Yosuke Kawai

Morphological changes in the masseter muscle and its motoneurons during postnatal development.

It has been suggested that the morphological properties of the masseter muscle are changed by the masticatory activity pattern. In the rat, the activity pattern of the muscle alters from sucking to biting around 3 weeks after birth. The working hypothesis in this study is that the unique alteration in masticatory activity has an important influence on the development of the masseter muscle and its motoneurons.

Changes in myosin heavy chain isoform expression of overloaded rat skeletal muscles.

1. The effect of functional overload produced by tenotomy of synergistic gastrocnemius muscle on the expression of myosin heavy chain (MHC) isoforms in the plantaris and soleus muscles of the rat was studied using gradient sodium dodecyl sulfate-acrylamide gel electrophoresis. 2. Five weeks tenotomy, the plantaris and soleus muscle weights induced by tenotomy of the gastrocnemius muscle were 44.3% (P < 0.005) and 37.4% (P < 0.005), respectively, heavier than the contralateral control muscles. 3. Although four types of MHC isoforms were observed in both control and experimental plantaris, the percentage of MHC isoforms in the control and experimental muscles differed; the hypertrophied plantaris muscle contained more HCI (P < 0.05), HCIIa and HCIId (P < 0.05) and less HCIIb (P < 0.05) than the control muscle. 4. The control soleus muscle contained two MHC isoforms, HCI and HCIIa. However, there was only a single HCI isoform in the hypertrophied soleus muscle. 5. These results indicate that overloading a skeletal muscle by removing its synergists produces not only the muscle hypertrophy but also the changes in the expression of MHC isoforms.

Relationship between soma diameter and oxidative enzyme activity of α-motoneurons

Abstract In order to determine whether there is a relationship between soma diameter and oxidative enzyme activity of α-motoneurons, we investigated the α-motoneurons innervating the different portions within a rat gluteus medius muscle. Two fluorescent neuronal tracers, Nuclear yellow and Fast blue, were used for labeling motoneurons innervating the deep (predominance of oxidative fibers) and superficial (predominance of non-oxidative fibers) portions of the muscle. An inverse relationship between soma diameter and oxidative enzyme activity was not seen in the motoneuron pool innervating either the deep or superficial portions. When the two portions were taken together, however, the inverse relationship was seen. The inverse relationship seemed to be demonstrated in a motoneuron pool which intermingled motoneurons innervating oxidative fibers and motoneurons innervating non-oxidative fibers. These results suggest that the oxidative enzyme activity of α-motoneurons is not correlated strictly with their soma size. We consider that the oxidative enzyme activity of α-motoneurons is correlated with oxidative enzyme activity of the muscle unit.

Soma diameter and oxidative enzyme activity of identified α-motoneurons: application of a retrograde fluorescent neuronal tracer

Using a fluorescent compound as a retrograde neuronal tracer for the identification of a specific motoneuron pool, we examined the soma diameter and oxidative enzyme activity of alpha-motoneurons innervating the extensor digitorum longus (EDL) and the soleus (Sol) muscles in the rat. An inverse relationship between soma diameter and oxidative enzyme activity was demonstrated in the EDL, while there was no significant relationship between these variables in the Sol. These results suggest that an inverse relationship between soma diameter and oxidative enzyme activity cannot always be demonstrated in all of the motoneuron pools.

Effect of hypothalamic cooling on the tonic vibration reflex of gastrocnemius muscle in cats

The tonic vibration reflex (TVR) in a medial gastrocnemius muscle (MG) was analyzed in 28 cats anesthetized with pentobarbital sodium while the preoptic-anterior hypothalamic region (PO-AH) was locally cooled or warmed in a thermoneutral environment. Cooling of the PO-AH did not produce shivering or changes in rectal and skin temperatures, but it brought about facilitation (16/28 cats) or inhibition (9/28) of the TVR in the MG. A few cats (3/28) showed inconsistent alteration of the TVR. Warming of the PO-AH did not produce any changes in the TVR of the MG. Histological identification of thermode placements revealed that in cats having the facilitatory response of the TVR, the thermodes were localized at regions caudal to A 14.0 mm and most of the thermodes in those having the inhibitory response were rostral to A 14.0 mm, except for two cases. Bilateral microinjection of a small quantity of anesthetic agent into the medial forebrain bundle (MFB) at the level of the mamillary body abolished the facilitatory effect of PO-AH cooling on the TVR. The results suggest that the spinal motor system is influenced by the PO-AHs sensitivity to local temperature and that the MFB constitutes part of the efferent pathway of the hypothalamic influence.