Fast Fatigable Research Articles

Effects of chronic spinalization on ankle extensor motoneurons. III. Composite Ia EPSPs in motoneurons separated into motor unit types.

1. In this paper we continue an examination of changes in composite Ia excitatory postsynaptic potentials (EPSPs) in ankle extensor motoneurons after 6-wk (L1-L2) spinal cordotomy. The ratio of rheobase to input resistance was used to divide motoneurons into three groups approximating fast-fatigable (FF), fast fatigue-resistant (FR), and slow (S) motor units in barbiturate-anesthetized cats. Homonymous monosynaptic Ia EPSPs evoked by low-strength [1.2 times threshold (T)] electrical stimulation and heteronymous EPSPs evoked by 2T stimulation were compared between groups of motoneurons in unlesioned and chronic spinal preparations. 2. The distribution of motor unit types of triceps surae and plantaris (PL) motoneurons according to the present classification scheme agrees well with that obtained elsewhere using mechanical typing. Chronic spinalization resulted in an increased proportion of type FF motoneurons in PL and type FR motoneurons in lateral gastrocnemius (LG) motoneurons. There was a numeric but insignificant increase in the proportion of fast medial gastrocnemius motor units. 3. Membrane time constant (tau m) and estimated total cell capacitance were significantly reduced in FF and S motoneurons in chronic spinal preparations. FF motoneurons from chronic spinal animals also had a reduced afterhyperpolarization duration. Mean values of membrane electrical properties in FR motoneurons were unaltered after spinalization. 4. Homonymous Ia EPSP changes after chronic spinalization occurred preferentially in type FR and S motor units. Amplitudes increased 69% in type FR and 38% type S motor units but were unchanged in type FF units. Furthermore, the amplitudes of heteronymous Ia EPSPs in type FF and S units in the chronic spinal preparation were almost double those in unlesioned preparations. 5. Homonymous EPSP 10-90% rise times decreased 25% in type FR motor units and 15% in type S motor units and were unchanged in type FF motor units. Homonymous EPSP half-width decreased in all three motoneuron groups. Normalization of EPSP rise time and half-width to tau m reduced the difference between EPSP shape indexes in unlesioned and chronic spinal preparations in type FF and S motoneurons but less so in type FR motoneurons. Normalized EPSP shape indexes in some type FR units were shorter after chronic spinalization than any in unlesioned preparations. 6. The increased amplitude and decreased rise time of Ia EPSPs in type FR motoneurons after spinalization occurred without changes in the electrical properties of type FR motor units.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of motor units after immobilization of cat peroneus longus muscle

Changes in contractile properties of cat peroneus longus motor units were studied 2, 5, and 8 wk after selective immobilization of this muscle, which was achieved by fixing the distal tendon of the peroneus longus to the fibula either at the muscle minimal physiological length ("short" length) or at the length for a 90 degree ankle joint ("neutral" length). In each muscle, 75-90% of the units [slow (S), fast resistant to fatigue (FR), fast intermediate (FI), and fast fatigable (FF)] were studied. Immobilization elicited a permanent decrease in tetanic force developed by single motor units, which was larger for resistant-to-fatigue units (S, FR). In most instances this decrease was not related to the immobilization length. In all units, twitch contraction and half-relaxation times underwent a transient increase, the extent and time course of which were influenced by immobilization length. The relationship between the frequency of motor units activation and the ratio of unfused to maximal tetanic force was studied. For fast units, there was a transient shift of the relation toward low frequencies after 2 and 5 wk of immobilization at neutral and short length, respectively.

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 responses of rat plantaris motor units to overload induced by surgical removal of its synergists.

1. Rat plantaris muscles were subjected to chronic overload by the surgical removal of the soleus and most of the gastrocnemius muscles. Twelve to 16 wk later whole muscle and motor unit (ventral root dissection technique) contractile properties as well as histochemistry were determined. 2. Motor units were categorized as fast, fatigable (FF), fast, intermediate fatigue-resistant (FI), fast, fatigue-resistant (FR), and slow (S) based on contractile characteristics. Muscle fibers were identified as type I and type II according to myofibrillar ATPase staining. 3. Whole muscles demonstrated increases in wet weight, tetanic force, proportion of type I fibers, and mean cross-sectional areas of both type I and II fibers, as a result of chronic overload. 4. Tetanic tension increased by the same relative magnitude in all motor units whereas twitch tension remained unchanged. A significant change in the proportions of the motor unit types occurred in overloaded muscles, such that the latter contained higher proportions of FF and S units, and lower proportions of FI and FR units, than normal muscles. 5. The fatigue profile of a composite constructed from a summation of motor unit responses revealed that the overloaded plantaris displayed fatigue resistance similar to that of the normal plantaris for a given absolute force output. 6. Glycogen-depleted fibers of hypertrophied single motor units demonstrated uniform myofibrillar ATPase and SDH staining characteristics suggesting that metabolic adaptations among fibers of the same unit were similar after 12-16 wk of overload. 7. The finding that overload caused a uniform increase in the tetanic strength of all motor units, whereas alterations in fatigue resistance varied in degree and direction among unit types, demonstrate that these two properties are not controlled in parallel in this model. The smallest units maintain or even increase their fatigue resistance during the hypertrophic process, whereas high threshold units actually decrease in fatigue resistance.

Proportion of fatigue-resistant motor units in hindlimb muscles of cat and their relation to axonal conduction velocity.

1. A study of motor units to hindlimb muscles of cat has been made, with as complete a sample as possible of the motor axons to an individual muscle. In single experiments as much as 95% of the motor supply to a muscle has been examined. 2. The following muscles have been studied: peroneus brevis, peroneus tertius, peroneus longus, plantaris, gastrocnemius medialis, soleus, tenuissimus and lumbricalis superficialis. 3. Units were identified as slow resistant (S), fast resistant (FR), fast fatigable (FF) and fast intermediate (FI). The proportion of various motor unit types differs from one muscle to another. There is also some variation in the proportions to a given muscle from one animal to another. With the exceptions of soleus, which is entirely slow resistant, and gastrocnemius, which has relatively fewer resistant units, most muscles contain 60% or more of resistant (S and FR) units. 4. The conduction velocity ranges of FF, FR and FI units overlapped. There was little overlap between the conduction velocity ranges of these F units and of S units. 5. In individual experiments there was a strong and significant positive correlation between the logarithm of maximal tetanic tension and axonal conduction velocity in S and in S+FR units. In terms of contractile response the total fatigue-resistant population appeared to be a continuum. The correlation coefficient between maximal tetanic tension and conduction velocity was also high in the totality of units of all types, although within the FF group there appeared to be little or no correlation. In pooled data there was much more scatter and these relations were less clear. This resulted largely from differences in the ranges of axonal conduction velocity for a given motor unit type from one animal to another. 6. There was a highly significant negative correlation between isometric twitch contraction time and axonal conduction velocity in individual experiments. This relationship could also be seen, but less clearly, in pooled data. 7. The possible bases for these relationships are discussed.

Distribution of physiological types of motor units in the cat peroneus tertius muscle

Motor units of the cat peroneus tertius muscle were systematically analyzed using the criteria established by Burke et al. (1973). On the basis of their speed of contraction and resistance to fatigue, 121 (97%) of 125 motor units examined in ten adult cats could be classified as belonging to one of four types: fast-fatigable (FF), fast-resistant (FR), fast-intermediate (FI), and slow-resistant (S). Peroneus tertius was found to contain 30% FF motor units, 9% FI units, 39% FR units, and 22% S units. Contraction times of fast motor units (FF, FR, and FI) ranged from 15 to 27 ms and those of S units from 26 to 42 ms. The mean tetanic tensions were 37 g for FF units, 29 g for FI units, 7.5 g for FR units, and 1.1 g for S units. Fast motor units displayed considerable post-tetanic potentiation of twitch tension. Under similar conditions of stimulation, FF units appeared able to potentiate more and faster than FR units.

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.