ABSTRACT
Spatial displacements of spins between radio frequency pulses in a Double-Quantum (DQ) nuclear magnetic resonance pulse sequence generate additional terms in the effective DQ Hamiltonian. We derive a simple expression that allows the estimation and control of these contributions to the initial rise of the DQ build up function by variation of experimental parameters in systems performing anomalous diffusion. The application of polymers is discussed.
ABSTRACT
A self-consistent approximation beyond the Redfield limit and without using the Anderson-Weiss approximation for the Free Induction Decay (FID) of deuteron spins belonging to polymer chains undergoing reptation is formulated. The dynamical heterogeneity of the polymer segments created by the end segments is taken into account. Within an accuracy of slow-changing logarithmic factors, FID can be qualitatively described by a transition from an initial pseudo-Gaussian to a stretched-exponential decay at long times. With an increase in observation time, the contribution from end effects to the FID increases. In the regime of incoherent reptation, contributions to the FID from central segments yield an exponent of 1/4 for the stretched decay and contributions from end segments yield an exponent of 3/16. In the regime of coherent reptation, the central segments generate a stretching exponent of 1/2, whereas the end segments contribute with an exponent of 1/4. These predictions are shown to be in qualitative agreement with the experimental FIDs of perdeuterated poly(ethylene oxide) with molecular masses of 132 kg/mol and 862 kg/mol.
ABSTRACT
A model of the postsynaptic current generation in response to a release of the quantum mediator from the nerve terminal is suggested. In its terms the law of the current density attenuation is determined as j = I/rb, where I is the current density in the site of generation, while j--current densities at the distance r from the site of generation. Experiments with extracellular recording have shown that coefficient b equals approximately 1. Assuming that sites of the quantum release and a site of the postsynaptic current generation are spatially identical, the new method is suggested to determine coordinates of the transmitter release sites in the motor nerve terminal. This method consists in the measuring of a uniquantal signal amplitude by three extracellular microelectrodes, arranged at a distance of 5-10 microns from each other. The construction of spatial pictures of the transmitter secretion on the basis of the analysis of several hundreds of signals in the cutaneous pectoris frog muscle has shown that release sites are organized in groups transversal to the nerve terminal. It is supposed that these groups of sites reflect the transmitter secretion in the active zones of the nerve ending. Advantages, shortcomings and errors of the method are shown.
Subject(s)
Nerve Endings/metabolism , Neurotransmitter Agents/metabolism , Animals , In Vitro Techniques , Mathematics , Membrane Potentials/physiology , Microelectrodes , Microscopy, Interference , Microscopy, Polarization , Models, Neurological , Motor Endplate/physiology , Neuromuscular Junction/physiology , Rana ridibunda , Receptors, Neurotransmitter/physiologyABSTRACT
The topography of transmitter release sites at the motor-nerve terminal of the cutaneous-pectoris frog muscle has been determined using three extracellular electrodes. It is shown that release sites are united in groups arranged transversally to the nerve endings and reflecting the transmitter release in the active zones (AZ) of the nerve terminal. The quantitative analysis of revealed groups has permitted concluding that the maximal level of secretion is at the centre of AZ, decreasing to the edge and aside from AZ. At the low extracellular Ca2+ concentration all the AZ take part in the spontaneous release process, while in the evoked one--only some of AZ. Advantages of the three-microelectrode method over the two-microelectrode one are analyzed. It is found that the transmitter secretion in spatially isolated AZ leads to the polymodality in uniquantal signal amplitude distribution at extracellular recording. The role of AZ in the transmitter release process is discussed.
