Biomedical subjects

Arthur W English

Publications and source records attributed to Arthur W English.

4 recordsLinked to original sources

PubMed2003

Sex differences in rabbit masseter motoneuron firing behavior.

To evaluate whether sex differences in the proportions of fibers of different phenotypes in the masseter muscle might be the result of differences in the behavior of their motoneurons, we studied the firing patterns of masseter motoneurons in adult male and female rabbits. Activity in individual motoneurons was determined from high spatial resolution EMG recordings made during cortically evoked rhythmic activation of the masticatory muscles. Although some motoneurons could be said to fire according to slow-tonic or fast-phasic patterns, most did not. In both sexes a substantial range of median firing rates and median firing durations was found. In adult males, masseter motoneurons fired more rapidly than those recorded from adult females. No significant sex differences in motoneuron firing duration were found. These results are consistent with the hypothesis that androgen-induced differences in rabbit masseter muscle fiber phenotype are a reflection of differences in motoneuron firing rate. Whether this effect of androgen is directly upon the motoneurons or is the result of a response of muscle fibers to androgen remains to be investigated.

Action Potentials↗

PubMed2003

Sex differences in rabbit masseter muscle function.

The proportions of fibers of different phenotypes in the rabbit masseter muscle differ strikingly in adult males and females. Muscles from females contain similar proportions of small fibers that express both the slow/beta and cardiac alpha myosin heavy chain (MyHC) isoforms and larger fibers containing the IIa MyHC isoform. In muscles from males, nearly 80% of fibers are of the IIa phenotype. To evaluate the functional significance of these sex differences, we used finely graded intramuscular microstimulation to study the contractile properties of masseter motor units in >6-month-old male and female rabbits. Twitch forces and torques in males were significantly greater in magnitude than those of females. Greater proportions of units that produced larger forces/torques were encountered in the males. The same motor units produced force or torque more rapidly in males than in females, principally because units in which twitch rise times were >22 ms were found only in females. The forces applied to the mandible and the torques generated about the right mandibular condyle were studied during cortically evoked rhythmic activation of the masticatory muscles. The overall range of torque rise times and the magnitudes of the peak torques did not differ between sexes. The mean rise time was significantly shorter and the mean peak torque was significantly greater in males. We conclude that sex differences in fiber phenotype proportions are reflected in sex differences in motor unit properties and in the function of these motor units during rhythmic activation.

Animals↗

PubMed2003

Neurotrophin 4/5 is required for the normal development of the slow muscle fiber phenotype in the rat soleus.

During normal postnatal development, rat soleus (SOL) muscle fibers undergo a dramatic fast-to-slow myosin heavy chain (MyHC) isoform transformation. We exploited this phenomenon to evaluate the role of neurotrophin 4/5 (NT-4/5) in the regulation of muscle fiber phenotype. Intramuscular injections of recombinant NT-4/5 into the SOL muscle of rat neonates significantly accelerated the normal fast-to-slow MyHC isoform transformation. Sequestration of endogenous NT-4/5 with TrkB-IgG prevented this transformation from occurring. Administration of the other TrkB ligand, brain-derived neurotrophic factor (BDNF), did not affect the normal course of the MyHC isoform transformation in this muscle, indicating that the observed effect is NT-4/5 specific. Botulinum toxin blockade of synaptic transmission significantly disrupted the normal fast-to-slow MyHC isoform switch. Because administration of NT-4/5 to paralyzed muscles failed to restore the normal course of this MyHC transformation, we believe that the effect of NT-4/5 is not directly on the muscle fibers but that it probably activates or forms a type of retrograde signal to motoneurons. The developmental upregulation of NT-4/5 mRNA in rat SOL muscle fibers occurred earlier than the upregulation of MyHC I/b mRNA associated with muscle fiber transformation. This timing is consistent with the idea that NT-4/5 is involved in early events that lead to the upregulation of the slow MyHC isoform in this muscle.

Animals↗

PubMed2002

Development of sex differences in the rabbit masseter muscle is not restricted to a critical period.

The proportions of muscle fibers of different phenotype in the adult rabbit masseter differ greatly in different sexes. These sex differences are not apparent in young adults, but arise under the influence of testosterone in the males. We examined whether this switch occurred during a critical period of postnatal development. Testosterone was administered to young adults 1, 2, or 4 mo after castration, and also to adult females. Samples of masseter muscle were taken at four monthly intervals after the onset of treatment and examined for the expression of different myosin heavy chain (MyHC) isoforms by using a panel of monoclonal antibodies. Despite the length of androgen deprivation, treatment with testosterone produced a marked MyHC isoform switch from alpha-slow/beta to IIa. This male proportion of fibers of different phenotypes persisted well beyond the return of serum testosterone levels to pretreatment levels. Thus brief exposure to testosterone produces a permanent change in the proportions of masseter muscle fibers of different phenotypes, and the capacity for this change is not restricted to a critical period.

Animals↗