scholarly journals Common Synaptic Input to the Human Hypoglossal Motor Nucleus

2011 ◽  
Vol 105 (1) ◽  
pp. 380-387 ◽  
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.

scholarly journals Activation properties of trigeminal motoneurons in participants with and without bruxism

2013 ◽  
Vol 110 (12) ◽  
pp. 2863-2872 ◽  
Author(s):  
Jessica M. D'Amico ◽  
Ş. Utku Yavuz ◽  
Ahmet Saraçoğlu ◽  
Elif Sibel Atiş ◽  
Monica A. Gorassini ◽  
...  
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Synaptic Input ◽  
Motor Units ◽  
Sleep Bruxism ◽  
Test Unit ◽  
Motor Activities ◽  
Inward Currents ◽  
Trigeminal Motoneurons ◽  
Voluntary Contractions

In animals, sodium- and calcium-mediated persistent inward currents (PICs), which produce long-lasting periods of depolarization under conditions of low synaptic drive, can be activated in trigeminal motoneurons following the application of the monoamine serotonin. Here we examined if PICs are activated in human trigeminal motoneurons during voluntary contractions and under physiological levels of monoaminergic drive (e.g., serotonin and norepinephrine) using a paired motor unit analysis technique. We also examined if PICs activated during voluntary contractions are larger in participants who demonstrate involuntary chewing during sleep (bruxism), which is accompanied by periods of high monoaminergic drive. In control participants, during a slowly increasing and then decreasing isometric contraction, the firing rate of an earlier-recruited masseter motor unit, which served as a measure of synaptic input to a later-recruited test unit, was consistently lower during derecruitment of the test unit compared with at recruitment (ΔF = 4.6 ± 1.5 imp/s). The ΔF, therefore, is a measure of the reduction in synaptic input needed to counteract the depolarization from the PIC to provide an indirect estimate of PIC amplitude. The range of ΔF values measured in the bruxer participants during similar voluntary contractions was the same as in controls, suggesting that abnormally high levels of monoaminergic drive are not continually present in the absence of involuntary motor activity. We also observed a consistent “onion skin effect” during the moderately sized contractions (<20% of maximal), whereby the firing rate of higher threshold motor units discharged at slower rates (by 4–7 imp/s) compared with motor units with relatively lower thresholds. The presence of lower firing rates in the more fatigue-prone, higher threshold trigeminal motoneurons, in addition to the activation of PICs, likely facilitates the activation of the masseter muscle during motor activities such as eating, nonnutritive chewing, clenching, and yawning.

Evidence for Plateau Potentials in Tail Motoneurons of Awake Chronic Spinal Rats With Spasticity

2001 ◽  
Vol 86 (4) ◽  
pp. 1972-1982 ◽  
Author(s):  
David J. Bennett ◽  
Yunru Li ◽  
Philip J. Harvey ◽  
Monica Gorassini
Keyword(s):  
Spinal Cord ◽  
Firing Rate ◽  
Motor Unit ◽  
Synaptic Input ◽  
Control Unit ◽  
Motor Units ◽  
Test Unit ◽  
Plateau Potentials ◽  
Motor Unit Firing ◽  
Chronic Spinal Rats

Motor units of segmental tail muscles were recorded in awake rats following acute (1–2 days) and chronic (>30 days) sacral spinal cord transection to determine whether plateau potentials contributed to sustained motor-unit discharges after injury. This study was motivated by a companion in vitro study that indicated that after chronic spinal cord injury, the tail motoneurons of the sacrocaudal spinal cord exhibit persistent inward currents ( I PIC) that cause intrinsically sustained depolarizations ( plateau potentials) and firing ( self-sustained firing). Importantly, in this companion study, the plateaus were fully activated at recruitment and subsequently helped sustain the firing without causing abrupt nonlinearities in firing. That is, after recruitment and plateau activation, the firing rate was modulated relatively linearly with injected current and therefore provided a good approximation of the input to the motoneuron despite the plateau. Thus in the present study, pairs of motor units were recorded simultaneously from the same muscle, and the firing rate ( F) of the lowest-threshold unit (control unit) was used as an estimate of the synaptic input to both units. We then examined whether firing of the higher-threshold unit (test unit) was intrinsically maintained by a plateau, by determining whether more synaptic input was required to recruit the test unit than to maintain its firing. The difference in the estimated synaptic input at recruitment and de-recruitment of the test unit (i.e., change in control unit rate, Δ F) was taken as an estimate of the plateau current ( I PIC) that intrinsically sustained the firing. Slowly graded manual skin stimulation was used to recruit and then de-recruit the units. The test unit was recruited when the control unit rate was on average 17.8 and 18.9 Hz in acute and chronic spinal rats, respectively. In chronic spinal rats, the test unit was de-recruited when the control unit rate (re: estimated synaptic input) was significantly reduced, compared with at recruitment (Δ F = −5.5 Hz), and thus a plateau participated in maintaining the firing. In the lowest-threshold motor units, even a brief stimulation triggered very long-lasting firing (seconds to hours; self-sustained firing). Higher-threshold units required continuous stimulation (or a spontaneous spasm) to cause firing, but again more synaptic input was needed to recruit the unit than to maintain its firing (i.e., plateau present). In contrast, in acute spinal rats, the stimulation did not usually trigger sustained motor-unit firing that could be attributed to plateaus because Δ F was not significantly different from zero. These results indicate that plateaus play an important role in sustaining motor-unit firing in awake chronic spinal rats and thus contribute to the hyperreflexia and hypertonus associated with chronic injury.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. III. Muscle-unit force modulation

1996 ◽  
Vol 75 (1) ◽  
pp. 51-59 ◽  
Author(s):  
K. E. Tansey ◽  
A. K. Yee ◽  
B. R. Botterman
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Muscle Force ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Rate Variation ◽  
Unit Force ◽  
Rate Modulation ◽  
Force Modulation ◽  
Whole Muscle

1. The aim of this study was to examine the extent of muscle-unit force modulation due to motoneuron firing-rate variation in type-identified motor units of the cat medial gastrocnemius (MG) muscle, and to investigate the contribution of muscle-unit force modulation to whole-muscle force regulation. The motoneuron discharge patterns recorded from 8 pairs of motor units during 12 smoothly graded muscle contractions evoked by stimulation of the mesencephalic locomotor region (MLR) were used to reactivate those units in isolation to estimate what their force profiles would have been like during the evoked whole-muscle contractions. 2. For most motor units, muscle-unit force modulation was similar to motoneuron firing-rate modulation, in that muscle-unit force increased over a limited range (120-600 g) of increasing whole-muscle tension and was then maintained at a near maximal (> 70%) output level as muscle force continued to rise. Most muscle units also decreased their force outputs over a slightly larger range of declining whole-muscle force before relaxing. This second finding was best explained by the counterclockwise hysteresis recorded in the motor units' frequency-tension (f-t) relationships. 3. In those instances when whole-muscle force fluctuated just above the recruitment threshold of a motor unit, a substantial percentage (10-25%) of the change in whole-muscle force could be accounted for by force modulation in that motor unit alone. This finding suggested that few motor units in the pool were simultaneously simultaneously undergoing force modulation. To evaluate this possibility, the extent of parallel muscle-unit force modulation within the 8 pairs of simultaneously active motor units was evaluated. As with parallel motoneuron firing-rate modulation, the extent of parallel muscle-unit force modulation was limited to unit pairs of the same physiological type and recruitment threshold. In several instances, pairs of motor units displayed parallel motoneuron firing-rate modulation but did not show parallel muscle-unit force modulation because of the nature of the motor units' f-t relationships. 4. The limited extent of parallel muscle-unit force modulation seen in these experiments implies that the major strategy for force modulation in the cat MG muscle, involving contractions estimated to reach 30-40% of maximum, may be motor-unit recruitment rather than motor-unit firing-rate variation with resulting force modulation. Given, however, that the majority of motor units are already recruited at these output levels (< 40%), it is proposed that motor-unit firing-rate variation with resulting force modulation may take over as the major muscle force modulating strategy at higher output levels.

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

2016 ◽  
Vol 116 (2) ◽  
pp. 611-618 ◽  
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.

scholarly journals Blood Flow Restriction Alters Motor Unit Behavior During Resistance Exercise

2019 ◽  
Vol 40 (09) ◽  
pp. 555-562 ◽  
Author(s):  
Pedro Fatela ◽  
Goncalo V. Mendonca ◽  
António Prieto Veloso ◽  
Janne Avela ◽  
Pedro Mil-Homens
Keyword(s):  
Blood Flow ◽  
Firing Rate ◽  
Motor Unit ◽  
Motor Units ◽  
Blood Flow Restriction ◽  
Motor Unit Recruitment ◽  
Recruitment Threshold ◽  
Firing Rates ◽  
Flow Restriction ◽  
Voluntary Contractions

AbstractWe aimed to determine whether blood flow restriction (BFR) alters the characteristics of individual motor units during low-intensity (LI) exercise. Eight men (26.0±3.8 yrs) performed 5 sets of 15 knee extensions at 20% of one-repetition maximum (with and without BFR). Maximal isometric voluntary contractions (MVC) were performed before and after exercise to quantify force decrement. Submaximal isometric voluntary contractions were additionally performed for 18 s, matching trapezoidal target-force trajectories at 40% pre-MVC. EMG activity was recorded from the vastus lateralis muscle. Then, signals were decomposed to extract motor unit recruitment threshold, firing rates and action potential amplitudes (MUAP). Force decrement was only seen after LI BFR exercise (–20.5%; p<0.05). LI BFR exercise also induced greater decrements in the linear slope coefficient of the regression lines between motor unit recruitment threshold and firing rate (BFR: –165.1±120.4 vs. non-BFR: –44.4±33.1%, p<0.05). Finally, there was a notable shift towards higher values of firing rate and MUAP amplitude post-LI BFR exercise. Taken together, our data indicate that LI BFR exercise increases the activity of motor units with higher MUAP amplitude. They also indicate that motor units with similar MUAP amplitudes become activated at higher firing rates post-LI BFR exercise.

Voluntary control of motor units in human antagonist muscles: coactivation and reciprocal activation

1987 ◽  
Vol 58 (3) ◽  
pp. 525-542 ◽  
Author(s):  
C. J. De Luca ◽  
B. Mambrito
Keyword(s):  
Firing Rate ◽  
Motor Units ◽  
Interphalangeal Joint ◽  
Time Shift ◽  
Extensor Pollicis Longus ◽  
Firing Rates ◽  
Antagonist Muscles ◽  
Control Scheme ◽  
Common Drive ◽  
Flexion And Extension

1. Myoelectric (ME) activity of several motor units was detected simultaneously from the human flexor pollicis longus and extensor pollicis longus muscles, the only two muscles that control the interphalangeal joint of the thumb. The ME signals were detected while the subjects produced isometric force outputs to track three different paradigms: triangular trajectories, random-force trajectories requiring both flexion and extension contractions, and net zero force resulting from stiffening the joint by voluntarily coactivating both muscles. 2. The ME signals were decomposed into their constituent motor-unit action potential trains. The firing rate behavior of the concurrently active motor units was studied using cross-correlation techniques. 3. During isometric contractions, the firing rates of motor units within a muscle were greatly cross-correlated with essentially zero time shift with respect to each other. This observation confirms our previous report of this behavior, which has been called common drive. Common drive was also found among the motor units of the agonist and antagonist muscles during voluntary coactivation to stiffen the interphalangeal joint. This observation suggests two interesting points: 1) that the common drive mechanism has a component of central origin, and 2) that the central nervous system may control the motoneuron pools of an agonist-antagonist muscle pair as if they were one pool when both are performing the same task. 4. During force reversals, the firing rates of motor units reverse in an orderly manner: earlier recruited motor units decrease their firing rate before later recruited motor units. This orderly reversal of firing rates is consistent with the concept of orderly recruitment and derecruitment. 5. A control scheme is suggested to explain the behavior of the motor units in both muscles during force reversal. It consists of centrally mediated reciprocally organized flexion and extension commands along with a common coactivation command to both muscles. This control scheme allows for coactivation and reciprocal activation of an agonist-antagonist set. 6. The agonist-antagonist pair was observed to generate a net force in two control modalities: proportional activation and reciprocal activation. In proportional activation, the agonist-antagonist set is coactivated during either of two states: when uncertainty exists in the required task or when a compensatory force contraction is perceived to be required.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Influence of common synaptic input to motor neurons on the neural drive to muscle in essential tremor

2015 ◽  
Vol 113 (1) ◽  
pp. 182-191 ◽  
Author(s):  
Juan A. Gallego ◽  
Jakob L. Dideriksen ◽  
Ales Holobar ◽  
Jaime Ibáñez ◽  
José L. Pons ◽  
...  
Keyword(s):  
Motor Unit ◽  
Motor Neurons ◽  
Synaptic Input ◽  
Motor Units ◽  
Spike Trains ◽  
Neural Drive ◽  
Systematic Analysis ◽  
Synaptic Inputs ◽  
Tremor Frequency ◽  
Unit Spike

Tremor in essential tremor (ET) is generated by pathological oscillations at 4–12 Hz, likely originating at cerebello-thalamo-cortical pathways. However, the way in which tremor is represented in the output of the spinal cord circuitries is largely unknown because of the difficulties in identifying the behavior of individual motor units from tremulous muscles. By using novel methods for the decomposition of multichannel surface EMG, we provide a systematic analysis of the discharge properties of motor units in nine ET patients, with concurrent recordings of EEG activity. This analysis allowed us to infer the contribution of common synaptic inputs to motor neurons in ET. Motor unit short-term synchronization was significantly greater in ET patients than in healthy subjects. Furthermore, the strong association between the degree of synchronization and the peak in coherence between motor unit spike trains at the tremor frequency indicated that the high synchronization levels were generated mainly by common synaptic inputs specifically at the tremor frequency. The coherence between EEG and motor unit spike trains demonstrated the presence of common cortical input to the motor neurons at the tremor frequency. Nonetheless, the strength of this input was uncorrelated to the net common synaptic input at the tremor frequency, suggesting a contribution of spinal afferents or secondary supraspinal pathways in projecting common input at the tremor frequency. These results provide the first systematic analysis of the neural drive to the muscle in ET and elucidate some of its characteristics that determine pathological tremulous muscle activity.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. II. Motoneuron firing-rate modulation

1996 ◽  
Vol 75 (1) ◽  
pp. 38-50 ◽  
Author(s):  
K. E. Tansey ◽  
B. R. Botterman
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Muscle Force ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Firing Rates ◽  
Rate Modulation ◽  
The Mean ◽  
Fast Twitch ◽  
The Relationship

1. The aim of this study was to examine the nature of motoneuron firing-rate modulation in type-identified motor units during smoothly graded contractions of the cat medial gastrocnemius (MG) muscle evoked by stimulation of the mesencephalic locomotor region (MLR). Motoneuron discharge patterns, firing rates, and the extent of firing-rate modulation in individual units were studied, as was the extent of concomitant changes in firing rates within pairs of simultaneously active units. 2. In 21 pairs of simultaneously active motor units, studied during 41 evoked contractions, the motoneurons' discharge rates and patterns were measured by processing the cells' recorded action potentials through windowing devices and storing their timing in computer memory. Once recruited, most motoneurons increased their firing rates over a limited range of increasing muscle tension and then maintained a fairly constant firing rate as muscle force continued to rise. Most motoneurons also decreased their firing rates over a slightly larger, but still limited, range of declining muscle force before they were derecruited. Although this was the most common discharge pattern recorded, several other interesting patterns were also seen. 3. The mean firing rate for slow twitch (type S) motor units (27.8 imp/s, 5,092 activations) was found to be significantly different from the mean firing rate for fast twitch (type F) motor units (48.4 imp/s, 11,272 activations; Student's t-test, P < 0.001). There was no significant difference between the mean firing rates of fast twitch, fatigue-resistant (type FR) and fast twitch, fatigable (type FF) motor units. When the relationship between motoneuron firing rate and whole-muscle force was analyzed, it was noted that, in general, smaller, lower threshold motor units began firing at lower rates and reached lower peak firing rates than did larger, higher threshold motor units. These results confirm both earlier experimental observations and predictions made by other investigators on the basis of computer simulations of the cat MG motor pool, but are in contrast to motor-unit discharge behavior recorded in some human motor-unit studies. 4. The extent of concomitant changes in firing rate within pairs of simultaneously active motor units was examined to estimate the extent of simultaneous motoneuron firing-rate modulation across the motoneuron pool. A smoothed (5 point sliding average) version of the two motoneurons' instantaneous firing rates was plotted against each other, and the slope and statistical significance of the relationship was determined. In 16 motor-unit pairs, the slope of the motoneurons' firing-rate relationship was significantly distinct from 0. Parallel firing-rate modulation (< 10-fold difference in firing rate change reflected by a slope of > 0.1) was noted only in pairs containing motor units of like physiological type and then only if they were of similar recruitment threshold. 5. Other investigators have demonstrated that changes in a motoneuron's "steady-state" firing rate predictably reflect changes in the amount of effective synaptic current that cell is receiving. The finding in the present study of limited parallel firing-rate modulation between simultaneously active motoneurons would suggest that changes in the synaptic drive to the various motoneurons of the pool is unevenly distributed. This finding, in addition to the findings of orderly motor-unit recruitment and the relationship between motor-unit recruitment threshold and motoneuron firing rate, cannot be adequately accommodated for by the existing models of the synaptic organization in motoneuron pools. Therefore a new model of the synaptic organization within the motoneuron pool has been proposed.

VASTUS LATERALIS MOTOR UNIT RECRUITMENT THRESHOLDS ARE COMPRESSED TOWARDS LOWER FORCES IN OLDER MEN

2020 ◽  
pp. 1-6
Author(s):  
R.M. Girts ◽  
J.A. Mota ◽  
K.K. Harmon ◽  
R.J. MacLennan ◽  
M.S. Stock
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Vastus Lateralis ◽  
Motor Units ◽  
Older Men ◽  
Force Level ◽  
Recruitment Threshold ◽  
Rate Versus ◽  
Younger Men ◽  
The Mean

Background: Aging results in adaptations which may affect the control of motor units. Objective: We sought to determine if younger and older men recruit motor units at similar force levels. Design: Cross-sectional, between-subjects design. Setting: Controlled laboratory setting. Participants: Twelve younger (age = 25 ± 3 years) and twelve older (age = 75 ± 8 years) men. Measurements: Participants performed isometric contractions of the dominant knee extensors at a force level corresponding to 50% maximal voluntary contraction (MVC). Bipolar surface electromyographic (EMG) signals were detected from the vastus lateralis. A surface EMG signal decomposition algorithm was used to quantify the recruitment threshold of each motor unit, which was defined as the force level corresponding to the first firing. Recruitment thresholds were expressed in both relative (% MVC) and absolute (N) terms. To further understand age-related differences in motor unit control, we examined the mean firing rate versus recruitment threshold relationship at steady force. Results: MVC force was greater in younger men (p = 0.010, d = 1.15). Older men had lower median recruitment thresholds in both absolute (p = 0.005, d = 1.29) and relative (p = 0.001, d = 1.53) terms. The absolute recruitment threshold range was larger for younger men (p = 0.020; d = 1.02), though a smaller difference was noted in relative terms (p = 0.235, d = 0.50). These findings were complimented by a generally flatter slope (p = 0.070; d = 0.78) and lower y-intercept (p = 0.009; d = 1.17) of the mean firing rate versus recruitment threshold relationship in older men. Conclusion: Older men tend to recruit more motor units at lower force levels. We speculate that recruitment threshold compression may be a neural adaptation serving to compensate for lower motor unit firing rates and/or denervation and subsequent re-innervation in aged muscle.

The motor-unit population of the cat tenuissimus muscle

1988 ◽  
Vol 59 (4) ◽  
pp. 1128-1142 ◽  
Author(s):  
A. Lev-Tov ◽  
C. A. Pratt ◽  
R. E. Burke
Keyword(s):  
Motor Unit ◽  
Motor Units ◽  
Retrograde Transport ◽  
Length Change ◽  
Single Type ◽  
Motor Nucleus ◽  
Single Motor Unit ◽  
Hindlimb Muscles ◽  
Typical Type ◽  
Fast Twitch

1. We studied the organization of motor units in the tenuissimus (TEN) muscle of pentobarbital-anesthetized cats. The cat TEN is a long, delicate straplike muscle that spans hip and knee, which has a very flat length-tension curve through 22 mm of length change. 2. The TEN motor nucleus, labeled by retrograde transport of several forms of horseradish peroxidase, was composed of 8-31 cells in different cats, of which about half were, on average, in the size range of alpha-motoneurons. TEN motoneurons were scattered through the ventrolateral portion of lamina IX, over a rostrocaudal distance of up to 6.5 mm, making it relatively easy to isolate individual TEN motor axons for single motor-unit stimulation. 3. Individual TEN muscle units were classified into four groups [fast-twitch, fatigable (FF), intermediate, fatigue-resistant (Fint), fast-twitch, fatigue-resistant (FR), and slow-twitch, fatigue resistant (S)] on the basis of "sag" and fatigue index mechanical properties, as in other cat hindlimb muscles. There was a relatively large proportion of Fint units (28%) in the TEN sample, and the range of tetanic tension (approximately 19-fold) was much smaller than found in other cat hindlimb muscles. 4. A majority of TEN muscle fibers could be classified into the three major histochemical types (IIB, IIA, and I) found in other cat muscles, but a substantial minority remained "unclassified." A single type Fint muscle unit was successfully depleted of glycogen for histochemical study. It exhibited a typical type IIB histochemical profile. 5. Despite its unusual morphology, the cat TEN contains the same types of motor units found in larger, more "typical" limb muscles.

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