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Biomedical subjects

D M Lewis

Publications and source records attributed to D M Lewis.

At least 163 records · Page 9Linked to original sources
PubMed1974

Dynamic properties of denervated fast and slow twitch muscle of the cat.

1. Isometric and isotonic contractions were recorded from cat flexor digitorum longus (FDL) and soleus muscles denervated for 28-35 days and compared with the responses of contralateral control muscles.2. Isometric twitch time to peak was longer in the denervated muscles than in the controls. Tetanic tension was reduced whether expressed as absolute units, per gram or per unit area. Absolute twitch tensions were reduced less below the control values with a consequent increase of twitch-tetanus ratio. These results agree with earlier reports; but, in contrast, there was no reduction in the relative rate of rise of tension in the isometric tetanus of denervated compared with control FDL. The maximum rate of rise of tension in the twitch was only a little less than that in the fully fused tetanus in the denervated muscles.3. Whole muscle shortening velocity was reduced in the denervated soleus with no change in the number of sarcomeres per fibre. There was a smaller change in FDL muscles. This was the result of an increase in the number of sarcomeres per fibre which followed denervation in FDL, compensating for a decreased velocity of shortening of the sarcomere. Maximum sarcomere shortening velocity was reduced by 30% relative to control values in both muscles.4. Measurements were made of the isometric contractions resulting from pairs of stimuli in an attempt to assess the period of maximal activation in the twitch. Maximum summation occurred when the second stimulus was given close to the time of the peak of the twitch in the denervated muscles.

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↗
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↗
PubMed1972

The effect of denervation on the mechanical and electrical responses of fast and slow mammalian twitch muscle.

1. The soleus (slow twitch), flexors digitorum and hallucis longus muscles (fast twitch) of the cat were denervated. Isometric contractions and electrical responses were examined 2-160 days after the operation.2. In the first week the time course of the twitch and the ratio of tetanus tension to twitch tension were normal in both muscles. The maximum rate of rise of tension in the tetanus was reduced in fast muscles.3. In the second week all the twitches showed a normal contraction phase but relaxations were interrupted by a repetitive after-contraction. This became less marked after longer periods of denervation.4. During the third and subsequent weeks, the contraction and relaxation phases of the twitches in all muscles became slower than normal. These changes were greater in fast muscles which, nevertheless, remained quite distinct from soleus. The ratio of tetanus tension to twitch tension fell below normal. It is suggested that these changes are brought about by more complete activation of the contractile proteins in a twitch. In flexor hallucis longus the rate of rise of tension in isometric tetani was found to be further reduced. No change was found in soleus.5. In extracellular and intracellular records the initial response was a single action potential. An after-discharge occurred in a proportion of fibres during the relaxation phase of the twitch.6. The intracellularly recorded action potential was smaller and had a longer duration than that of normal muscle. Refractory period increased. Conduction velocity decreased. These changes were greater in fast muscle and differences between fast and slow twitch muscle were less marked than in normal muscle. Fibrillation potentials occurred at the same rate in fast and slow muscle.7. The mechanisms of the mechanical changes are discussed and the possible relevance to the question of motor nerve influence on muscle is indicated.

Action Potentials↗
PubMed1972

The effect of muscle length and rate of fusimotor stimulation on the frequency of discharge in primary endings from muscle spindles in the cat.

1. Responses from the primary endings of muscle spindles in the soleus muscle of the cat were recorded during repetitive fusimotor stimulation at a number of different muscle lengths.2. An increase in the rate of stimulation increased the size of both the peak and the plateau of the responses to stimulation of both static and dynamic fusimotor fibres.3. Responses, with the exception of the peak frequency of the discharge during dynamic fusimotor stimulation, increased in size on raising the muscle length up to maximum body length. The peak of the dynamic response reached its highest value at intermediate lengths.4. The effect of increasing stimulation rate and muscle length was to reduce both the latency and time to peak of fusimotor responses. The change in latency with muscle length was particularly dramatic at low stimulus rates.5. In an attempt to compare fusimotor responses with the behaviour of extrafusal muscle fibres, a model is proposed which consists of a mixture of extrafusal tension and rate of change of tension. This model could simulate the static fusimotor responses reported here.

Action Potentials↗