The Experts below are selected from a list of 51 Experts worldwide ranked by ideXlab platform
Peter S. Allen - One of the best experts on this subject based on the ideXlab platform.
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multicomponent water proton transverse relaxation and t2 discriminated water diffusion in myelinated and Nonmyelinated Nerve
Magnetic Resonance Imaging, 1998Co-Authors: Christian Beaulieu, F R Fenrich, Peter S. AllenAbstract:Abstract The influence of compartmental boundaries on water proton transverse relaxation and diffusion measurements was investigated in three distinct excised Nerves, namely, the non-myelinated olfactory Nerve, the Schwann cell myelinated trigeminal Nerve, and the oligodendrocyte myelinated optic Nerve of the garfish. The transverse relaxation decay curves were multiexponential and their decomposition yielded three primary components with T2 values ∼30–50, 150, and 500 ms, which were subsequently assigned to water protons in the myelin, axoplasm, and interaxonal compartments. The short T2 component was absent in the non-myelinated olfactory Nerve, but present in both myelinated Nerves and thus provides supporting evidence for the use of quantitative T2 measurements to measure the degree of myelination. The signal contribution of each T2 component to the apparent diffusion coefficient measurements was varied by incrementing the spin-echo time with a preparatory CPMG train of radiofrequency pulses. The apparent diffusion coefficient and its anisotropy were shown to be independent of the spin-echo time over the range of 70 to 450 ms.
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WATER DIFFUSION IN THE GIANT AXON OF THE SQUID : IMPLICATIONS FOR DIFFUSION-WEIGHTED MRI OF THE NERVOUS SYSTEM
Magnetic resonance in medicine, 1994Co-Authors: Christian Beaulieu, Peter S. AllenAbstract:To clarify the result that marked diffusional anisotropy had been found in Nonmyelinated Nerve, and in completion of an evaluation of the role of all longitudinal axonal structures, we report NMR measurements of water diffusion in the giant axon of the squid, where diffusional anisotropy is determined by the neurofilamentary structure. The diffusion coefficients of water parallel and perpendicular to the long axis of the squid giant axon at 20°C are (1.61 ± 0.06) × 10−5 cm2 s−1 and (1.33 ± 0.09) × 10−5 cm2 s−1, respectively, which yield an anisotropic diffusion ratio of 1.2 2 0.1. Water diffusion in the squid giant axon is therefore quite rapid and nearly isotropic, thus eliminating the possibility of a significant role for the longitudinally oriented neurofilaments in producing diffusional anisotropy within the axoplasm. In conjunction with our work on garfish Nerves therefore, only membranes, either as numerous axonal membranes or as myelin (if present), remain to fulfill the role of the primary determinant of anisotropic water diffusion in Nerve and in white matter.
Christian Beaulieu - One of the best experts on this subject based on the ideXlab platform.
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multicomponent water proton transverse relaxation and t2 discriminated water diffusion in myelinated and Nonmyelinated Nerve
Magnetic Resonance Imaging, 1998Co-Authors: Christian Beaulieu, F R Fenrich, Peter S. AllenAbstract:Abstract The influence of compartmental boundaries on water proton transverse relaxation and diffusion measurements was investigated in three distinct excised Nerves, namely, the non-myelinated olfactory Nerve, the Schwann cell myelinated trigeminal Nerve, and the oligodendrocyte myelinated optic Nerve of the garfish. The transverse relaxation decay curves were multiexponential and their decomposition yielded three primary components with T2 values ∼30–50, 150, and 500 ms, which were subsequently assigned to water protons in the myelin, axoplasm, and interaxonal compartments. The short T2 component was absent in the non-myelinated olfactory Nerve, but present in both myelinated Nerves and thus provides supporting evidence for the use of quantitative T2 measurements to measure the degree of myelination. The signal contribution of each T2 component to the apparent diffusion coefficient measurements was varied by incrementing the spin-echo time with a preparatory CPMG train of radiofrequency pulses. The apparent diffusion coefficient and its anisotropy were shown to be independent of the spin-echo time over the range of 70 to 450 ms.
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WATER DIFFUSION IN THE GIANT AXON OF THE SQUID : IMPLICATIONS FOR DIFFUSION-WEIGHTED MRI OF THE NERVOUS SYSTEM
Magnetic resonance in medicine, 1994Co-Authors: Christian Beaulieu, Peter S. AllenAbstract:To clarify the result that marked diffusional anisotropy had been found in Nonmyelinated Nerve, and in completion of an evaluation of the role of all longitudinal axonal structures, we report NMR measurements of water diffusion in the giant axon of the squid, where diffusional anisotropy is determined by the neurofilamentary structure. The diffusion coefficients of water parallel and perpendicular to the long axis of the squid giant axon at 20°C are (1.61 ± 0.06) × 10−5 cm2 s−1 and (1.33 ± 0.09) × 10−5 cm2 s−1, respectively, which yield an anisotropic diffusion ratio of 1.2 2 0.1. Water diffusion in the squid giant axon is therefore quite rapid and nearly isotropic, thus eliminating the possibility of a significant role for the longitudinally oriented neurofilaments in producing diffusional anisotropy within the axoplasm. In conjunction with our work on garfish Nerves therefore, only membranes, either as numerous axonal membranes or as myelin (if present), remain to fulfill the role of the primary determinant of anisotropic water diffusion in Nerve and in white matter.
M C Schneider - One of the best experts on this subject based on the ideXlab platform.
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mechanism of antinociceptive action of clonidine in Nonmyelinated Nerve fibres
European Journal of Pharmacology, 1999Co-Authors: Francoise Ernebrand, Petr Jirounek, Jurgen Drewe, K F Hampl, M C SchneiderAbstract:Abstract Despite a large body of clinical evidence in favour of a local anesthetic effect of clonidine, the underlying mechanism has not yet been elucidated. In this study we have used the sucrose-gap method to measure the effects of clonidine on the electrophysiological properties of Nonmyelinated Nerve fibers in the rabbit vagus Nerve. The results showed that clonidine enhanced the hyperpolarizing and reduced the depolarizing afterpotential that follow compound action potentials during electrical activity. We showed that summation of these afterpotentials shifts the membrane potential toward more negative values, thus creating a region of low safety conduction, where the local circuit currents might fail to depolarize the axonal membrane to the threshold value needed to open voltage-dependent Na+ channels. Yohimbine did not reverse the inhibitory effects of clonidine on impulse propagation, indicating that the observed effects of clonidine relies on mechanisms not mediated by α2-adrenoceptors.
Ichiji Tasaki - One of the best experts on this subject based on the ideXlab platform.
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a note on the local current associated with the rising phase of a propagating impulse in Nonmyelinated Nerve fibers
Bulletin of Mathematical Biology, 2006Co-Authors: Ichiji TasakiAbstract:To extend our recent paper dealing with the cable properties and the conduction velocity of Nonmyelinated Nerve fibers (Bull. Math. Biol. 64, 1069; 2002), the behavior of the local current associated with the rising phase of a propagating action potential is discussed. It is shown that the process of charging the membrane capacity by means of the local current plays a crucial role in determining the velocity of Nerve conduction. The symmetry of the local current with respect to the boundary between the resting and active regions of the Nerve fiber is emphasized. It is noted that there are several simple quantitative rules governing the intensities of the capacitive, resistive and total membrane currents observed during the rising phase of an action potential.
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on the conduction velocity of Nonmyelinated Nerve fibers
Journal of Integrative Neuroscience, 2004Co-Authors: Ichiji TasakiAbstract:Nerve impulse conduction in Nonmyelinated Nerve fibers is analyzed by considering this process as a direct consequence of the coexistence of two structurally distinct regions, active and resting. Assuming that the active (i.e. swollen) region of the fiber is in direct contact with the resting (i.e. shrunken) region, a simple procedure for deriving the conduction velocity equation is described. The physico-chemical significance of the quantities in this velocity equation is briefly discussed.
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On the cable theory of Nerve conduction
Bulletin of Mathematical Biology, 2002Co-Authors: Ichiji Tasaki, Gen MatsumotoAbstract:Conduction of an impulse in the Nonmyelinated Nerve fiber is treated quantitatively by considering it as a direct consequence of the coexistence of two structurally distinct regions, resting and active, in the fiber. The profile of the electrical potential change induced in the vicinity of the boundary between the two regions is analyzed by using the cable equations. Simple mathematical formulae relating the conduction velocity to the electrical parameters of the fiber are derived from the symmetry of the potential profile at the boundary. The factors that determine the conduction velocity in the myelinated Nerve fiber are reexamined.
Francoise Ernebrand - One of the best experts on this subject based on the ideXlab platform.
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mechanism of antinociceptive action of clonidine in Nonmyelinated Nerve fibres
European Journal of Pharmacology, 1999Co-Authors: Francoise Ernebrand, Petr Jirounek, Jurgen Drewe, K F Hampl, M C SchneiderAbstract:Abstract Despite a large body of clinical evidence in favour of a local anesthetic effect of clonidine, the underlying mechanism has not yet been elucidated. In this study we have used the sucrose-gap method to measure the effects of clonidine on the electrophysiological properties of Nonmyelinated Nerve fibers in the rabbit vagus Nerve. The results showed that clonidine enhanced the hyperpolarizing and reduced the depolarizing afterpotential that follow compound action potentials during electrical activity. We showed that summation of these afterpotentials shifts the membrane potential toward more negative values, thus creating a region of low safety conduction, where the local circuit currents might fail to depolarize the axonal membrane to the threshold value needed to open voltage-dependent Na+ channels. Yohimbine did not reverse the inhibitory effects of clonidine on impulse propagation, indicating that the observed effects of clonidine relies on mechanisms not mediated by α2-adrenoceptors.
