Whole-Muscle and Motor-Unit Contractile Properties of the Styloglossus Muscle in Rat

1999 ◽  
Vol 82 (2) ◽  
pp. 584-592 ◽  
Author(s):  
Thomas G. Sutlive ◽  
J. Ross McClung ◽  
Stephen J. Goldberg
Keyword(s):  
Motor Units ◽  
Contractile Properties ◽  
Contraction Time ◽  
Fusion Frequency ◽  
Single Motor Units ◽  
The Mean ◽  
Whole Muscle ◽  
Tetanic Tension ◽  
Contractile Characteristics ◽  
Stimulation Of

Investigations of whole muscle and motor-unit contractile properties have provided valuable information for our understanding of the spinal cord and extraocular motor systems. However, no previous investigation has examined these properties in an isolated tongue muscle. The purpose of this study was to determine the contractile properties and muscle fiber types of the rat styloglossus muscle. The styloglossus is one of three extrinsic tongue muscles and serves to retract the tongue within the oral cavity. Adult male Sprague-Dawley rats ( n = 19) were used in these experiments. The contractile characteristics of the whole styloglossus muscle ( n = 9) were measured in response to stimulation of the hypoglossal nerve branch to the muscle. The average twitch tension produced was 3.30 g with a mean twitch contraction time of 13.81 ms. The mean maximum tetanic tension was 19.66 g and occurred at or near the fusion frequency, which averaged 109 Hz. The styloglossus muscle was resistant to fatigue [fatigue index (F. I.) = 0.76]. In separate experiments ( n = 7), the contractile characteristics of 37 single motor units were measured in response to extracellular stimulation of hypoglossal motoneurons. The twitch tension generated by styloglossus motor units averaged 35.7 mg, and the mean twitch contraction time was 12.46 ms. The mean fusion frequency was 92 Hz. Maximum tetanic tension averaged 177.8 mg. Styloglossus single motor units were resistant to fatigue (F. I. = 0.74). The sites of stimulation that yielded a contractile response in the styloglossus muscle were consistent with the location of the styloglossus motoneuron pool reported in earlier anatomy studies. Muscle fiber typing was determined in three animals based on the myofibrillar ATPase reaction at pH 9.8, 4.6, and 4.3. The styloglossus muscle was composed of ≈99% type IIA fibers with a few scattered type I fibers present in the study sample. On the basis of the combined findings of the physiology and histochemistry experiments, the styloglossus muscle appeared to be a homogeneous muscle composed almost exclusively of fast, fatigue-resistant motor units. These properties of the styloglossus muscle and its motor units were compared with findings in other rat skeletal muscles.

Appearance of "transitional" motor units in overloaded rat skeletal muscle

1989 ◽  
Vol 67 (5) ◽  
pp. 2049-2054 ◽  
Author(s):  
E. G. Noble ◽  
F. P. Pettigrew
Keyword(s):  
Skeletal Muscle ◽  
Motor Units ◽  
Contraction Time ◽  
Rat Skeletal Muscle ◽  
Short Term ◽  
Single Motor Units ◽  
Whole Muscle ◽  
Tetanic Tension ◽  
Large Decline ◽  
Contractile Characteristics

The contractile characteristics of single motor units, isolated from rat plantaris muscles subjected to short-term (30 days) compensatory overload, were assessed to determine whether motor units in transition could be detected. In the control plantaris 88% of the motor units were classified as fast. After overload, a large decline (26.5%) in the proportion of typical fast motor units was noted. The estimated contribution of fast fatigable units to whole muscle tetanic tension (Po 200) also declined (from 55 to 25%), whereas that of fast intermediate motor units increased (from 33 to 55%). In the overloaded plantaris, motor units that exhibited unusual “sag” and contraction time characteristics were detected. These motor units, which could be further subdivided into two distinct types by a variety of indexes, exhibited characteristics intermediate to fast and slow units and therefore were termed “transitional.” Transitional units accounted for 12% of the estimated whole muscle Po200 after overload. These experiments characterize novel classifications of motor units undergoing transformation and further detail the motor unit shift that accompanies compensatory overload.

Physiological and histochemical characteristics of motor units in cat tibialis anterior and extensor digitorum longus muscles

1980 ◽  
Vol 43 (6) ◽  
pp. 1615-1630 ◽  
Author(s):  
R. P. Dum ◽  
T. T. Kennedy
Keyword(s):  
Motor Unit ◽  
Tibialis Anterior ◽  
Extensor Digitorum Longus ◽  
Motor Units ◽  
Input Resistance ◽  
Histochemical Staining ◽  
Glycogen Depletion ◽  
Single Motor Units ◽  
The Mean ◽  
Tetanic Tension

1. Intracellular recording and stimulation techniques were used to study the normal motor-unit population of tibialis anterior (TA) and extensor digitorum longus (EDL) muscles in the cat. Histochemical staining of the whole muscle and glycogen depletion of single motor units were performed. These results may be compared to those of their extensor antagonist, medial gastrocnemius (MG), as reported in studies by Burke and co-workers (7, 11, 13). 2. On the basis of two physiological properties, “sag” and fatigue resistance, the motor units in both TA and EDL could be classified into the same categories (types FF, F(int), FR, and S) as in MG (11). In contrast to MG, TA and EDL had nearly twice as many type-FR motor units and only half as many type-S motor units. 3. Glycogen depletion of representative single motor units of types FF and FR suggests a close correspondence between the physiological classification and a unique histochemical profile. No type-S units were depleted. 4. On the basis of histochemical staining, the muscle fibers in TA were presumed to belong to type-FF, -FR, or -S motor units. TA had a higher proportion of type-FR and a lower proportion of type-S muscle fibers than are found in MG. A striking feature was the variation in the proportion of each fiber type in different regions of TA. The anterolateral portion had mostly types FF and FR, while the posteriomedial portion had more types FR and S. 5. The twitch time to peak (TwTP) of isometric motor-unit contractions was generally quite fast with none having TwTP greater than 55 ms. The mean TwTP (not in EDL) and the mean tetanic tension of each motor-unit type were significantly different from each other. Most of the motor units exhibited significant postetanic potentiation of twitch tension and a corresponding lengthening of half-relaxation time and to a lesser degree, twitch contraction time. 6. There was a significant relationship between the inverse of motoneuronal input resistance and either tetanic tension or twitch contraction time. These relationships were not apparent when axonal conduction velocity rather than input resistance was used as an index of motoneuron size. The mean input resistances of the three major motor-unit types were significantly different while the mean conduction velocities of types FF and FR were nearly identical. A weak positive correlation was observed between the TwTP and the afterhyperpolarization of TA and EDL motoneurons. 7. In general, the mechanical characteristics and intrinsic motoneuronal properties of TA and EDL appear to parallel the organization of their extensor antagonist, MG, with some important quantitative differences that may reflect their different functional roles.

Contractile properties of single motor units in human toe extensors assessed by intraneural motor axon stimulation

1996 ◽  
Vol 75 (6) ◽  
pp. 2509-2519 ◽  
Author(s):  
V. G. Macefield ◽  
A. J. Fuglevand ◽  
B. Bigland-Ritchie
Keyword(s):  
Stimulus Frequency ◽  
Motor Units ◽  
Extensor Digitorum ◽  
Force Frequency ◽  
Contraction Time ◽  
Single Motor ◽  
Single Motor Units ◽  
Extensor Muscles ◽  
Stimulus Train ◽  
The Mean

1. Single motor axons innervating human toe extensor muscles were selectively stimulated through a tungsten microelectrode inserted percutaneously into the peroneal nerve. Twitch and tetanic forces were measured from a strain gauge over the proximal phalanx of the toe generating the greatest force. Twitch data were obtained from 19 single motor units in nine subjects: 8 motor units supplied extensor hallucis longus (EHL), 5 motor units supplied extensor digitorum longus (EDL), and 6 motor units supplied extensor digitorum brevis (EDB). Unpotentiated twitch forces ranged from 6.3 to 78.1 mN (20.0 +/- 4.0 mN, mean +/- SE), with the distribution highly skewed toward small forces. Twitch contraction and half-relaxation times were 74.8 +/- 3.9 and 78.6 +/- 6.0 ms, respectively. Compared with motor units in human thenar muscles, those in human toe extensor muscles were stronger but slower. However, as in thenar motor units, twitch force and contraction time were not related. 2. Force-frequency relationships were determined for 13 units (5 EDL, 5 EHL, 3 EDB) by stimulating each unit with short trains (1.0-5.0 s) of constant frequency (2-100 Hz). Peak force was related to stimulus frequency in a sigmoid fashion. The steep region of the curve extended from 5.5 +/- 0.7 (SE) Hz to 16.3 +/- 1.1 Hz for all units, and the stimulus frequency required to generate half-maximal force (9.6 +/- 0.6 Hz) was close to the center of the steep range. This frequency, which was inversely related to twitch contraction time, was lower than the frequency required to develop half-maximal force of human thenar motor units (12 +/- 4 Hz, mean +/- SD). The slopes of the regression lines relating force to frequency, computed over the steep range for each unit, were also lower for the toe extensors (3.7 +/- 0.7 mN/Hz) than for the thenar muscles (6 +/- 1 mN/Hz). 3. Maximal tetanic forces ranged from 29.9 to 188.1 mN (89.0 +/- 16.5 mN, mean +/- SE), and were generated at stimulus frequencies from 15 to 100 Hz (median 50 Hz). The stimulation frequency required for fused tetani (absence of noticeable force fluctuation) was generally less than that required for maximum tetanic force. The mean twitch-tetanus ratio, calculated for unpotentiated twitches, was 0.22 +/- 0.02 (range 0.15-0.41). This ratio was higher than for human thenar motor units (0.14 +/- 0.06, mean +/- SE). After twitch potentiation of 10 units, the mean twitch-tetanus ratio increased to 0.28 +/- 0.04. 4. The effects of preceding each stimulus train with a short interstimulus interval (10 ms) on force production at each frequency were examined in nine motor units. Peak forces at the onset of each contraction were higher when such an “initial doublet” preceded stimulus trains of < or = 20 Hz, but the mean force at the end of each stimulus train was not significantly affected at any frequency. 5. Eight units were stimulated with a train that increased in frequency continuously from 2 to 80 Hz, and then decreased symmetrically. This pattern resulted in peak forces that were higher on the descending limb of the stimulus train, the force-frequency relationship tracing a hysteresis loop. Hysteresis was exhibited because damping in the neuromuscular system causes the mechanical output of muscle to lag behind neural input. Thus, in non-steady-state conditions (as in most forms of natural activity), somewhat higher firing rates may be required to attain a particular level of force; once attained, force output will be transiently unresponsive to diminution of firing rate. 6. We conclude that there are differences in the contractile properties of single motor units in human toe extensor muscles (involved in posture and locomotion) and thenar muscles (involved in prehension and manipulation). Twitch-tetanus ratios were greater for motor units in the toe extensors, and this property accounted for the lower force sensitivity of these units to increases in frequency. (ABSTRACT TRUNCATED)

Summation of motor unit tensions in the tibialis posterior muscle of the cat under isometric and nonisometric conditions

1991 ◽  
Vol 66 (6) ◽  
pp. 1838-1846 ◽  
Author(s):  
R. K. Powers ◽  
M. D. Binder
Keyword(s):  
Motor Unit ◽  
Muscle Fibers ◽  
Stimulus Frequency ◽  
Muscle Length ◽  
Motor Units ◽  
Force Transducer ◽  
Tibialis Posterior ◽  
Single Motor Units ◽  
Individual Motor ◽  
Stimulation Of

1. The tension produced by the combined stimulation of two to four single motor units of the cat tibialis posterior muscle was compared with the algebraic sum of the tensions produced by each individual motor unit. Comparisons were made under isometric conditions and during imposed changes in muscle length. 2. Under isometric conditions, the tension resulting from combined stimulation of units displayed marked nonlinear summation, as previously reported in other cat hindlimb muscles. On average, the measured tension was approximately 20% greater than the algebraic sum of the individual unit tensions. However, small trapezoidal movements imposed on the muscle during stimulation significantly reduced the degree of nonlinear summation both during and after the movement. This effect was seen with imposed movements as small as 50 microns. 3. The degree of nonlinear summation was not dependent on motor unit size or on stimulus frequency. The effect was also unrelated to tendon compliance because the degree of nonlinear summation of motor unit forces was unaffected by the inclusion of different amounts of the external tendon between the muscle and the force transducer. 4. Our results support previous suggestions that the force measured when individual motor units are stimulated under isometric conditions is reduced by friction between the active muscle fibers and adjacent passive fibers. These frictional effects are likely to originate in the connective tissue matrix connecting adjacent muscle fibers. However, because these effects are virtually eliminated by small movements, linear summation of motor unit tensions should occur at low force levels under nonisometric conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

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

1991 ◽  
Vol 66 (5) ◽  
pp. 1483-1492 ◽  
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)

Reflex responses in active muscles elicited by stimulation of low-threshold afferents from the human foot

1992 ◽  
Vol 67 (5) ◽  
pp. 1375-1384 ◽  
Author(s):  
A. M. Aniss ◽  
S. C. Gandevia ◽  
D. Burke
Keyword(s):  
Motor Units ◽  
Medial Gastrocnemius ◽  
Emg Activity ◽  
Onset Latency ◽  
Single Motor ◽  
Reflex Responses ◽  
Posterior Tibial ◽  
Single Motor Units ◽  
Low Threshold ◽  
Stimulation Of

1. Reflex responses were elicited in muscles that act at the ankle by electrical stimulation of low-threshold afferents from the foot in human subjects who were reclining supine. During steady voluntary contractions, stimulus trains (5 pulses at 300 Hz) were delivered at two intensities to the sural nerve (1.2-4.0 times sensory threshold) or to the posterior tibial nerve (1.1-3.0 times motor threshold for the intrinsic muscles of the foot). Electromyographic (EMG) recordings were made from tibialis anterior (TA), peroneus longus (PL), soleus (SOL), medial gastrocnemius (MG), and lateral gastrocnemius (LG) muscles by the use of intramuscular wire electrodes. 2. As assessed by averages of rectified EMG, stimulation of the sural or posterior tibial nerves at nonpainful levels evoked a complex oscillation with onset latencies as early as 40 ms and lasting up to 200 ms in each muscle. The most common initial responses in TA were a decrease in EMG activity at an onset latency of 54 ms for sural stimuli, and an increase at an onset latency of 49 ms for posterior tibial stimuli. The response of PL to stimulation of the two nerves began with a strong facilitation of 44 ms (sural) and 49 ms (posterior tibial). With SOL, stimulation of both nerves produced early inhibition beginning at 45 and 50 ms, respectively. With both LG and MG, sural stimuli produced an early facilitation at 52-53 ms. However, posterior tibial stimuli produced different initial responses in these two muscles: facilitation in LG at 50 ms and inhibition in MG at 51 ms. 3. Perstimulus time histograms of the discharge of 61 single motor units revealed generally similar reflex responses as in multiunit EMG. However, different reflex components were not equally apparent in the responses of different single motor units: an individual motor unit could respond slightly differently with a change in stimulus intensity or background contraction level. The multiunit EMG record represents a global average that does not necessarily depict the precise pattern of all motor units contributing to the average. 4. When subjects stood erect without support and with eyes closed, reflex patterns were seen only in active muscles, and the patterns were similar to those in the reclining posture. 5. It is concluded that afferents from mechanoreceptors in the sole of the foot have multisynaptic reflex connections with the motoneuron pools innervating the muscles that act at the ankle. When the muscles are active in standing or walking, cutaneous feedback may play a role in modulating motoneuron output and thereby contribute to stabilization of stance and gait.

Anatomic and physiological characteristics of the ferret lateral rectus muscle and abducens nucleus

2007 ◽  
Vol 103 (5) ◽  
pp. 1706-1714 ◽  
Author(s):  
Keith N. Bishop ◽  
J. Ross McClung ◽  
Stephen J. Goldberg ◽  
Mary S. Shall
Keyword(s):  
Motor Units ◽  
Lateral Rectus ◽  
Contraction Time ◽  
Fatigue Index ◽  
Muscle Twitch ◽  
Abducens Nucleus ◽  
Single Motor Unit ◽  
Twitch Contraction ◽  
Whole Muscle ◽  
Individual Motor

The ferret has become a popular model for physiological and neurodevelopmental research in the visual system. We believed it important, therefore, to study extraocular whole muscle as well as single motor unit physiology in the ferret. Using extracellular stimulation, 62 individual motor units in the ferret abducens nucleus were evaluated for their contractile characteristics. Of these motor units, 56 innervated the lateral rectus (LR) muscle alone, while 6 were split between the LR and retractor bulbi (RB) muscle slips. In addition to individual motor units, the whole LR muscle was evaluated for twitch, tetanic peak force, and fatigue. The abducens nucleus motor units showed a twitch contraction time of 15.4 ms, a mean twitch tension of 30.2 mg, and an average fusion frequency of 154 Hz. Single-unit fatigue index averaged 0.634. Whole muscle twitch contraction time was 16.7 ms with a mean twitch tension of 3.32 g. The average fatigue index of whole muscle was 0.408. The abducens nucleus was examined with horseradish peroxidase conjugated with the subunit B of cholera toxin histochemistry and found to contain an average of 183 motoneurons. Samples of LR were found to contain an average of 4,687 fibers, indicating an LR innervation ratio of 25.6:1. Compared with cat and squirrel monkeys, the ferret LR motor units contract more slowly yet more powerfully. The functional visual requirements of the ferret may explain these fundamental differences.

scholarly journals Nonlinear summation of contractions in cat muscles. I. Early depression.

1981 ◽  
Vol 78 (3) ◽  
pp. 277-293 ◽  
Author(s):  
R B Stein ◽  
F Parmiggiani
Keyword(s):  
Motor Units ◽  
Repetitive Stimulation ◽  
Actin Binding ◽  
Linear Summation ◽  
First Order ◽  
Twitch Response ◽  
Single Motor Units ◽  
Sodium Hydrate ◽  
Slow Twitch ◽  
Stimulation Of

Nerves to fast- and slow-twitch cat muscles were stimulated with various numbers of supramaximal pulses under isometric conditions. By subtracting the force produced by j - 1 pulses from that produced by j pulses, the contribution of the j th pulse could be compared with the response to one pulse (twitch response). A less-than-linear summation (depression) was observed during the rising phase of the twitch. This depression became increasingly prominent and longer in duration with repetitive stimulation. A more-than-linear summation (facilitation) was observed during the falling phase of the twitch, which became increasingly delayed and smaller in amplitude with repetitive stimulation. The early depression could be abolished for the first few pulses by Dantrolene [1-(5-p-nitrophenyl) furfurilidene amino hydantoin sodium hydrate], which reduced Ca++ release from the sarcoplasmic reticulum. The depression was less prominent at short muscle lengths or with stimulation of single motor units. A first-order, saturable reaction such as Ca++ binding to troponin or actin binding to myosin can quantitatively account for the early depression.

scholarly journals Thermal Sensitivity of Swimming Performance and Muscle Contraction in Northern and Southern Populations of Tree Frogs (Hyla Crucifer)

1989 ◽  
Vol 142 (1) ◽  
pp. 357-372 ◽  
Author(s):  
HENRY B. JOHN-ALDER ◽  
M. CHRISTOPHER BARNHART ◽  
ALBERT F. BENNETT
Keyword(s):  
Muscle Contraction ◽  
Contractile Properties ◽  
Swimming Velocity ◽  
Tree Frog ◽  
Contraction Time ◽  
Thermal Dependence ◽  
Stroke Frequency ◽  
Muscle Contractile Properties ◽  
Muscle Contractile ◽  
Tetanic Tension

The effects of temperature on sprint swimming ability and muscle contractile properties were examined in northern and southern populations of the holarctic tree frog, Hyla crucifer Wied-Neuwied, acclimated to 20–23°C. Maximal swimming velocities of 29 (southern) and 32 (northern) cms−1 and stroke frequencies of 4.1 (southern) and 5.5 (northern) strokes s−1 were attained at 30°C, and maximal stroke lengths (i.e. distance moved per stroke) of 8.0 (southern) and 7.4 (northern) cm at 20°C. The thermal dependence of swimming velocity decreases with increasing temperature (e.g. Q10 = 4.0 from 6 to 10°C, 1.2 from 20 to 30°C), as reported for locomotion in other ectothermic vertebrates. Over a temperature range of l.5-30°C, velocity increases by a factor of 5.5, frequency by a factor of 4, and length by a factor of 1.7. Thus, increased velocity at higher temperatures can be attributed mostly to increased stroke frequency; increments in stroke length contribute less. Muscle contractile properties have similar thermal dependencies to those of other vertebrates: rate processes [including twitch time-to-peak tension (TPT), twitch half-relaxation time (RT½), maximal rate of tetanic tension development and isotonic shortening velocity] are much more sensitive to temperature than is force generation (twitch and tetanic tension). Below 8°C, stroke frequency is limited by twitch contraction time (TPT + RT½), and leg extension in a swimming stroke by TPT. At higher temperatures, the thermal dependence of stroke time is lower than that of contraction time. Neither locomotor nor muscle contractile properties are different between the two populations (except for twitch tension at low temperatures). Inflexibility in the thermal dependence of muscle contraction and locomotion in this species may help to explain differences in breeding phenologies between northern and southern populations.

Motoneuron electrophysiological and muscle contractile properties of superior oblique motor units in cat

1986 ◽  
Vol 55 (4) ◽  
pp. 715-726 ◽  
Author(s):  
J. S. Nelson ◽  
S. J. Goldberg ◽  
J. R. McClung
Keyword(s):  
Conduction Velocity ◽  
Stimulus Frequency ◽  
Single Pulse ◽  
Motor Units ◽  
Contractile Properties ◽  
Oblique Muscle ◽  
Inferior Oblique Muscle ◽  
Homogeneous Population ◽  
Single Motor Units ◽  
Superior Oblique

Intracellular techniques were used to study single motor units of the trochlear nucleus and superior oblique muscle in the cat. Motoneuron electrophysiological properties were correlated with muscle-unit contractile characteristics assessed under isometric conditions. Two distinct motor-unit types were identified and designated as twitch and nontwitch. Nontwitch units made up 5% of the total population studied. They responded only to tetanic stimulation with graded force that increased as stimulus frequency was increased up to 300-400 Hz. These units made up a homogeneous population in that they were innervated by slowly conducting axons, produced weak tetanic tensions, and were extremely fatigue resistant. Twitch units made up the majority (95%) of units studied. These units responded to single pulse stimulation with typical twitch contractions. The contraction speed and tension ranges for these units were comparable with those obtained from other extraocular muscle single units. Superior oblique twitch units, mechanically comparable with multiply innervated conducting units, identified in the cat inferior oblique muscle (31) were not observed. The twitch-unit population was heterogeneous in terms of neuromuscular fatigue resistance. Unit fatigability was inversely related to maximal tetanic tension. Motoneuron conduction velocity was related to muscle-unit contractile properties in a way similar to that seen in extremity motor units. The slowest twitch units were weak, fatigue resistant, and innervated by slow conducting axons. The fastest units were, in general, innervated by faster conducting axons, produced greater tetanic tensions, and were more susceptible to fatigue. Correlations among input resistance, rheobase, and conduction velocity were also observed. At present, subdivisions of the twitch-unit population on the basis of any one or combination of unit properties does not seem appropriate.

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