Multicomponent proton NMR relaxation in nerve

Abstract

The NMR relaxation of water protons in peripheral nerve departs from log-linear decay, and can be viewed as a composite of multiple exponential components. To gain insight into the structural features of nerve which give rise to multicomponent relaxation, the time course of longitudinal (T{dollar}\sb1{dollar}) and transverse (T{dollar}\sb2{dollar}) proton relaxation was studied in a variety of peripheral and cranial nerves.;Ten types of myelinated nerves from different animals were found to have strongly multicomponent relaxation profiles in studies at 20 MHz. As many as four exponential components could be resolved for T{dollar}\sb1{dollar} as well as T{dollar}\sb2{dollar} in the bullfrog sciatic nerve. In contrast, the majority of protons in the garfish olfactory nerve, which is completely unmyelinated, decayed along a simple exponential time course.;Myelin may contribute to multicomponent relaxation by acting as a diffusion barrier to the movement of water between intra-axonal and extracellular compartments. After addition of a paramagnetic manganese chelate to the extracellular space of myelinated bullfrog sciatic nerves, approximately 24% of the water protons proved inaccessible to the paramagnetic label. In unmyelinated garfish nerves, virtually the entire nerve water pool was accessible to externally applied paramagnetic solutions. The 'inaccessible' protons in myelinated nerves were shown to be predominantly intra-axonal in origin, and could be subdivided into at least two groups on the basis of their relaxation properties.;The physical mechanisms underlying proton relaxation in nerve were explored using two additional NMR techniques, field-cycling relaxometry and selective excitation. Relaxometry studies corroborated that at least two relaxation components can be distinguished in the bullfrog sciatic nerve. Low-field anomalies in the frequency dispersion of 1/T{dollar}\sb1{dollar} were also revealed, which may represent a hitherto undescribed proton-proton dipolar interaction occurring in nerve. Selective excitation experiments established the presence of a significant cross relaxation effect in the bullfrog sciatic nerve.;To assess whether demyelination alters the multicomponent relaxation properties of myelinated nerves, we examined sciatic nerves from rabbits afflicted with Experimental Allergic Neuritis. Preliminary studies suggest that inflammatory changes affect fast and intermediate relaxation components, while demyelination alters the slowest relaxing proton components as well. (Abstract shortened with permission of author.).