THE ROLE OF BRAIN-STEM DISCENDING INHIBITORY SYSTEM IN THE ANTINOCICEPTIVE EFFECT ELICITED BY MUSCLE SPINDLE AFFERENTS

Objective To analyse the antinociceptive effect of muscle spindle afferents and the involved mechanism.Methods The single unit of wide dynamic range neurons in the spinal cord dorsal horn were recorded extracelluarly.The effects of muscle spindle afferents elicited by intravenous administration of succinylcholine on nociceptive responses (C fibres evoked responses,C responses) of WDR neurons were observed before and after bilateral lesions of ventrolateral periaqueduct gray .And the effects of muscle spindle afferents on the spontaneous discharge of the tail flick related cell in the rostral ventro medial medulla and on the spontaneous discharge of the PAG neurons were observed.Results The C responses of WDR neurons were significantly inhibited by muscle spindle afferents,and the inhibitory effects were reduced by bilateral lesions of ventrolateral PAG.The spontaneous discharge of the off cell in the RVM was excited while the on cell was inhibited by intravenous administration of Sch.The spontaneous discharge of the PAG neurons were excited by muscle spindle afferents.Conclusion Muscle spindle afferents show a distinct effect of antinociception.PAG RVM descending inhibitory system may play an important role in this nociceptive modulative mechanism.

THE MORPHOLOGICAL CHANGES IN MUSCLE SPINDLES AND ALTERATIONS IN CELL ACTIVITY OF THE RATS' RED NUCLEUS AFTER 2 WEEKS' SIMULATED WEIGHTLESSNESS

Objective To study the morphological changes of soleus muscle spindle and electrical activity of neurons in Red Nucleus(RN) of the rat after 2 weeks’ simulated weightlessness, and to reveal the interaction between proprioceptive inputs of muscle spindles and reciprocal alterations in RN under simulated weightlessness. Methods Twenty female rats were exposed to weightlessness simulated by tail-suspension for 14 days (SW-14d). Body weight(200-220g) matched female rats were control group(Con). The morphological changes in isolated muscle spindle of soleus muscle, the discharges of red nucleus neurons were observed after 14d tail-suspensions by silver staining and extracellular recording respectively. Results Compared with control group ,the nerve ending of muscle spindle in SW-14d was distorted, degenerated and dissolved; the diameters of intrafusal fibers and capsule in equatorial region of soleus muscle spindles were diminished(P<0.05). The spontaneous cell activity and discharge of RN neurons (spikes/s) induced by afferent firing from muscle spindles after injection of succinylcholine were reduced after 2 weeks’ simulated weightlessness respectively (18.44±5.96 vs. 10.19±6.88, 32.50±8.08 vs. 16.86±5.97, P<0.01). Conclusion The degeneration of muscle spindle induced by simulated weightlessness may be one of the causes that led to alterations in discharges of RN.

THE ROLE OF NUCLEUS RAPHE MAGNUS IN THE ANTINOCICEPTIVE EFFECT OF MUSCLE SPINDLE AFFERENTS IN THE RAT

Objective To investigate the role of NRM in the antinociceptive effect of muscle spindle afferents,the influence of NRM lesion on the inhibitory effect of muscle spindle afferents on the nociceptive responses of wide dynamic range (WDR) neurons and the effects of the muscle spindle afferents on the NRM neuronal activities were observed. Methods The single units of WDR neurons in the spinal dorsal horn were recorded extracellularly, and the inhibitory effects of activating muscle spindle afferents by intravenous administration of succinylcholine (SCH) on the C fibers evoked responses (C responses) of WDR neurons were tested before and after lesion of NRM.The effects of the muscle spindle afferents activated by administrating SCH on the single NRM neurons were also examined.Results ①It was found that the C responses of WDR neurons were significantly inhibited by intravenously administration of SCH, and the inhibitory effect was reduced after lesion of NRM;②The activities of most of the NRM neurons could be changed significantly by administrating SCH. According to their responses, NRM neurons could be classified into three types:excitatory, inhibitory and non responsive neurons, and the responses were dose dependent. Conclusion These results suggest that the muscle spindle afferents evoked by SCH may activate the NRM neurons, which plays an important role in the antinociception of muscle spindle afferents.