Biomedical Engineering Reference
In-Depth Information
10
Imaging Membrane
Potential with SHG
10.1 Introduction: Membrane Potential Measurements
with Imaging................................................................................... 229
10.2 SHG: Theoretical Background ........................................................230
10.3 SHG Voltage Chromophores...........................................................230
10.4 Experimental Setup ..........................................................................231
General Description of the SHG Setup • Imaging Electrical
Signals with SHG
10.5 Applications.......................................................................................233
SHG Imaging of Somatic Voltages • SHG Imaging of
Dendritic Spines • SHG Imaging of Axons
10.6 Conclusions and Perspectives.........................................................238
Applications • Genetic Approaches • Hardware Devices
References......................................................................................................240
Mutsuo Nuriya
Keio University
Rafael Yuste
Columbia University
10.1 introduction: Membrane Potential Measurements
with imaging
The functioning of the central nervous system is arguably one of the major challenges in contemporary
science. After more than a century of neuroscience research, it is clear that neurons are the fundamental
units of the nervous system and that they integrate electrical signals, that is, changes in the membrane
voltage, and in turn communicate with other neurons by generating electrical action potentials. Thus,
methods that can measure these electrical input and output properties are crucial to the understanding
of how the central nervous system functions.
The traditional method to investigate the electrical properties of neurons is to insert microelectrodes
into neurons, or perform whole-cell recordings, or use extracellular probes, to measure the membrane
potential and its change on receiving stimuli. Although electrical recordings remain the work horse of
functional studies in neuroscience, they are not usable to probe small (<1 μm) neuronal structures or to
record the activity of assemblies of hundreds or thousands of neurons. As an example of their limitation,
it is not possible at this time to measure the membrane potential of dendritic spines, small structures
(~1 fl in volume), which are of special significance because they are the major sites of excitatory inputs
[1]. Indeed, even 120 years after their discovery by Cajal, it is still unclear whether or not they have an
active electrical function [2]. A similar case could be made for the electrical measurement from axons or
axonal terminals. Although their function is essential, aside from particularly large preparations, like
the squid giant axon, it is still unclear exactly how they propagate and integrate electrical signals and
their functional neurobiology is a practically unexplored territory in terms of experimental data [3].
The development of optical methods, which are less invasive than microelectrodes, can obviate
the spatial restriction of electrical recordings and therefore have the potential to greatly advance
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