Abstract
Intrinsic optical properties, such as optical birefringence, may serve as a tool for minimally invasive neuroimaging methods with high spatiotemporal resolution to aid in the study of neuronal activation patterns. To facilitate imaging neuronal activity by sensing dynamic birefringence, temporal characteristics behind the signal must be better understood. We have developed a novel nerve chamber to investigate changes in birefringence at the stimulation site, and at distances ~4-28 mm from that site. Using crustacean nerves with either heterogeneous or homogeneous size distributions of axon diameters, we found that the gradual (slow) recovery of the crossed-polarized signal is not explained by the arrival times of action potentials in smaller axons. Through studying the effects of stimulating current and voltage pulses, we hypothesize that the recovery may be caused by a capacitive-like coupling between firing axons and adjacent tissue structures, and we report data consistent with this hypothesis. This study will aid in the utilization of action-potential-related changes in birefringence to study fast changes in neuronal network activity.
Highlights
The development of minimally invasive, high-resolution modalities for imaging neuronal activity, both for research and pre-clinical applications, is an ongoing endeavor that will allow for a greater understanding of neuronal network processes involved in complex functions, as well as neuropathies such as epilepsy, stroke and cortical spreading depressions associated with migraines [1,2,3,4]
Beyond approximately the range of −2 V to + 2 V, the applied-field effect saturates for both polarities. The results of these experiments further characterize the dynamic birefringence signal associated with both action potential activity and applied electric fields in complex, heterogeneous nerves, and with nerves having narrow axon size distributions, while providing new information regarding the temporal characteristics of the spreading and gradual recovery of the crossed-polarized signal
The data suggest that the gradual recovery of the XPS cannot be explained only by the spread of arrival times of individual axonal action potentials (APs)
Summary
The development of minimally invasive, high-resolution modalities for imaging neuronal activity, both for research and pre-clinical applications, is an ongoing endeavor that will allow for a greater understanding of neuronal network processes involved in complex functions, as. The XPS was found to predominantly be isolated to areas near or through the membrane; and in nerve fibers, an appreciable contribution to static birefringence from Schwann cells and connective tissue was noted [15] This complexity in optical signals caused by contributions from adjacent tissue structures presents a significant barrier to the full understanding of the underlying physiological origins of the XPS in higher-order neuronal structures. Additional information would lead to a more complete understanding of the causes of the XPS in nerves and brain tissues To this end, we carried out a direct comparison of the XPS in two different crustacean species with different axon size distributions (different distributions of action-potential propagation velocities), and we designed a new nerve chamber (see Fig. 1(B)) with a longer viewing window than that in previous reports, which allows for an investigation of nerve XPS at the sites of electrical stimulation and recording (~24 mm from stimulation), as well as a range of distances of ~4-20 mm from the stimulus site. Our results are consistent with others in demonstrating that the birefringence changes in nerves are closely associated with membrane potential, and provide significant novel information that can contribute toward a fuller understanding of the basis of the XPS
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