Detection of Neural Action Potentials Using Optical Coherence Tomography: Intensity and Phase Measurements with and without Dyes

We review the use of optical coherence tomography (OCT) for detection of neural activity, and present a new approach for depth-localization of neural action potentials (APs) using voltage-sensitive dyes as contrast agents in OCT. A stained squid giant axon is imaged by spectral-domain OCT. Changes in the intensity and phase of back-scattered light coming from regions around the membrane are measured during AP propagation. The depth-resolved change in back-scattered intensity coincides with the arrival of AP at the measurement area, and is synchronous with the changes in transmitted light intensity and reflection-mode cross-polarized light intensity measured independently. The system also provides depth-resolved phase changes as an additional indication of activity. With further investigation our results could open a new era in functional imaging technology to localize neural activity at different depths in situ

Optical coherence tomography phase measurement of transient changes in squid giant axons during activity

Noncontact optical measurements reveal that transient changes in squid giant axons are associated with action potential propagation and altered under different environmental (i.e., temperature) and physiological (i.e., ionic concentrations) conditions. Using a spectral-domain optical coherence tomography system, which produces real-time cross-sectional images of the axon in a nerve chamber, axonal surfaces along a depth profile are monitored. Differential phase analyses show transient changes around the membrane on a millisecond timescale, and the response is coincident with the arrival of the action potential at the optical measurement area. Cooling the axon slows the electrical and optical responses and increases the magnitude of the transient signals. Increasing the NaCl concentration bathing the axon, whose diameter is decreased in the hypertonic solution, results in significantly larger transient signals during action potential propagation. While monophasic and biphasic behaviors are observed, biphasic behavior dominates the results. The initial phase detected was constant for a single location but alternated for different locations; therefore, these transient signals acquired around the membrane appear to have local characteristics

Exponential Sum-Fitting of Dwell-Time Distributions without Specifying Starting Parameters

AbstractFitting dwell-time distributions with sums of exponentials is widely used to characterize histograms of open- and closed-interval durations recorded from single ion channels, as well as for other physical phenomena. However, it can be difficult to identify the contributing exponential components. Here we extend previous methods of exponential sum-fitting to present a maximum-likelihood approach that consistently detects all significant exponentials without the need for user-specified starting parameters. Instead of searching for exponentials, the fitting starts with a very large number of initial exponentials with logarithmically spaced time constants, so that none are missed. Maximum-likelihood fitting then determines the areas of all the initial exponentials keeping the time constants fixed. In an iterative manner, with refitting after each step, the analysis then removes exponentials with negligible area and combines closely spaced adjacent exponentials, until only those exponentials that make significant contributions to the dwell-time distribution remain. There is no limit on the number of significant exponentials and no starting parameters need be specified. We demonstrate fully automated detection for both experimental and simulated data, as well as for classical exponential-sum-fitting problems

OCT Measurement of Neural Structure and Function

Optical coherence tomography produces high-resolution cross-sectional images of tissue microstructure using backscattered light that allows non-invasive or non-contact applications. These include novel phase sensitive applications that utilize sub-wavelength changes in optical path length. The techniques are promising to image neural structure and functional changes during action potential propagation

Low coherence interferometer for sensing retardance change during neural activity

A variety of optical measurements, including retardance/birefringence change, have revealed transient optical and structural changes associated with action potential propagation. Those changes can be understood better by developing new techniques and improving the current approaches. To detect transient retardance changes in a stimulated nerve, we propose a differential phase technique utilizing two orthogonal polarization channels of a polarization-maintaining fiber based interferometer. The superior sensitivity of the system (10.4 pm) is promising to achieve a non-contact optical measurement of action potential propagation in reflection mode, and to study the transient retardance changes during neural activity

Calcium Influx Mediates the Voltage-Dependence of Sperm Entry into Sea Urchin Eggs

Sperm entry was monitored in voltage-clamped sea urchin eggs following insemination in a variety of artificial seawaters. In regular seawater, maintaining the membrane potential at increasingly negative values progressively inhibits sperm entry. Reducing [Ca2+]o relieves the inhibition, shifting the sperm entry vs voltage relationship toward more negative potentials. Raising [Ca2+]o shifts the relationship in the other direction. Large changes in [Na+]o or [Mg2+]o do not affect sperm entry although changing [Na+]o dramatically changes the currents following sperm attachment. Applying one of seven different calcium channel blockers or replacing Ca2+ with Ba2+ or Sr2+ or microinjecting calcium chelators into the cytoplasm relieves the block to sperm entry at negative potentials. We conclude that the block to sperm entry at negative potentials is mediated by calcium which crosses the membrane and acts at an intracellular site

Optical Measurements on Squid Axons

A combination of two factors makes the squid giant axon a favorable preparation for optical measurements: there is a relatively large membrane area and one can obtain good control of membrane potential. The large membrane area makes for relatively large signal-to-noise ratios in the optical measurements and the ability to control the membrane potential allows one to investigate the physiological origins of the signals in some detail. Using giant axons it is relatively straight-forward to determine if a given optical signal depends on changes in membrane potential, or the membrane currents, or the increases in membrane permeability (Cohen et al.,1968, 1972b)