Akihiko Ishihara

Effects of ageing on the total number of muscle fibers and motoneurons of the tibialis anterior and soleus muscles in the rat

The age-related changes in the total number of muscle fibers and motoneurons of the tibialis anterior and soleus muscles were studied using 10-, 65-, and 135-week-old rats. The number of fast twitch muscle fibers was decreased at age 65 weeks, while the numbers of slow twitch fibers and of alpha motoneurons were decreased only later, at age 135 weeks. Therefore, the degenerative process of muscle fibers differs with the fiber type.

Soma size and oxidative enzyme activity of motoneurons supplying the fast twitch and slow twitch muscles in the rat

The relationship between soma size and oxidative enzyme activity of motoneurones supplying the fast twitch muscle and the slow twitch muscle was examined. Horseradish peroxidase was injected into the extensor digitorum longus (fast twitch) muscle and the soleus (slow twitch) muscle to retrogradely label corresponding motoneurones of the rat spinal cord. There was a negative relation between soma size and oxidative enzyme activity of motoneurones in a particular neurone pool. The alpha motoneurones supplying the slow twitch muscle had higher oxidative enzyme activities than identical size motoneurones supplying the fast twitch muscle. The present results suggest that there is a difference between oxidative capacities of fast twitch and slow twitch neurone pools.

Effects of age on the number and histochemical properties of muscle fibers and motoneurons in the rat extensor digitorum longus muscle

The number and histochemical properties of muscle fibers and motoneurons were studied in the extensor digitorum longus muscle in female albino rats at the ages of 10, 60 and 120 weeks. The number of fast twitch glycolytic (FG) fibers was decreased at 60 weeks, while that of fast twitch oxidative glycolytic (FOG) fibers was decreased at 120 weeks. The number and oxidative enzyme activity of motoneurons were decreased later at 120 weeks. Thus, it is suggested that the decrease in FG fibers at 60 weeks was due to selective muscle fiber atrophy or to degeneration of neuromuscular junctions, while on the other hand, the decrease in FOG fibers at 120 weeks was due mainly to both a decrease in motoneurons and a type shift of fibers from FOG to FG.

Oxidative metabolism of the rat soleus neuron pool following hypobaric hypoxia

After 7 weeks of hypoxic acclimation, oxidative enzyme (succinate dehydrogenase) activity of motoneurons in the rat soleus neuron pool was examined. Although the total oxidative metabolism of the neuron pool was not changed, the oxidative enzyme activity of small to medium sized (25-45 microns average soma diameter) alpha motoneurons was increased. These results indicate that hypoxia preferentially enhances the oxidative capacity of specific motoneurons in the neuron pool.

Menadione-linked α-glycerophosphate dehydrogenase activity of motoneurons in rat soleus and extensor digitorum longus neuron pools

After injection of horseradish peroxidase into the soleus (slow twitch) and extensor digitorum longus (fast twitch) muscles, glycolytic enzyme activity as reflected by α-glycerophosphate dehydrogenase activity of labeled motoneurons in the neuron pool was examined. No differences were found in glycolytic enzyme activity of motoneurons between slow twitch and fast twitch neuron pools.

Changes in Central Somatosensory Pathways Accompanying Reaction Movements

The changes within central somatosensory pathways accompanying reaction movements were examined by subtracting the peak latency of the major response from the cervical area (14 msec.) from that of the primary cortical response (20 msec.) and also measuring the CNV which is dependent on the direction of the subjects attention and level of arousal. These parameters were measured before (at rest), during fast and slow reaction movements, and mental readiness to act. Analysis showed that the difference in N20-N14 potential latencies is the shortest during fast reaction movements and is shorter during movement tasks than during mental readiness and intention to act. Moreover, we confirmed that the CNV amplitudes are significantly higher during fast than during slow reaction movements and also that the difference in N20-N14 potential latencies decreases as the CNV amplitude increases. Accordingly, from the present results and a series of studies conducted by others, it may be concluded that not only are the changes in the central motor area confirmed by the movement-associated cerebral potentials but also those in the central somatosensory area occur prior to reaction movements.