Fast Fatigue-resistant Research Articles

Physiological properties of motoneurons innervating different muscle unit types in rat gastrocnemius

1. The contractile properties of gastrocnemius muscle units and the electrophysiological properties of their innervating motoneurons were examined in anesthetized adult male Sprague-Dawley rats in situ, using conventional microelectrode techniques. 2. Muscle units (n = 70) were classified as fast (F) or slow (S) on the basis of the degree of force summation during stimulation at 25 Hz, as well as fatigue resistance, in response to current injection into innervating motoneurons. S units categorized using these criteria were also found to invariably demonstrate twitch half-relaxation times (RT1/2) > 28 ms, with F units demonstrating RT1/2 < 27 ms. Some overlap was present between F and S units in twitch time-to-peck tension (TPT). 3. S muscle units were innervated by motoneurons with significantly higher afterhyperpolarization (AHP) times-to-peak and half-decay times, AHP amplitudes, and input resistances than F units. Motoneurons innervating S units also demonstrated slower mean axon conduction velocity than F units. 4. F units were further classified as fast fatiguing (FF), fast fatigue-resistant (FR), or fast intermediate (FI) on the basis of their fatigue resistance. Muscle unit forces were different among the unit types such that FF > FI > FR > S. Twitch TPT and RT1/2 differed such that FF < FI, FR < S. 5. No differences among FF, FI, and FR units were found for measures of AHP time course, AHP amplitude, rheobase, or input resistance. The only motoneuron property that differed among F unit types was axon conduction velocity, which was lower for FF than for FI and FR units. 6. The best relationships between muscle unit and motoneuron physiological properties were between expression of twitch time course, which included RT1/2 and AHP half-decay time (r = 0.73 to 0.74). S units always had AHP half-decay times > 20 ms, whereas for F units, values never exceeded 19 ms. Correlations between these variables increased when only S units were considered (r = 0.86-0.97), and were weak and not significant among F units only. 7. The only other significant correlations between muscle unit and motoneuron properties > 0.5 were between input resistance and expressions of twitch time course (r = 0.56-0.66). These relationships were nonexistent when type S units were not included in the analysis. 8. A sample of tibial motoneurons (n = 98) that innervated hindlimb muscles other than gastrocnemius was combined with the gastrocnemius motoneurons to examine interrelationships among motoneuron properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Extraocular motor units: type classification and motoneuron stimulation frequency-muscle unit force relationships

Extracellular and intracellular techniques were used to study single motor units of the abducens nucleus and lateral rectus muscle in the cat. Using a combination of two motor unit properties, the fusion frequency and an index of fatigability, the population of twitch motor units could be separated into 4 subgroups: fast fatigable (FF), fast fatigue resistant (FR), slow fatigable (SF) and slow fatigue resistant (S). Nontwitch motor units, a fifth subgroup (NT), formed 10% of the total studied population. The twitch tension and the maximum tetanic tension of the FF motor unit type were significantly stronger than all other motor unit types. The use of frequency varying stimulation patterns did not further differentiate the motor unit types. The relation between a series of single motoneuron stimulation frequencies and the resultant single muscle unit forces generated a slope defined as a motor unit's kt value. Motor units with low kt values had higher twitch tensions, higher maximum tetanic tensions, higher fusion frequencies and lower fatigue indices than motor units with high kt values. Motoneuron recruitment was tested by electrical stimulation of the medial rectus subdivision of the contralateral oculomotor nucleus. No correlations were seen between recruitment order and the mechanical parameters of the single abducens motor units.

Effects of tetanic contraction of motor units of similar type on the initial stiffness to ramp stretch of the cat peroneus longus muscle

1. The stiffness during the initial portion of a ramp stretch was measured in cat peroneus longus muscle at rest and during maximal tetanic contractions produced by increasing numbers of motor units of the same type [slow (S), fast fatigue resistant (FR), or fast fatigable (FF)]. 2. This initial ramp stiffness was defined as the ratio between tension and length change over the limited range of constant velocity extension during which tension rose linearly with length change. This stiffness was reduced by tetanic contraction of a number of motor units while other units remained inactive. The reduction had different characteristics in contractions produced by S, FR, or FF units. 3. Two brief ramp (triangular) stretches were applied at short intervals to evaluate the contribution of stable cross bridges to the changes in ramp stiffness. When the amplitude of the first stretch exceeded the presumed elastic limits of the stable cross bridges, the second ramp stretch showed a reduction of 20-60% in initial stiffness. This was seen both in passive muscles and in muscles in which several motor units were contracting. 4. When increasing numbers of motor units of the same type were activated, the initial ramp stiffness to the second of a pair of triangular stretches delivered during contraction increased almost linearly with the developed tension. The slope of this increase was 2.5 times steeper for S units than for FR units. This reflects the fact that contraction produced by S units causes a proportionally greater resistance to stretch than that of fast units.

Changes in muscle stiffness produced by motor units of different types in peroneus longus muscle of cat

1. The effects of maximal tetanic contractions of varying numbers of motor units of the same type [slow (S), fast fatigue-resistant (FR), or fast fatigable (FF)] on the mechanical responses to muscle stretch were studied in the peroneus longus muscle of anesthetized cats. 2. Two types of stiffness measurements were made: 1) an average stiffness, defined as the tension change from the beginning to end of a 0.5-mm ramp stretch; and 2) a dynamic stiffness, defined as the ratio of peak-to-peak tension to amplitude of a maintained 85-microns sinusoidal stretch at frequencies of 10-80 Hz. 3. Contractions of slow and fast units elicited different increases in average stiffness. Type S units, although developing much smaller tetanic tensions than fast ones, produced a resistance to stretch comparable with or greater than that of fast units developing much higher tensions. 4. For comparable tetanic tensions, slow units also elicited a greater dynamic stiffness than fast units. During sinusoidal stretch, changes in muscle tension led changes in muscle length during contraction of S units, but the reverse was observed for frequencies 30-50 Hz during contraction of FF units. This suggests that the latter perform oscillatory work on the driving apparatus. 5. Type S units, whose low-threshold motoneurons are the first to be recruited, appear well adapted to play a role in posture and in slow movements because of the resistance they offer to forces tending to change joint position or to oppose the progression of slow movements.

Properties of motor units in nerve-intact autografts of cat extensor digitorum longus muscles.

1. In cats, isometric contractile properties were measured on five extensor digitorum longus (EDL) muscles and four EDL muscle grafts 150-270 days after autografting with nerves intact. Comparisons were made between the properties of whole muscles and grafts and between 36 motor units in control EDL muscles and 41 motor units in grafts. 2. The time-to-peak twitch force (TPT) of 23 +/- 1.7 (SE) ms for grafts was significantly prolonged compared with the value of 17 +/- 0.7 ms observed for whole EDL muscles. The mean values for the TPT of motor units were not different from the respective values for whole grafts or for whole muscles. The maximum specific force of whole grafts of 19.7 +/- 0.6 (SE) N/cm2 was significantly less than the control value of 23.6 +/- 0.6 N/cm2, an observation consistent with all previous data on the maximum specific force of grafts and control muscles. 3. Based on the presence or absence of sag and an index of fatigue, motor units were classified as fast fatigable (FF), fast intermediate (FI), fast fatigue-resistant (FR), and slow (S). Motor units were classified 33% FF, 22% FI, 27% FR, and 17% S in control muscles and 17% FF, 43% FI, 29% FR, and 12% S in autografted muscles. Compared with control muscles, the number of small FF units increased significantly in the autografts, but no significant difference was observed in the fatigue properties of motor units.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological alterations of motor units in hemiplegia

Isometric contractions of single motor units in the first dorsal interosseous muscle were recorded with an intramuscular microstimulation technique in patients with short- or long-term hemiplegia and compared with controls. In the hemiplegic patients motor units could be classified as in controls, utilizing twitch contraction time and fatigue sensitivity, as FF (fast fatigable), FR (fast fatigue resistant) and S (slow fatigable). The mean twitch contraction time of fast-twitch but not slow-twitch units was slightly prolonged in patients with spastic hemiplegia and motor units, especially type S, tended to generate larger twitch tensions. A fourth type of motor unit, characterized by slow-twitch contraction times and increased fatigability (SF units), was recorded in long-term hemiplegia. The data demonstate that during long-term spastic hemiplegia in man some motor units develop increased fatigability and prolonged-twitch contraction times, reflecting the dynamic properties of muscle.

A quantitative study of skeletofusimotor innervation in the cat peroneus tertius muscle.

1. Physiological tests were used to identify skeletofusimotor or beta axons to the cat peroneus tertius muscle in order to assess the proportion of beta axons in the motor supply to this muscle. 2. Static beta axons (beta S) were identified by: (a) observation of a delay between the complete block of extrafusal contraction and the failure of spindle activation upon prolonged stimulation, (b) increase of spindle excitation with stimulation frequencies above that eliciting maximal extrafusal contraction, (c) observation of 'unfused' frequencygram of spindle primary afferent discharge during stimulation of the axon at frequencies above that eliciting complete fusion of extrafusal contraction and (d) static action exerted on the response of the spindle afferent to ramp stretch. 3. Dynamic beta axons (beta D) were identified by the persistence of spindle activation after selective block of extrafusal neuromuscular junctions and by their dynamic action on spindle primary endings. 4. The actions of 116 motor axons (conduction velocity 56-104 m/sec) on ninety-five spindle afferents (fifty-seven from primary and thirty-eight from secondary endings) were examined in ten experiments. Thirty-six beta axons (31% of the total sample) were identified: twenty-four beta S (conduction velocity 69-104 m/sec) and twelve beta D (conduction velocity 56-91 m/sec). 5. Twenty (35%) primary endings were activated by a beta S and sixteen (28%) by a beta D axon. Nineteen (45%) secondary endings were activated by a beta S and five (13%) by a beta D axon. Convergence of beta D and beta S axons on the same spindle occurred in 10% of instances. beta-innervated spindles were also supplied by gamma axons. 6. Most of the beta S motor units were of the fast-fatigue resistant (FR) type, with a few units of the fast-fatigable (FF) type, and nearly all the beta D motor units were of the slow (S) type.

Histochemistry of fibres of masseter and temporalis muscles of edentulous monkeys Macaca mulatta

Muscle fibres of the masseter and temporalis muscles of female rhesus monkeys which had been edentulous for 4.5 yr were analysed histochemically. Fibres were classified on the basis of activities of succinic acid dehydrogenase and myofibrillar adenosine triphosphatase. Capillaries were visualized by phosphatase activity. Succinate oxidase activity was determined by differential respirometry of aliquots of whole muscle homogenates. These data were compared to data obtained from the muscles of control adult female animals. Relative to control values, the muscles of edentulous animals had lower oxidative capacity, more fast fatigable (FF) and fewer fast fatigue resistant (FR) and slow fatigue resistant (S) fibres, smaller S fibres and slightly reduced capillarity. These data indicate relative disuse of elevator muscle following removal of teeth.