Different Sensitivity of action potential generation to the rate of depolarization in vagal afferent A-fiber versus C-fiber neurons

The vagal afferent nerves innervate the visceral organs and convey sensory information from the internal environment to the central nervous system. A better understanding of the mechanisms controlling the activation of vagal afferent neurons bears physiological and pathological significance. Although it is generally believed that the magnitude and the rising rate of membrane depolarization are both critical for the action potential generation, no direct or quantitative evidence has been documented so far for the sensitivity of vagal afferent neuron activation to the rate of depolarization and for its underlying ionic mechanisms. Here, by measuring the response of mouse nodose neurons to the suprathreshold current stimuli of varying rising rates, the slowest depolarization capable of evoking action potentials, the rate-of-depolarization threshold (dV/dtthreshold), was determined and found to be ~20 fold higher in the A-fiber neurons compared to the C-fiber neurons classified based on the capsaicin responsiveness and characteristics of action potential waveforms. Moreover, although the dV/dtthreshold varied substantially among individual neurons it was not different in any one neuron in response to different intensities of current stimuli. Finally, inhibition of low-threshold activated D-type potassium current (IK.D) by α-dendrotoxin or low concentration of 4-aminopyrydine nearly abrogated the sensitivity of action potential generation to the depolarization rate. Thus, the depolarization rate is an important independent factor contributing to the control of action potential discharge, which is particularly effective in the vagal afferent A-fiber neurons. The IK.D channel may regulate the excitability of vagal sensory neurons by setting the dV/dtthreshold for action potential discharge.

Mechanical and Electrophysiological Properties of the Sarcolemma of Muscle Fibers in Two Murine Models of Muscle Dystrophy: Col6a1−/−and Mdx

This study aimed to analyse the sarcolemma of Col6a1−/−fibers in comparison with wild type and mdx fibers, taken as positive control in view of the known structural and functional alterations of their membranes. Structural and mechanical properties were studied in single muscle fibers prepared from FDB muscle using atomic force microscopy (AFM) and conventional electrophysiological techniques to measure ionic conductance and capacitance. While the sarcolemma topography was preserved in both types of dystrophic fibers, membrane elasticity was significantly reduced in Col6a1−/−and increased in mdx fibers. In the membrane of Col6a1−/−fibers ionic conductance was increased likely due to an increased leakage, whereas capacitance was reduced, and the action potential (ap) depolarization rate was reduced. The picture emerging from experiments on fibers in culture was consistent with that obtained on intact freshly dissected muscle. Mdx fibers in culture showed a reduction of both membrane conductance and capacitance. In contrast, in mdx intact FDB muscle resting conductance was increased while resting potential and ap depolarization rate were reduced, likely indicating the presence of a consistent population of severely altered fibers which disappear during the culture preparation.

Magnetism of Multi-Layer HgBa2Ca4Cu5Oy Superconductor Studied by μSR Measurements

μSR measurements were performed on HgBa 2 Ca 4 Cu 5 O y (Hg-1245) samples with Tc of 108 K. Rapid enhancement of muon-spin depolarization rate and losses of initial asymmetry were found in ZF-μSR time spectra below 60 K. These indicate enhancement of magnetic correlation between Cu spins. Moreover, muons precession signals were observed below 45 K, indicating that the Hg-1245 samples were in long range ordered state. These results reveal that antiferromagnetic order is developing in the Hg-1245 superconductor below Tc.

Postnatal Changes in Membrane Properties of Mice Trigeminal Ganglion Neurons

Intracellular recordings from neurons in the mouse trigeminal ganglion (TG) in vitro were used to characterize changes in membrane properties that take place from early postnatal stages (P0–P7) to adulthood (>P21). All neonatal TG neurons had uniformly slow conduction velocities, whereas adult neurons could be separated according to their conduction velocity into Aδ and C neurons. Based on the presence or absence of a marked inflection or hump in the repolarization phase of the action potential (AP), neonatal neurons were divided into S- (slow) and F-type (fast) neurons. Their passive and subthreshold properties (resting membrane potential, input resistance, membrane capacitance, and inward rectification) were nearly identical, but they showed marked differences in AP amplitude, AP overshoot, AP duration, rate of AP depolarization, rate of AP repolarization, and afterhyperpolarization (AHP) duration. Adult TG neurons also segregated into S- and F-type groups. Differences in their mean AP amplitude, AP overshoot, AP duration, rate of AP depolarization, rate of AP repolarization, and AHP duration were also prominent. In addition, axons of 90% of F-type neurons and 60% of S-type neurons became faster conducting in their central and peripheral branch, suggestive of axonal myelination. The proportion of S- and F-type neurons did not vary during postnatal development, suggesting that these phenotypes were established early in development. Membrane properties of both types of TG neurons evolved differently during postnatal development. The nature of many of these changes was linked to the process of myelination. Thus myelination was accompanied by a decrease in AP duration, input resistance ( R in), and increase in membrane capacitance (C). These properties remained constant in unmyelinated neurons (both F- and S-type). In adult TG, all F-type neurons with inward rectification were also fast-conducting Aδ, suggesting that those F-type neurons showing inward rectification at birth will evolve to F-type Aδ neurons with age. The percentage of F-type neurons showing inward rectification also increased with age. Both F- and S-type neurons displayed changes in the sensitivity of the AP to reductions in extracellular Ca2+ or substitution with Co2+ during the process of maturation.