Modeling non-linear kinetics of hyperpolarized [1-(13)C] pyruvate in the crystalloid-perfused rat heart.

Hyperpolarized (13)C MR measurements have the potential to display non-linear kinetics. We have developed an approach to describe possible non-first-order kinetics of hyperpolarized [1-(13)C] pyruvate employing a system of differential equations that agrees with the principle of conservation of mass of the hyperpolarized signal. Simultaneous fitting to a second-order model for conversion of [1-(13)C] pyruvate to bicarbonate, lactate and alanine was well described in the isolated rat heart perfused with Krebs buffer containing glucose as sole energy substrate, or glucose supplemented with pyruvate. Second-order modeling yielded significantly improved fits of pyruvate-bicarbonate kinetics compared with the more traditionally used first-order model and suggested time-dependent decreases in pyruvate-bicarbonate flux. Second-order modeling gave time-dependent changes in forward and reverse reaction kinetics of pyruvate-lactate exchange and pyruvate-alanine exchange in both groups of hearts during the infusion of pyruvate; however, the fits were not significantly improved with respect to a traditional first-order model. The mechanism giving rise to second-order pyruvate dehydrogenase (PDH) kinetics was explored experimentally using surface fluorescence measurements of nicotinamide adenine dinucleotide reduced form (NADH) performed under the same conditions, demonstrating a significant increase of NADH during pyruvate infusion. This suggests a simultaneous depletion of available mitochondrial NAD(+) (the cofactor for PDH), consistent with the non-linear nature of the kinetics. NADH levels returned to baseline following cessation of the pyruvate infusion, suggesting this to be a transient effect.

Mitochondrial hexokinase and cardioprotection of the intact heart.

The interaction of hexokinase with mitochondria has emerged as a powerful mechanism in protecting many cell types against cell death. However, the role of mitochondrial hexokinase (mitoHK) in cardiac ischemia-reperfusion injury has as of yet received little attention. In this review we examine whether increased binding of hexokinase to the mitochondrion is also an integral component of cardioprotective signalling. We discuss observations in cardiac mitochondrial activation that directed us to the hypothesis of hexokinase cellular redistribution with reversible, cardioprotective ischemia, summarize the data showing that many cardioprotective interventions, such as ischemic preconditioning, insulin, morphine and volatile anesthetics, increase mitochondrial hexokinase binding within the intact heart, and discuss similarities between mitochondrial hexokinase association and ischemic preconditioning. Although most data indicate that mitochondrial hexokinase may indeed be an integral part of cardioprotection, a definitive proof for a causal relation between the amount of mitoHK and cardiac ischemia-reperfusion injury in the intact heart is eagerly awaited. When such relationship is indeed observed, the association of hexokinase with mitochondria will offer an opportunity to develop new therapies to combat ischemic cardiac diseases.

Inhibition of the pentose phosphate pathway decreases ischemia-reperfusion-induced creatine kinase release in the heart.

OBJECTIVE: The oxidative pentose phosphate pathway (oxPPP) produces NADPH, which can be used to maintain glutathione in its reduced state (anti-oxidant; beneficial effects) or to produce radicals or nitric oxide (NO) through NADPH oxidase/NO synthase (detrimental effects). Changes in cytosolic redox status have been implicated in ischemic preconditioning (PC). This study investigates whether (1) PC affects mitochondrial redox state, (2) the oxPPP plays a protective or detrimental role in ischemia (I)-reperfusion (R) injury in the intact heart and (3) PPP is altered with PC. METHODS: Isolated rat hearts were subjected to 40-min global I and 30-min R (CO, control). Ischemia was either preceded by three 5-min I/R periods (PC) and/or oxPPP inhibition by 6-aminonicotinamide (6AN) or NADPH oxidase/NO synthase inhibition by diphenyleneiodonium (DPI). NADH videofluorometry was used to determine mitochondrial redox state. PPP intermediates were determined in CO and PC hearts using tandem mass spectrometry. RESULTS: PC reduced ischemic damage (creatine kinase, CK, release from 337+/-64 to 147+/-41 U/R/gdw) and contracture (from 59+/-5 to 31+/-3 mm Hg) and increased recovery of contractility (from 48+/-10% to 88+/-8%), as compared to CO. PC was without effect on NADH fluorometry. Inhibition of the oxPPP reduced injury (CK release: 91+/-24 U/R/gdw) to similar levels as PC, without improving contractility. Inhibition of NADPH oxidase/NO synthase mimicked the effects of oxPPP inhibition on injury (CK release: 140+/-22 U/R/gdw). Although levels of ribose-5P and (ribulose-5P+xylulose-5P) rose several fold during ischemia with minor changes in sedoheptulose-7P, demonstrating an active PPP in the heart, PC did not affect these levels. CONCLUSIONS: (1) PC can attenuate cardiac reperfusion injury without alterations in mitochondrial redox state; (2) inhibition of the oxPPP protects the heart against I/R-induced CK release; and (3) PC does not result in altered activity of the PPP.

Relation between fibre composition and daily duration of spontaneous activity in ankle muscles of the cat.

UNLABELLED: This study concerns the relation between use and fibre type composition among limb muscles. The histochemical properties were investigated for ankle muscles from cats that had previously been studied in 24 hr electromyographic (EMG) recordings of daily spontaneous activity. We then reported average daily "duty times" (i.e. the percentage of total sampling time filled with EMG activity) of 1.9% for extensor digitorum longus (EDL), 2.1 and 4.0% for anterior and posterior sites of tibialis anterior (TA), 6.6 and 9.5% for anterior and posterior sites of peroneus longus (PL), and 13.9% for soleus (SOL). In the present experiments, muscles from which these data had been obtained were sectioned in a cryostat and stained for myofibrillar ATPase. Fibres were classified as type I (presumably slow) or II (presumably fast), the latter fibres being further categorized as IIA, IIB and a minor portion of transitional IIAB fibres. As expected, SOL was 100% type I. Among the muscles of mixed fibre-type composition ("mixed muscles"), a statistically significant difference in the mean percentages of type I fibres was found between TA or EDL (2.9-6.0%) vs. PL (11.8-14.6%). For TA the percentage of type I fibres was higher in posterior (deep) than in anterior (superficial) sampling regions; for PL no clear antero-posterior difference was found. A significant correlation was obtained between the percentage of type I fibres and the total duration of daily activity recorded from corresponding mixed muscle sites (5 different recording sites in 4 cats, totally 15 cases of successfully combined physiological and histochemical measurements, r = 0.76, P < 0.001). Similarly, within TA the total duration of daily activity was higher for sites with high (posteriorly) than for those with low (anteriorly) percentages of type I fibres. IN CONCLUSION: a "coarse-grain" relationship was found between fibre type composition and the duration of daily activity among mixed muscles. Possible mechanisms underlying this relationship are discussed.

External recording of twitch time course in cat ankle muscles.

In chronic experiments concerning the activity-dependent plasticity of muscle properties, a simple and noninvasive method was used for monitoring changes of twitch speed in conscious adult cats. The animals had been provided with implanted electrodes for nerve stimulation, and a hand-held force transducer was pressed against the fully extended ankle joint while single test pulses were delivered to the common peroneal nerve. In the present report, this technique for the recording of ankle twitches is subjected to critical analysis and evaluation. The measurements were highly reproducible with respect to contraction time (time-to-peak) but less so for half-relaxation time and twitch amplitude; other methods should be used for the long-term monitoring of contractile force. The total force (torque) of the ankle twitch was mainly produced by tibialis anterior (about 45%), peroneus longus (PerL; 27%) and extensor digitorum longus (23%). The ankle twitch produced by PerL alone had about the same contraction time as that of all the muscles together. Among muscles that had become changed as a result of long-term electrical stimulation there was, in general, a good correspondence between the contraction times from simple external recordings of ankle twitches and those separately measured for PerL under general anesthesia (force transducer then directly connected to PerL tendon).

Recovery after intense chronic stimulation: a physiological study of cat's fast muscle.

Adult cats were used to study the recovery of muscles that had become altered by long-term electrical stimulation. Chronic activation was delivered to the deafferented common peroneal nerve (no pain, no reflexes), and contractile properties were measured for peroneus longus muscle. After 4 wk of great daily amounts of treatment at moderately high pulse rates (30-40 Hz delivered during 50% of daily time), the peroneus longus became considerably weaker, demonstrated a longer time course of twitches and a slower rate of rise of tetanic force, and became less fatigable. Furthermore, its twitch-to-tetanus ratio decreased, and there was no longer any depression of electromyogram (EMG) amplitude during fatigue tests. After 4 wk of subsequent rest it was found that 1) twitch speed and maximum tetanic force had returned to nearly normal values, 2) fatigue resistance showed some return toward normal but was still significantly enhanced, and 3) no significant recovery had yet occurred of the altered twitch-to-tetanus ratio, the abolished EMG depression, or the slowed rate of rise of tetanic tension. During the poststimulation recovery period, the progressive increase of isometric twitch speed was not promoted by the administration of small daily amounts of high-rate stimulation (100-Hz bursts). The results support the conclusions that 1) the time course of recovery differs among physiological properties, 2) the EMG and force reactions that occur during a fatigue test are not strongly coupled, as demonstrated by the alterations of their relationship during poststimulation recovery, and 3) in cat's fast muscles, there is still no evidence for rate-specific effects of chronic stimulation on isometric twitch speed.

Effects of physiological amounts of high- and low-rate chronic stimulation on fast-twitch muscle of the cat hindlimb. I. Speed- and force-related properties.

1. Long-term electrical stimulation was given during 4 or 8 wk to the peroneal nerve of deafferented hindlimbs in hemispinalized adult cats. Four different stimulation patterns were compared: 100-Hz bursts covering 5% of daily time (F1), 10-Hz bursts covering 5% of daily time (S1), pattern S1 plus added 100-Hz bursts during 0.5% of daily time (S1F2), and, finally, only the latter 100-Hz bursts (F2), again during 0.5% of daily time. 2. During the course of chronic stimulation, frequent noninvasive measurements were made of the twitch of the ankle dorsiflexors. In a terminal acute experiment under general anesthesia, performed after 4 or 8 wk of treatment, measurements were made of isometric contractile properties (speed, force) for one of the stimulated peroneal muscles, m. peroneus longus (PerL). Thereafter, the PerL muscle was removed for further histochemical/histological analysis. 3. Findings from chronically stimulated PerL muscles were compared with three kinds of control PerL muscles: 1) those from the contralateral (control) hindlimb of chronically treated animals, 2) those from the operated side of animals that had been deafferented and hemispinalized but not subjected to chronic stimulation, 3) those from normal animals that had not been subjected to chronic treatment. With respect to the presently studied parameters, the three kinds of control muscles rendered very similar results. 4. All the presently used patterns of chronic stimulation made the PerL muscles slower with respect to twitch contraction time, half-relaxation time, and tension-frequency relation. Patterns covering 5-5.5% of daily time (F1, S1, S1F2) also caused an increase in the percentage of fibers classified as 'slow' (type I) on basis of their staining for myosin adenosine triphosphatase (ATPase). 5. Among patterns covering 5% of daily time, the change in ATPase histochemistry and the degree of physiological slowing was at least as pronounced after chronic stimulation at 100 Hz (F1) as after treatment at 10 Hz (S1). The slowing produced by pattern S1 was not more pronounced than that caused by this pattern (10 Hz) plus an equal number of pulses at 100 Hz (S1F2). 6. The slowing produced by the presently used patterns of chronic stimulation took place within the initial 2-3 wk. 7. Patterns F1 and S1 caused a decrease in maximum tetanic force as well as in mean fiber diameter.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of physiological amounts of high- and low-rate chronic stimulation on fast-twitch muscle of the cat hindlimb. II. Endurance-related properties.

1. Long-term electrical stimulation was given to the peroneal nerve of deafferented hindlimbs in hemispinalized adult cats. The amount of stimulation covered 0.5-5.5% of total time per day, different in different animals. For some aspects of the present study, use was also made of cats subjected to "tonic" patterns of chronic stimulation (typically covering 50% of total time; 10, 16). 2. In a terminal acute experiment under general anesthesia, performed after 4 or 8 wk of long-term stimulation, one of the treated peroneal muscles (m. peroneus longus, PerL) was used for measurements of the resistance to contractile fatigue. The fatigue test consisted of 0.33-s bursts of motor-nerve stimulation at 40 Hz, repeated once a second for 4 min (6, 7). During this fatigue test, the evoked compound spikes of the muscle were recorded by electromyographic (EMG) techniques. Following the physiological procedures, PerL was removed for further histochemical analysis. In transverse sections, measurements of optical density were made in central regions of single fibers after staining for the activity of an oxidative enzyme, succinate dehydrogenase (core SDH). 3. Findings from chronically stimulated PerL muscles were compared with three kinds of control PerL muscles: 1) those contralateral to the stimulated ones, 2) those from the operated side of animals that had been deafferented and hemispinalized but not subjected to chronic stimulation, and 3) those from untreated normal animals. 4. Stimulation patterns covering both greater than or equal to 50% and 5-5.5% of daily time gave a marked improvement of fatigue resistance. Pulse rate seemed of little importance for these effects. The pattern covering only 0.5% of total daily time caused no increase of contractile endurance beyond that of normal muscles. 5. During the fatigue test of a control muscle (see above), the amplitude of the compound EMG spikes typically showed a marked decline. This "EMG depression" was effectively counteracted by all the present patterns of chronic stimulation, including the 0.5% pattern. 6. Fibers of chronically stimulated muscles became more similar to each other with respect to their density of core SDH staining. However, among muscles treated during 0.5-5.5% of total daily time, the degree and pattern of change in core SDH staining was not related to the amount and pattern of chronic stimulation or to the resulting degree of contractile endurance.

Fibre sizes and histochemical staining characteristics in normal and chronically stimulated fast muscle of cat.

1. Normal and chronically stimulated peroneus longus muscles of the cat's hind limb were studied with respect to fibre size and staining properties for myofibrillar (myosin) adenosine triphosphatase (ATPase) and succinate dehydrogenase (SDH) activity. The intensity of staining for SDH activity was measured by microphotometry from the central portions of the muscle fibres ('core-SDH staining'). For comparison, histochemical properties were also studied in non-stimulated soleus muscles. 2. On account of the pH sensitivity of their myofibrillar ATPase, about 18% of the fibres in normal peroneus longus muscles were classified as type I, and about half of the remainder as II A and II B respectively. 3. In the normal peroneus longus muscles, the mean diameter of single muscle fibres generally varied between about 25 and 75 micron, whereby the average size of type I less than type II. 4. In the normal peroneus longus muscles the staining intensity for core SDH varied over a wide range. The average heaviness of staining was clearly ranked in the order type I greater than type II A greater than type II B. 5. Chronic stimulation was given to the deafferented common peroneal nerve by aid of a portable and remotely controlled mini-stimulator. The stimulation was delivered in 'tonic' patterns (greater than or equal to 50% of total time taken up by activity) of 'fast' (20 or 40 Hz) or 'slow' (5 or 10 Hz) rates. 6. Prior to the period of long-term stimulation, the cats had been subjected to a dorsal rhizotomy and hemispinalization on the ipsilateral (left) side. In the absence of chronic stimulation, these operations had no evident effects on the sizes or staining properties of peroneus longus fibres. 7. After 8 weeks of treatment with tonic patterns of stimulation, the fibres of peroneus longus muscles clearly became more similar to each other with respect to their diameter as well as their staining for ATPase and SDH activity. With respect to ATPase staining, however, the chronically stimulated peroneus longus fibres had become more similar to non-stimulated soleus fibres than to non-stimulated type I fibres of peroneus longus. With respect to the staining for core SDH, the chronically stimulated fibres all became similar to normal II A fibres of peroneus longus. The 'fast' and 'slow' patterns of chronic stimulation had the same effects on the staining properties. 8. Chronically stimulated peroneus longus muscles showed a decrease in fibre diameter which corresponded, roughly, to the concomitant decrease in muscle weight.(ABSTRACT TRUNCATED AT 400 WORDS)

Soma size and oxidative enzyme activity in normal and chronically stimulated motoneurones of the cat's spinal cord.

In normal adult cats we measured the density of staining for the activity of succinate dehydrogenase (SDH staining) in ventral horn cells of different sizes. The measurements were restricted to that part of the lumbar ventral horn (L6-L7) which is known to contain motoneurones of the peroneal nerve. A statistically significant tendency was found for the SDH staining to be denser in smaller than in larger neurones within the size range of a motoneurones (soma diameter greater than 40 microns). These results are consistent with recently published evidence for ventral horn cells of rats and qualitatively similar relationships between size and SDH staining have also been observed among skeletal muscle fibres (confirmed for mixed muscle of cat in present study). In hindlimb muscles, size as well as SDH staining are known to be markedly activity-dependent. We tested whether this is the case for peroneal motoneurones as well by analyzing the effects of chronic nerve stimulation on the properties of neurones within the appropriate region of the ventral horn. Prior to the final acute experiment, these cats had been subjected to a left-side dorsal rhizotomy and hemispinalization. By aid of a portable mini-stimulator, the left-side common peroneal nerve was activated by repetitive pulses during 50% of total time per day (intra-activity rate: 10, 20 or 40 Hz). After 8 weeks of such treatment, cell sizes as well as the densities of SDH staining showed hardly any differences between peroneal ventral horn cells of the experimental and control sides of the spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal and muscle unit properties at different rostro-caudal levels of cat's motoneurone pool.

The rostrocaudal organization of motoneurones of the cat's m.peroneus longus (PerL) was investigated with respect to motoneuronal size (soma diameter) as well as muscle unit contractile properties (twitch speed, fatigue resistance, maximum force). Sizes of cell bodies were measured after retrograde labelling with horseradish peroxidase. Other properties were studied by aid of electrical stimulation of rostro-caudally distinct subfilaments of ventral roots. A weak but significant tendency was found for units to become somewhat slower and more fatigue-resistant at caudal than at more rostral levels within the pool. Neurones of different sizes and unit properties were, however, very widely intermingled at all levels of the pool. Rostro-caudally distinct root filaments gave rise to differentially located electromyographic signals in the muscle. The experimental results led to the following main conclusions: a type-specific intraspinal innervation of PerL motoneurones is unlikely to be arranged predominantly on topographical principles; and the intraspinal site of PerL motoneurones is related to the intramuscular site of their muscle fibres.

Somatotopic relations between spinal motoneurones and muscle fibres of the cat's musculus peroneus longus.

The cat's m.peroneus longus was analyzed with respect to the somatotopic relation between the rostro-caudal site of emergence of ventral root filaments (i.e. rostro-caudal site of motoneurones) and the intramuscular distribution of innervation. Rostro-caudally distinct fractions of ventral roots were stimulated repetitively in order to deplete their respective muscle fibres of glycogen. The intramuscular position of glycogen-depleted fibres was analyzed in transverse sections from different proximo-distal levels. At each level, depleted muscle fibres were dispersed across the whole muscle. No consistent relation was found between the spinal site of origin of a ventral root filament and the proximo-distal distribution of its fibres within the pennate muscle. A significant and evident tendency was found, however, for rostral root filaments (i.e. rostral motoneurones) to innervate a greater number of muscle fibres in anterior than in posterior muscle portions. For caudal root filaments, the opposite pattern of innervation was observed.

Effects of fast and slow patterns of tonic long-term stimulation on contractile properties of fast muscle in the cat.

Different physiological rates of 'tonic' long-term electrical stimulation (rates 5-40 Hz; activity greater than or equal to 50% total time) were delivered to the left-side common peroneal nerve of the cat hind limb. The duration of treatment was 8 weeks, and the animals had previously been subjected to a left-side hemispinalization and dorsal rhizotomy. In the absence of stimulation, these operations had no slowing or weakening effects on peroneal muscle contraction. The minimum two-pulse interval that gave a summation of tension (neuromuscular refractory period) was longer for stimulated than for non-stimulated muscles. Twitches of chronically stimulated muscles had become prolonged by more than 100%. Corresponding changes were found in the tension-frequency relation and in the 'sag'-behaviour of the stimulated muscles. There were no differences between the 'fast' (20 or 40 Hz pulse rates) and the 'slow' (5 or 10 Hz pulse rates) patterns of tonic stimulation with respect to their effects on speed-related muscle properties. Furthermore, during the period of chronic stimulation, the prolongation of twitch contraction time occurred along the same time course for the fast and slow patterns of tonic treatment. All chronically stimulated muscles had become weaker than normal. In comparison to the slow patterns, the present fast patterns of long-term activation caused (1) a smaller amount of decline in maximum muscle force, (2) a smaller twitch: tetanus ratio, and (3) the retention of a normal amount of post-tetanic potentiation of twitch size (decreased by the slow patterns). When tested by a series of 40 Hz bursts, force was better maintained in chronically stimulated muscles than in normal ones. These effects on fatigue resistance were the same for the fast and slow patterns of long-term activation. In peroneus longus muscles contralateral to the side of chronic activation, an evident impairment had commonly occurred in the capability to maintain force during tetani at the high rates needed for a maximum tetanic contraction. The results are discussed in relation to problems concerning the long-term effects of motoneuronal activity patterns on the contractile properties of their muscle units.

Motor activity and muscle properties in the hemidecerebellate cat.

Luciani's ipsilaterally acting 'trophic' cerebellar influence on striated muscle was reinvestigated in hemidecerebellate preparations of varying extent. Cats with hindlimb postural asymmetries for 4 or more days after the lesion developed a bilateral reduction of maximum tetanic tension and increased twitch/tetanus ratios of soleus. In addition, soleus on the side of the lesion lost force and weight, showed decreased twitch contraction and half-relaxation times, elevated myosin ATPase activity in part of its fibres, occasional fibre necrosis and a few snake coils. Protracted postural asymmetry occurred only if complete hemicerebellectomy included ablation of the lateral vestibular nucleus and extended for at least 3.0 mm across the midline into the contralateral vermal and intermediate cortex, especially of Larsell's lobuli IV and V. Most simply, the cerebellar effect on muscle is explained as the result of altered motoneuronal activation patterns. Comparison of the experimental soleus changes with Holmes's clinical findings in cases of cerebellar injury suggests that muscle itself participates in experimental and human cerebellar asthenia.

Relation between isometric force and stimulus rate in cat's hindlimb motor units of different twitch contraction time.

The relation between isometric force and rate (or pulse interval) of repetitive stimulation was studied for 77 motor units from m. peroneus longus of the cat. The units were activated by constant-frequency bursts of 1 s, and the stimulus interval needed for producing half the maximum tension was strongly correlated to twitch contraction time (twitch CT, non-potentiated values 13-42 ms). This remained true for comparisons within groups of fast and slow units respectively (fast/slow classification according to criteria of Burke et al. 1973). A mean contractile force of half maximum amplitude (0.5 PO) was produced by repetitive stimuli with a pulse interval of about 1.5 CT in fast and 2 CT in slow units. Among both kinds of unit, however, these stimulus rates corresponded to pulse intervals of about 1.4 times the half-relaxation time of the twitch. At half-maximum force, the rise of tension per Hz rise of stimulus frequency was about 2.5% PO for fast and 5.8% PO for slow units. Fast-twitch fatigue-sensitive (FF) and twitch fatigue-resistant (FR) units showed similar tension-frequency relations. Comparisons to results from m. gastrocnemius medialis showed that, for corresponding types of fast units (FF units), the twitch CT tended to be about 25% longer for gastrocnemius than for peroneus. The stimulus rate needed for a half-maximum contraction was, however, not lower for FF units from gastrocnemius than for those from the peroneus muscle.

Motor unit categorization on basis of contractile properties: an experimental analysis of the composition of the cat's m. peroneus longus.

Recordings were made of isometric contractions of single motor units of the cat's m. peroneus longus (PerL). The units were activated by stimulation of dissected filaments of ventral roots. In accordance with the general principles introduced by Burke et al. (1973), the 80 isolated PerL units were classified into three or four type-categories according to their contractile speed and endurance. Three currently used varieties of a "fatigue index" were calculated and found to give equivalent results. Units with a high, intermediate and low resistance to fatigue were responsible for about 22.5, 25.4 and 52.1% respectively of the total muscle force. Two alternative methods for fast/slow categorization were compared: (i) classifying all units as slow that failed to show a "sag" in partly fused contractions ("sag-criterion", Burke et al. 1973) and (ii) classifying all units as slow that had a more prolonged twitch contraction time than that of fatigue-sensitive units ('FF vs. S-criterion'). The relative contribution of slow units to total muscle force was about 2.8 times as great (14 versus 5%) for a classification by the FF vs. S-criterion than for a subdivision according to sagging behaviour. When compared to equivalent data from previously published studies of feline hindlimb muscles, peroneus longus was found to resemble gastrocnemius medialis in relative motor unit composition. The maximum force of individual PerL units was, however, on average less than or equal to 50% of that reported for corresponding types of gastrocnemius units.

Effects of cross-innervation and disuse on the twitch speed of fast and slow hindlimb muscles in the decerebellate cat.

In adult cats, we have investigated how nerve cross-union between slow and fast muscle affects twitch contraction and half-relaxation times in the presence and absence of motor disuse, produced by complete decerebellation. Cerebellectomy abolished the slowing of cross-innervated fast muscle and diminished the amount of speeding up of cross-innervated slow muscle. The results are consistent with the idea that cross-innervation between fast and slow muscles affects contraction speed through changes in the patterns of muscle fiber activation.