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Biomedical subjects

J Bagust

Publications and source records attributed to J Bagust.

51 records · Page 3Linked to original sources
PubMed1981

Motor units in cross-reinnervated fast and slow twitch muscle of the cat.

1. Isometric contractile properties of motor units were measured in cross-reinnervated fast (flexor digitorum longus) and slow (soleus) twitch muscles of the cat. All but one cross was at least 95% pure. 2. There was a reduction in the number of motor units in all muscles, but totals remained about equal in cross-reinnervated soleus and flexor digitorum longus. 3. Motor unit tensions (mean and maximum values) were higher in cross-reinnervated soleus than in cross-reinnervated flexor digitorum longus, reversing the differences between normal muscles. This was due to increases in muscle mass and in the tension developed per unit cross-sectional area. There were motor unit tensions larger and smaller than those seen in normal muscle, but the range was comparable with that seen in self-reinnervated muscle. 4. The changes in twitch time to peak of whole muscle following cross-reinnervations resulted from a change over the whole range of motor units. The conversion of soleus was less complete than that of flexor digitorum longus, and the time to peak of its fastest motor unit was twice as long as any seen in normal flexor digitorum longus. 5. In neither of the cross-reinnervated muscles were the fast contracting motor units larger than the slow contracting ones, and in cross-reinnervated soleus they were smaller. 6. Axonal conduction velocity was correlated with motor unit tension in both muscles and with twitch time to peak in cross-reinnervated flexor digitorum longus, but in all cases less clearly than in normal muscles. 7. The ratio of twitch to tetanic tension increased with increasing twitch time to peak, as in normal muscles.

Animals↗
PubMed1979

The effects of tenotomy upon the contraction characteristics of motor units in rabbit soleus muscle.

1. Isometric contractions of tenotomized rabbit solei have been compared with a group of unoperated control muscles. A large decrease in the tension developed by the tenotomized muscle was accompanied by a slight shortening of the twitch contraction time, and a larger, progressive, reduction in the time to half relaxation.2. A comparison of motor units obtained from muscles which had been tenotomized for six weeks with those of the control muscles showed a large reduction in the range of contraction and half relaxation times of the units from the tenotomized muscles.3. The mean motor unit tension (expressed as a percentage of the whole muscle tension) was similar for both those units from the control muscles and those from muscles which had been tenotomized for six weeks, indicating a uniform atrophy of motor units within the tenotomized muscles.4. It was concluded that the change in the pattern of motor unit contraction times was not the result of a process of differential atrophy favouring the preservation of the faster contracting motor units.5. A correlation between axon conduction velocity and both the speed of contraction and the size (tension) of the motor units was demonstrated in the control muscles. Following tenotomy the relationship between axon conduction velocity and motor unit tension was lost.

Animals↗
PubMed1974

The properties of motor units in a fast and a slow twitch muscle during post-natal development in the kitten.

1. Motor units were functionally isolated from a fast and a slow twitch muscle of kittens: flexor digitorum longus (FDL) and soleus. Both muscles were examined in two groups: one aged about 2 weeks and the other 6 weeks.2. A clear division into alpha and gamma axons could be made. The mean conduction velocity of alpha axons from FDL was higher than that of soleus axons at both ages.3. Tetanic tensions were expressed as a percentage of whole muscle tension. The distributions of motor unit tensions were symmetrical in soleus muscles but were skewed towards large values in FDL. In both types of muscle the scatter of motor unit tensions was greater at 2 weeks than at 6, the mean percentage tension also fell with age.4. The motor unit twitch times to peak were symmetrically distributed in soleus but skewed towards large values in FDL. The variance was less than in adults.5. Motor unit axonal conduction velocity was related to tetanic tension and twitch speed. The relationship between twitch time to peak and the ratio of twitch to tetanic tension varied with age but was not seen so clearly.

Action Potentials↗
PubMed1974

Isometric contractions of motor units in self-reinnervated fast and slow twitch muscles of the cat.

1. Isometric contractions of motor units have been studied in self-reinnervated soleus and flexor digitorum longus muscles, the motor nerves of which had been transected 6 months earlier.2. The distributions of motor unit tetanic tensions in both muscles were skewed towards large values and showed greater variance than normal: there were motor units which produced more tension and others which developed less tension than in control muscles. Mean tensions of motor units are compared with those of the controls.3. The variance of times to peak of motor units in both muscles was significantly smaller than normal.4. Conduction velocities of axons were slower than normal above the point of nerve section. A number of axons did not functionally cross this point and those that did conducted slowly below the neuroma.5. Many of the correlations seen in normal muscle between axon conduction velocity, motor unit tetanic tension, twitch time to peak and ratio of twitch to tetanic tension were demonstrated in reinnervated muscle.6. We suggest that the pattern of motor unit organization in self-reinnervated muscle can in part be due to random regrowth of axons but is largely produced by specific processes related to axon size which are not identical in fast and slow muscle.

Animals↗
PubMed1974

Post-tetanic effects in motor units of fast and slow twitch muscle of the cat.

1. Motor unit twitches were examined in cat flexor digitorum longus (fast twitch) and soleus (slow twitch) muscles. The time course of the effects of a standard tetanus on the peak twitch tension was plotted and the maximal potentiation or depression (the post-tetanic ratio) was measured.2. The post-tetanic ratio decreased continuously as the twitch time to peak of motor units increased; motor units from flexor digitorum longus and soleus could be described as a single population. The closest approximation to a linear relationship was found by plotting post-tetanic ratio against the reciprocal of time to peak.3. The post-tetanic ratio was also related to the ratio of tetanic to twitch tension. The time of maximum potentiation or depression occurred between 1 and 11 sec, but this variable was unrelated to the time to peak or any other characteristic of the motor unit.

Animals↗
PubMed1974

Relationships between motor nerve conduction velocities and motor unit contraction characteristics in a slow twitch muscle of the cat.

1. Isometric contractions of cat soleus muscles and of functionally isolated motor units have been measured under conditions similar to those of Bagust, Knott, Lewis, Luck & Westerman (1973).2. The muscle lengths at which motor unit twitch and tetanus tensions were maximal were close to the optimal lengths of the parent muscles.3. Motor unit contraction times and tensions were shown to be related to the conduction velocities of the nerve axons supplying them. These relationships were obscured when a number of small samples from several experiments were grouped together.4. The ratios of the twitch to the tetanic tensions of the motor units were directly related to their times to peak tension.

Animals↗
PubMed1973

Polyneuronal innervation of kitten skeletal muscle.

1. Isometric contractions of kitten soleus and flexor hallucis longus (FHL) muscles have been examined for evidence of polyneuronal innervation. The sum of tetanic tensions of two almost equal divisions of the ventral roots was greater than the tetanic tension elicited by stimulating both the divisions simultaneously. The difference was large in kittens aged about 3 days, was less at 2 weeks and was small or absent at 6 weeks.2. A tetanus elicited from one division of the ventral root potentiated a twitch elicited from the other root division. The time course of this potentiation was similar to that of post-tetanic potentiation induced from the same root from which the twitch was elicited.3. It is concluded that polyneuronal innervation exists in the kitten limb muscles.4. The observed degree of tension excess was less if the divisions of the root were not equal. The observations were compatible with a model which assumed a random distribution of nerve axons to the muscle fibres.

Animals↗
PubMed1973

Isometric contractions of motor units in a fast twitch muscle of the cat.

1. Isosmetric contractions of cat flexor digitorum longus whole muscles and of functionally isolated motor units have been measured under conditions similar to those used by Buller & Lewis (1965a).2. Motor unit twitch time to peak was inversely related to axonal conduction velocity. The logarithm of tetanic tension was directly related to conduction velocity. These relationships suggest that each motoneurone has an influence on the muscle fibres which it innervates.3. The ratio of twitch to tetanic tension was directly related to the time to peak of the motor unit. This fact might be explained by variation between motor units of the duration of ;active state'.4. The muscle length at which tension was maximal varied between motor units and the optima were found over the range of muscle lengths which could occur in the body. Slow motor units had longer optimal lengths.5. The sample of motor units was considered to be unbiased because the distribution of axon conduction velocities was compatible with reported motor fibre diameter spectra of the muscle nerve. The mean motor unit tetanic tension gave a reasonable estimate of the number of alpha-motor axons in the muscle nerve. Twitch tensions gave a value that was 40% higher.6. Motor unit and whole muscle data were in good agreement for length-tetanus tension curves, for times to peak and for twitch-tetanus ratios at long muscle lengths.

Action Potentials↗