51 research outputs found
Conduction Properties Distinguish Unmyelinated Sympathetic Efferent Fibers and Unmyelinated Primary Afferent Fibers in the Monkey
Different classes of unmyelinated nerve fibers appear to exhibit distinct conductive properties. We sought a criterion based on conduction properties for distinguishing sympathetic efferents and unmyelinated, primary afferents in peripheral nerves.In anesthetized monkey, centrifugal or centripetal recordings were made from single unmyelinated nerve fibers in the peroneal or sural nerve, and electrical stimuli were applied to either the sciatic nerve or the cutaneous nerve endings, respectively. In centrifugal recordings, electrical stimulation at the sympathetic chain and dorsal root was used to determine the fiber's origin. In centrifugal recordings, sympathetic fibers exhibited absolute speeding of conduction to a single pair of electrical stimuli separated by 50 ms; the second action potential was conducted faster (0.61 0.16%) than the first unconditioned action potential. This was never observed in primary afferents. Following 2 Hz stimulation (3 min), activity-dependent slowing of conduction in the sympathetics (8.6 0.5%) was greater than in one afferent group (6.7 0.5%) but substantially less than in a second afferent group (29.4 1.9%). In centripetal recordings, most mechanically-insensitive fibers also exhibited absolute speeding to twin pulse stimulation. The subset that did not show this absolute speeding was responsive to chemical stimuli (histamine, capsaicin) and likely consists of mechanically-insensitive afferents. During repetitive twin pulse stimulation, mechanosensitive afferents developed speeding, and speeding in sympathetic fibers increased.The presence of absolute speeding provides a criterion by which sympathetic efferents can be differentiated from primary afferents. The differences in conduction properties between sympathetics and afferents likely reflect differential expression of voltage-sensitive ion channels
CO 2 activation on single crystal based ceria and magnesia/ceria model catalysts
Novel multifunctional ceria based materials may show an improved performance
in catalytic processes involving CO2 activation and reforming of
hydrocarbons. Towards a more detailed understanding of the underlying
surface chemistry, we have investigated CO2 activation on single
crystal based ceria and magnesia/ceria model catalysts. All model systems are prepared starting from well-ordered and fully
stoichiometric CeO2(111) films on a Cu(111) substrate. Samples with
different structure, oxidation state and compositions are generated,
including CeO2-x/Cu(111) (reduced), MgO/CeO2-x/Cu(111) (reduced),
mixed MgO-CeO2/Cu(111) (stoichiometric), and mixed
MgO-CeO2-x/Cu(111) (reduced). The morphology of the model surfaces is
characterized by means of scanning tunneling microscopy (STM), whereas the
electronic structure and reactivity is probed by X-ray photoelectron
spectroscopy (XPS). The experimental approach allows us to compare the
reactivity of samples containing different types of Ce3+, Ce4+,
and Mg2+ ions towards CO2 at a sample temperature of 300 K.
Briefly, we detect the formation of two CO2-derived species, namely
carbonate (CO32-) and carboxylate (CO2-) groups, on the
surfaces of all investigated samples after exposure to CO2 at 300 K. In
parallel to formation of the carbonate species, slow partial reoxidation of
reduced CeO2-x/Cu(111) occurs at large doses of CO2. The
reoxidation of the reduced ceria is largely suppressed on MgO-containing
samples. The tendency for reoxidation of Ce3+ to Ce4+ by CO2
decreases with increasing degree of intermixing between MgO and CeO2-x.
Additionally, we have studied the stability of the formed carbonate species
as a function of annealing temperature
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