Differences in Ca 2+ Channels Governing Generation of Miniature and Evoked Excitatory Synaptic Currents in Spinal Laminae I and II

Many neurons of spinal laminae I and II, a region concerned with pain and other somatosensory mechanisms, display frequent miniature "spontaneous" EPSCs (mEPSCs). In a number of instances, mEPSCs occur often enough to influence neuronal excitability. To compare generation of mEPSCs to EPSCs evoked by dorsal root stimulation (DR-EPSCs), various agents affecting neuronal activity and Ca2+ channels were applied to in vitro slice preparations of rodent spinal cord during tight-seal, whole-cell, voltage-clamp recordings from laminae I and II neurons. The AMPA/kainate glutamate receptor antagonist CNQX (10-20 microM) regularly abolished DR-EPSCs. In many neurons CNQX also eliminated mEPSCs; however, in a number of cases a proportion of the mEPSCs were resistant to CNQX suggesting that in these instances different mediators or receptors were also involved. Cd2+ (10-50 microM) blocked evoked EPSCs without suppressing mEPSC occurrence. In contrast, Ni2+ (</=100 microM), a low-threshold Ca2+ channel antagonist, markedly decreased mEPSC frequency while leaving evoked monosynaptic EPSCs little changed. Selective organic antagonists of high-threshold (HVA) Ca2+ channels, nimodipine, omega-Conotoxin GVIA, and Agatoxin IVA partially suppressed DR-EPSCs, however, they had little or no effect on mEPSC frequency. La3+ and mibefradil, agents interfering with low-threshold Ca2+ channels, regularly decreased mEPSC frequency with little effect on fast-evoked EPSCs. Increased [K+]o (5-10 mM) in the superfusion, producing modest depolarizations, consistently increased mEPSC frequency; an increase suppressed by mibefradil but not by HVA Ca2+ channel antagonists. Together these observations indicate that different Ca2+ channels are important for evoked EPSCs and mEPSCs in spinal laminae I and II and implicate a low-threshold type of Ca2+ channel in generation of mEPSCs

Progressive collapse resistance mechanism of RC frame structure considering reinforcement corrosion

Corrosion causes reduction in cross-sectional area of reinforcement, deterioration of mechanical properties, and degradation of bonding properties between reinforced concrete, which are the most important factors leading to the degradation of structural service performance. In order to investigate the progressive collapse mechanism of a corroded reinforced concrete frame structure, the failure modes, characteristics of the vertical displacement, and load capacity are studied using the finite element method. Based on existing experimental research, the established model is verified, and the influence of different influencing factors on the progressive collapse mechanism is analyzed. The results show that the corrosion of the reinforcement affects the yield load, peak load, and ultimate load of the reinforced concrete substructure. As the corrosion rate increases, the tensile arch action shows a particularly severe deterioration. The variation of concrete strength and the height–span ratio affects the substructure’s load-bearing capacity much more significantly than the stirrup spacing