. For one MG type S unit it was possible to compare firing rates during soleus inhibition and excitation for trials of overlapping levels of MG force. For this unit, firing rate was similar, but still appreciably higher, during inhibition. 4. Soleus inhibition was also produced by stretch of the plantaris (PL) or lateral gastrocnemius (LG) muscles. Type S units in PL (n = 2) or in LG (n = 1) were recruited or increased firing rate even when stretch of these muscles produced soleus inhibition. 5. The firing behavior of 12 fast-twitch (type F) units was studied (11 from MG, 1 from PL). All type F units either were recruited or accelerated the rate of firing during soleus inhibition, as well as during soleus excitation. 6. These findings give evidence that reflex inhibition of type S motor units in the soleus muscle does not necessarily reflect an organizational scheme in which there is inactivation of type S units in other active muscles. In the DISCUSSION we point out the absence of direct evidence for selective inactivation of units on the basis of their type classification.

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Less common synaptic input between muscles from the same group allows for more flexible coordination strategies during a fatiguing task

Journal of Neurophysiology ◽

10.1152/jn.00453.2021 ◽

2022 ◽

Author(s):

Julien Rossato ◽

Kylie J. Tucker ◽

Simon Avrillon ◽

Lilian Lacourpaille ◽

Ales Holobar ◽

...

Keyword(s):

Synaptic Input ◽

Vastus Lateralis ◽

Rectus Femoris ◽

Motor Units ◽

The Other ◽

Triceps Surae ◽

Strong Positive Correlation ◽

Neural Drive ◽

Coordination Strategies ◽

Common Drive

This study aimed to determine whether neural drive is redistributed between muscles during a fatiguing isometric contraction, and if so, whether the initial level of common synaptic input between these muscles constrains this redistribution. We studied two muscle groups: triceps surae (14 participants) and quadriceps (15 participants). Participants performed a series of submaximal isometric contractions and a torque-matched contraction maintained until task failure. We used high-density surface electromyography to identify the behavior of 1874 motor units from the soleus, gastrocnemius medialis (GM), gastrocnemius lateralis(GL), rectus femoris, vastus lateralis (VL), and vastus medialis(VM). We assessed the level of common drive between muscles in absence of fatigue using a coherence analysis. We also assessed the redistribution of neural drive between muscles during the fatiguing contraction through the correlation between their cumulative spike trains (index of neural drive). The level of common drive between VL and VM was significantly higher than that observed for the other muscle pairs, including GL-GM. The level of common drive increased during the fatiguing contraction, but the differences between muscle pairs persisted. We also observed a strong positive correlation of neural drive between VL and VM during the fatiguing contraction (r=0.82). This was not observed for the other muscle pairs, including GL-GM, which exhibited differential changes in neural drive. These results suggest that less common synaptic input between muscles allows for more flexible coordination strategies during a fatiguing task, i.e., differential changes in neural drive across muscles. The role of this flexibility on performance remains to be elucidated.

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Nonuniform fatigue characteristics of slow-twitch motor units activated at a fixed percentage of their maximum tetanic tension

Journal of Neurophysiology ◽

10.1152/jn.1991.66.5.1483 ◽

1991 ◽

Vol 66 (5) ◽

pp. 1483-1492 ◽

Cited By ~ 7

Author(s):

T. C. Cope ◽

C. B. Webb ◽

A. K. Yee ◽

B. R. Botterman

Keyword(s):

Relaxation Time ◽

Conduction Velocity ◽

Motor Units ◽

Contraction Time ◽

Stimulation Rate ◽

Axonal Conduction ◽

Slow Twitch ◽

Force Clamp ◽

Highly Correlated ◽

Tetanic Tension

1. The endurance of slow-twitch motor units from the soleus (SOL) and medial gastrocnemius (MG) muscles of the cat were tested by determining the length of time (endurance time, Et) that a unit could maintain its tension output at 85% of maximum. Motor-unit tension was clamped at the target level by altering the stimulation rate of a unit's motor axon through computer feedback control. Tested in this way, units of both muscles displayed a wide range of Ets, approximately 40- to 50-fold. 2. Electromyographic (EMG) waveforms of motor units subjected to force-clamp contractions were analyzed to access whether any activity-dependent changes in their waveform shape might predict Et. Three measurements of waveform shape were determined: baseline-to-baseline duration, peak-to-peak amplitude, and area. Typically, amplitude decreased and duration increased as a contraction proceeded, whereas area remained fairly constant. Because changes in each measure were very similar for units of widely different Ets, it was concluded that neuromuscular junction failure and changes in the excitability of the sarcolemma (excluding the t-tubule system) play a minor role in determining Et. 3. Et was highly correlated with the mean stimulation rate (Et/number of stimuli) used during the force-clamp contractions. Mean rate was seen to progressively decrease with increasing Et. This correlation could not be explained by measures of isometric contractile speed or relaxation (e.g., twitch contraction time or half-relaxation time) measured before the force-clamp contractions. Both contraction time and half-relaxation time were found to be unrelated to both Et and the rate used to stimulate the unit during the force-clamp contraction. 4. Among type S units of SOL and MG, maximum tetanic tension and Et were not related. A significant relation (r = -0.49) was found between axonal conduction velocity and Et for SOL units (n = 38). In addition, a significant correlation (r = 0.47) was found between conduction velocity and tetanic tension for SOL units. Perhaps because of the small sample of type S units from MG (n = 10), conduction velocity was found not be related to either Et or tetanic tension. 5. Others have shown that a motor unit's maximum tetanic tension and axonal conduction velocity are correlated with its order of recruitment among motoneurons innervating a muscle. Recent work has further shown that among type F units the order in which a motoneuron is recruited is highly correlated with the fatigue resistance of its muscle unit.(ABSTRACT TRUNCATED AT 400 WORDS)

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Changes in diaphragm motor unit EMG during fatigue

Journal of Applied Physiology ◽

10.1152/jappl.1990.68.5.1917 ◽

1990 ◽

Vol 68 (5) ◽

pp. 1917-1926 ◽

Cited By ~ 34

Author(s):

G. C. Sieck ◽

M. Fournier

Keyword(s):

Motor Unit ◽

Fatigue Test ◽

Motor Units ◽

Evoked Force ◽

Slow Twitch ◽

The Right ◽

Fast Twitch ◽

Multiple Units ◽

Motor Unit Emg ◽

Stimulation Of

Fatigue-related changes in the waveform and root-mean-square (rms) values of evoked motor unit electromyographic (EMG) responses were studied in the right sternocostal region of the cat diaphragm. Motor units were isolated by microdissection and stimulation of C5 ventral root filaments and then classified as fast-twitch fatigable (FF), fast-twitch fatigue intermediate (FInt), fast-twitch fatigue resistant (FR), or slow-twitch (S) based on standard physiological criteria. The evoked EMG responses of S and FR units showed very little change during the fatigue test. The evoked EMG waveform and rms values of FF and FInt units displayed variable changes during the fatigue test. When changes were observed, they typically included a prolongation of the EMG waveform, a decrease in peak amplitude, and a decrease in rms value. The changes in EMG amplitude and rms values were not correlated. In more fatigable units, the decrease in force during the fatigue test generally exceeded the decrease in EMG rms values. Changes in the evoked force and EMG responses of multiple units innervated by C5 or C6 ventral roots were also examined during the fatigue test. The decrease in diaphragm force during the fatigue test closely matched the force decline predicted by the proportionate contribution of different motor unit types. However, the observed reduction in diaphragm EMG rms values during the fatigue test exceeded that predicted based on the aggregate contribution of different motor unit types. It was concluded that changes in EMG do not reflect the extent of diaphragm fatigue.

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Preferential distribution of nociceptive input to motoneurons with muscle units in the cranial portion of the upper trapezius muscle

Journal of Neurophysiology ◽

10.1152/jn.01117.2015 ◽

2016 ◽

Vol 116 (2) ◽

pp. 611-618 ◽

Cited By ~ 7

Author(s):

Jakob L. Dideriksen ◽

Ales Holobar ◽

Deborah Falla

Keyword(s):

Motor Unit ◽

Synaptic Input ◽

Isotonic Saline ◽

Trapezius Muscle ◽

Motor Units ◽

Noxious Stimulation ◽

Caudal Region ◽

Upper Trapezius ◽

Upper Trapezius Muscle ◽

Nociceptive Input

Pain is associated with changes in the neural drive to muscles. For the upper trapezius muscle, surface electromyography (EMG) recordings have indicated that acute noxious stimulation in either the cranial or the caudal region of the muscle leads to a relative decrease in muscle activity in the cranial region. It is, however, not known if this adaption reflects different recruitment thresholds of the upper trapezius motor units in the cranial and caudal region or a nonuniform nociceptive input to the motor units of both regions. This study investigated these potential mechanisms by direct motor unit identification. Motor unit activity was investigated with high-density surface EMG signals recorded from the upper trapezius muscle of 12 healthy volunteers during baseline, control (intramuscular injection of isotonic saline), and painful (hypertonic saline) conditions. The EMG was decomposed into individual motor unit spike trains. Motor unit discharge rates decreased significantly from control to pain conditions by 4.0 ± 3.6 pulses/s (pps) in the cranial region but not in the caudal region (1.4 ± 2.8 pps; not significant). These changes were compatible with variations in the synaptic input to the motoneurons of the two regions. These adjustments were observed, irrespective of the location of noxious stimulation. These results strongly indicate that the nociceptive synaptic input is distributed in a nonuniform way across regions of the upper trapezius muscle.

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Isometric contractions of motor units in self-reinnervated fast and slow twitch muscles of the cat

The Journal of Physiology ◽

10.1113/jphysiol.1974.sp010471 ◽

1974 ◽

Vol 237 (1) ◽

pp. 91-102 ◽

Cited By ~ 14

Author(s):

J. Bagust ◽

D. M. Lewis

Keyword(s):

Motor Units ◽

Isometric Contractions ◽

Slow Twitch

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Common Synaptic Input to the Human Hypoglossal Motor Nucleus

Journal of Neurophysiology ◽

10.1152/jn.00766.2010 ◽

2011 ◽

Vol 105 (1) ◽

pp. 380-387 ◽

Cited By ~ 20

Author(s):

Christopher M. Laine ◽

E. Fiona Bailey

Keyword(s):

Firing Rate ◽

Motor Unit ◽

Synaptic Input ◽

Motor Units ◽

Motor Nucleus ◽

Tongue Protrusion ◽

Ongoing Research ◽

Airway Patency ◽

Significant Difference ◽

Common Drive

The tongue plays a key role in various volitional and automatic functions such as swallowing, maintenance of airway patency, and speech. Precisely how hypoglossal motor neurons, which control the tongue, receive and process their often concurrent input drives is a subject of ongoing research. We investigated common synaptic input to the hypoglossal motor nucleus by measuring the coordination of spike timing, firing rate, and oscillatory activity across motor units recorded from unilateral (i.e., within a belly) or bilateral (i.e., across both bellies) locations within the genioglossus (GG), the primary protruder muscle of the tongue. Simultaneously recorded pairs of motor units were obtained from 14 healthy adult volunteers using tungsten microelectrodes inserted percutaneously into the GG while the subjects were engaged in volitional tongue protrusion or rest breathing. Bilateral motor unit pairs showed concurrent low frequency alterations in firing rate (common drive) with no significant difference between tasks. Unilateral motor unit pairs showed significantly stronger common drive in the protrusion task compared with rest breathing, as well as higher indices of synchronous spiking (short-term synchrony). Common oscillatory input was assessed using coherence analysis and was observed in all conditions for frequencies up to ∼5 Hz. Coherence at frequencies up to ∼10 Hz was strongest in motor unit pairs recorded from the same GG belly in tongue protrusion. Taken together, our results suggest that cortical drive increases motor unit coordination within but not across GG bellies, while input drive during rest breathing is distributed uniformly to both bellies of the muscle.

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Cutaneous stimulation fails to alter motor unit recruitment in the decerebrate cat

Journal of Neurophysiology ◽

10.1152/jn.1993.70.4.1433 ◽

1993 ◽

Vol 70 (4) ◽

pp. 1433-1439 ◽

Cited By ~ 30

Author(s):

B. D. Clark ◽

S. M. Dacko ◽

T. C. Cope

Keyword(s):

Motor Unit ◽

Conduction Velocity ◽

Motor Units ◽

Firing Frequency ◽

Medial Gastrocnemius ◽

Electromyographic Activity ◽

Size Principle ◽

Decerebrate Cat ◽

Slow Twitch ◽

Recruitment Order

1. An attempt was made to repeat the observation that cutaneous input to the cat medial gastrocnemius (MG) muscle sometimes had the differential effect of inhibiting motoneurons with slow axonal conduction velocity while simultaneously exciting others with fast conduction velocity. Dual microelectrode recording from intact ventral root filaments was used to study the effects of cutaneous inputs on recruitment order and on firing frequency of physiologically characterized MG motor units in decerebrate cats. Motor responses to pinch of the skin over the lateral surface of the ankle as well as electrical stimulation of the caudal cutaneous sural (CCS) nerve were contrasted with the responses to static muscle stretch as well as muscle vibration. 2. In contrast to the prediction, recruitment order in pairwise tests was the same for skin pinch or CCS stimulation as it was for MG stretch or vibration in all 32 tested pairs of motor units. This sample included seven pairs comprising one slow-twitch (S) and one fast-twitch motor unit, where the predicted reversal of recruitment should have been most apparent. Regardless of the source of excitation, recruitment of motor units of the MG was consistent with Henneman's size principle in approximately 90% of trials. 3. Skin pinch increased the firing rate of 30 of 32 individual motor units previously activated by stretch or vibration, including 7 slow-twitch units. In the remaining two units, skin pinch transiently (100-400 ms) slowed the firing of an S unit in 11 of 13 vibration + pinch trials. The other unit (type unknown) showed one or two retarded spikes in each of four vibration + pinch trials. In three S units, including the lone inhibitable unit and two others that were only excited by skin pinch, there was a significant positive rank correlation between change in unit firing frequency and change in soleus integrated electromyographic activity.(ABSTRACT TRUNCATED AT 400 WORDS)

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Effective synaptic current can be estimated from measurements of neuronal discharge

Journal of Neurophysiology ◽

10.1152/jn.1992.68.3.964 ◽

1992 ◽

Vol 68 (3) ◽

pp. 964-968 ◽

Cited By ~ 34

Author(s):

R. K. Powers ◽

F. R. Robinson ◽

M. A. Konodi ◽

M. D. Binder

Keyword(s):

Steady State ◽

Synaptic Input ◽

Discharge Rate ◽

Triceps Surae ◽

Repetitive Firing ◽

Basic Question ◽

Detailed Knowledge ◽

Synaptic Efficacy ◽

Synaptic Current ◽

Discharge Rates

1. The basic question of how motoneurons transform synaptic inputs into spike train outputs remains unresolved, despite detailed knowledge of their morphology, electrophysiology, and synaptic connectivity. We have approached this problem by making measurements of a synaptic input under steady-state conditions and combining them with quantitative assessments of their effects on the discharge rates of cat spinal motoneurons. 2. We used a modified voltage-clamp technique to measure the steady-state effective synaptic currents (IN) produced by rubrospinal input to cat triceps surae motoneurons. In the same motoneurons we measured the slope of the firing rate-injected current (f-i) relation in the primary range. We then reactivated the rubrospinal input during steady, repetitive firing to assess its effect on motoneuron discharge rate. 3. We found that changes in the steady-state discharge rate of a motoneuron produced by this synaptic input could be described simply as the product of the net effective synaptic current measured at the soma and the slope of the motoneuron's f-i relation. This expression essentially redefines synaptic efficacy in terms of a cell's basic input-output function. Further, measurements of effective synaptic current simplify the task of estimating synaptic efficacy, because detailed knowledge of neither the electrotonic architecture of the postsynaptic cell nor of the locations of the presynaptic boutons is required.

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