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During hind limb unloading (HU), the soleus is often in a shortened position and the natural physiological stimulus of muscle spindles is altered, such that muscle spindle activity also changes. Using isolated spindle conditions, the present study investigates the electrophysiological activity and ultrastructure of muscle spindles following HU. Results show that muscle spindle discharges fall into either of two main patterns, single spikes or spike clusters in shortened positions, with a steady frequency of 18-38 spikes/s (mean 29.08 +/- 2.45) in an extended position. Following 14-day HU, afferent discharge activity was significantly altered in soleus muscle spindles. Duration of individual spikes was significantly prolonged, from 0.54 +/- 0.05 ms for control rats to 1.53 +/- 0.25 ms for rats in the HU group. In a shortened position, regular rhythm afferent discharges were obviously depressed, and the majority of muscle spindles became silent, while in an extended position, the discharges remained continuous but with decreased frequency. Results also show that the ultrastructure of muscle spindles experience degenerative changes during HU. Altered muscle spindle afference could possibly modify the activity of motor neurons and further affect the activity of extrafusal fibers.

To determine whether the neuronal activity of the spindle cord in muscle spindle afferent pathways is altered after a period of hindlimb unloading (Hu), and after recovery, we focused on c-Fos-immunoreactivity in the spinal cord evoked by excitation of the muscle spindle in normal gravity and after 3, 7, and 14 days of Hu, and after 14 days of Hu with an additional 2, 5, and 9 days of unrestricted cage activity. High frequency sinusoidal vibration (HFV) applied to the Achilles tendon was used for activation of the muscle spindle. Results showed that c-Fos-immunoreactive neurons evoked by HFV were mainly concentrated in lamina IV-VII of the ipsilateral spinal cord. After 14 days of Hu, the total number of labeled neurons in the spinal cord was significantly increased (P0.05). The number of c-Fos-immunoreactive neurons in lamina IV and VI-VII were also significantly higher than that in normal rats (P0.05). After 9 days of reloading, the total number of c-Fos-immunoreactive neurons and the pattern of their lamellar distribution were both recovered to pre-experimental levels. Our findings suggest that simulated weightlessness induces a large increase in neuronal excitation of the spinal cord in muscle spindle afferent pathways. Weight bearing immediately after Hu could induce completed recovery of excitability of the spinal cord in muscle spindle afferent pathways.

To study the effects of inosine on myosin heavy chain (MHC) activity in rats soleus muscle after tail-suspension.Weightlessness was simulated by tail-suspension of rats. Using immunohistochemistry technique, changes of expression of MHC in intrafusal and extrafusal fibers in soleus muscle were detected.Expression of fast MHC in intrafusal and extrafusal fibers in soleus muscle increased after 14 d of simulated weightlessness, whereas in tail-suspended rats given inosine, the expression of fast MHC was not detected.Inosine could counteract the changes in expression of MHC in intrafusal and extrafusal fibers of rats soleus under simulated weightless situation.

To study the effects of Ligustrazine (Lig) and Radix Astragali (Rad) on activities of myosin adenosine triphosphatase (mATPase) of soleus muscle and atrophy in tail-suspended rat.Weightlessness was simulated by tail suspension in female rats. The activities of mATPase of intrafusal and extrafusal fibres in soleus muscle were detected by method of Ca2(+)-ATPase.1) Compared with tail-suspended (TS) group, the percentage of type II fibres of SOL in both Ligustrazine (Lig) group and Rad group decreased distinctly. Furthermore, the percentage of type I and type II fibres of SOL in Rad group showed no difference with control (CON) group. 2) The type I CSA of Lig group was markedly larger than that in TS group, and there was no difference as compared with that of CON group. CSA of type I, II and average CSA of Rad group were remarkably larger than those in TS group, and there were no difference as compared with those of CON group. 3) mATPase activities of intrafusal fibres in both Lig group and Rad group were approximate to that of CON group.Lig and Rad are both able to effectively prevent muscle atrophy caused by tail suspension, restrain the slow-twitch muscle transform to fast-twitch muscle and control the increase of mATPase activities caused by weightlessness.

To study the effects of high frequency vibration on expression of myosin heavy chain (MHC) in intrafusal and extrafusal fibers in soleus muscles of tail-suspended rats.Weightlessness was simulated by tail suspension of female rats. Using immunohistochemistry technique, changes of expression of MHC in intrafusal and extrafusal fibers of soleus muscles were detected.Expression of fast MHC in intrafusal and extrafusal fibers of soleus muscles increased during 7 d of simulated weightlessness, whereas during 7 d of tail suspension plus high frequency vibration, these changes were not detected.High frequency vibration can counteract the changes in expression of MHC in intrafusal and extrafusal fibers of rats soleus under muscles of rats in simulated weightlessness situation.

To study changes in electrophysiological characteristics of muscle spindles induced by simulated weightlessness in isolated rats soleus.Weightlessness was simulated by tail-suspension in female rats. Using electro-physiological technique, the spontaneous discharge of muscle spindle and its response to ramp-and-hold stretch were observed in isolated soleus of 7 d, 14 d tail-suspended and control rats.Soleus muscle spindles of rat manifest a sharp decrease in spontaneous discharge frequency and the response to ramp-and-hold stretch after 7 d tail-suspension. More significant changes were observed in rats after 14 d tail-suspension.Simulated weightlessness could induce time-related changes in electrophysiological characteristics of soleus muscle spindles in rat.

To examine the myosin heavy chain expression in intrafusal fibers in rat soleus muscle before and after simulated weightlessness and to analyse the mechanism involved in muscle spindle deterioration during simulated weightlessness.The rats were divided into 3 groups: 7 d, 14 d tail-suspension group and control group. Control and experimental spindles were examined using monoclonal antibodies specific for myosin heavy chains of slow-tonic (ALD58) and fast-twitch (MF30) chicken muscles by the immunoperoxidase reaction utilizing the ABC (avidin-biotin-complex) method.The extrafusal muscle fibers did not exhibit immunoreactivity to ALD58 and MF30. Only intrafusal fibers bound these antibodies. The bag 1 fiber and bag 2 fiber reacted to ALD58. The bag 2 fiber and chain fibers stained with MF30. After 7 d suspension the immunoreactivity of the bag 1 fiber and bag 2 fiber to ALD58 decreased, while the immunoreactivity of the bag 2 fiber and chain fibers to MF30 increased. After 14 d suspension the immunoreactivity of the bag 1 fiber and bag 2 fiber to ALD58 decreased remarkably, even disappeared. While all of the intrafusal fibers reacted to MF30 and the immunoreactivity increased obviously.The expression level and patterns of MHC in intrafusal fibers of rat soleus muscle changed after simulated weightlessness.

To study changes in structure and innervation of soleus muscle spindle in rats induced by simulated weightlessness.Weightlessness was simulated by tail suspension in female rats. The technique of Fawosky's Staining was used to detect the morphology of nerve endings of soleus muscle spindle in 7 d, 14 d, 21 d tail-suspended and control rats.The intrafusal muscle fibers were regular and nerve endings were intact and clear in the soleus spindles of control rats. No obvious change was observed in muscle spindles of 7 d tail-suspended rats. Intrafusal muscle fibers became irregular, with rough and vague nerve endings in 14 d tail-suspended rats. The distinct retrograde change of muscle spindles and nerve endings were observed in 21 d tail-suspended rats. Degeneration and rupture were found in nerve endings, which made it unclear and pale in staining.Simulated weightlessness could induce time-related changes in structure and innervation of soleus muscle spindle in rats.

Objective. To analyze the mechanism involved in muscle spindle deterioration during simulated weightlessness. Method. Using the immunoperoxidase reaction utilizing the ABC (avidin-biotin-complex) method, Calbindin D28K-like immunoreactivity (CaBP-LI) of intrafusal fibres in soleus muscle were detected in 7 d, 14 d tail-suspended rats and compared with control rats. Result. The extrafusal muscle fibres and nerve fibres did not exhibit immunoreactivity to CaBP. CaBP-LI was found in some of the intrafusal muscle fibres in all the muscle spindles. After 14 d suspension, the immunoreactivity to CaBP of the intrafusal fibres decreased markedly. Conclusion. Simulated weightlessness could induce changes in the immunoreactivity of rat soleus muscle spindle to CaBP, which may contribute to the deterioration of muscle spindle.

Objective. To explore the mechanisms involved in muscle atrophy and conversion of the fiber types induced by simulated weightlessness. Method. Weightlessness was simulated by tail suspension of female rats. Intrafusal and extrafusal fibers of soleus muscles in the rat were examined histochemically for their activity of acetylcholinesterase (AChE) and succinic dehydrogenase (SDH) in 7 d, 14 d, 21 d tail-suspended groups and control groups. Result. Staining for succinic dehydrogenase showed that simulated weightlessness caused obvious atrophy and change in fiber type composition in soleus muscle, with decrease of the proportion of type I fiber and increase of type II fiber. Acetylcholinesterase activities of intrafusal and extrafusal fibers were both decreased significantly after 21 d tail suspension. Conclusion. Simulated weightlessness could induce decrease of AChE activity in neuromuscular junctions, which might be linked with decrease in motor neuron activity.

To study changes in ultrastructure of soleus muscle spindle induced by simulated weightlessness in rats.Weightlessness was simulated by tail suspension in female rats. The ultrastructure of isolated soleus muscle spindle were observed in 4 d, 7 d, 14 d tail-suspended and 14 d recovered rats.Chaotic myofibril, hyperplasia of mitochondria, and enlargement of terminal cisterna were found in the ultrastructure of isolated soleus muscle spindle in 4 d tail-suspended rats. The ultrastructure of isolated soleus muscle spindle in 7 d tail-suspended rats showed more distinct changes, while that in 14 d tail-suspended rats showed obvious retrograde changes. The ultrastructure of isolated muscle spindle in 14 d recovered rats after 14 d tail-suspension were similar to that of control group.Simulated weightlessness could induce time-related and reversible changes in ultrastructure of soleus muscle spindle in rats.