Biomedical Engineering Reference
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potential depolarisation (ADP) by comparison with neurons grown on control
glass substrates. ADP was found to be dependent on the degree of dendritic
branching. Indeed, neurons with minimal dendritic branching grown on the
CNT substrate did not display ADP. The backpropagating current induces a
voltage change that increases the concentration of Ca
2+
in the dendrites and
can be measured through the presence of a slow membrane depolarisation
following repetitive action potentials. Therefore, the interactions between
CNTs and membranes of neurons can affect single-cell activity. These
experiments were also conducted on two other types of substrates: indium
tin oxide substrate, displaying high conductivity, and a non-conductive
nanostructured substrate containing peptides that self-assemble into
nanofibres. In both cases, no signiicant enhancement of ADP was observed,
indicating that the ADP enhancement effect is speciic to CNT substrate.
The authors suggested the electrotonic hypothesis to explain the physical
neuron-SWNT interactions and elucidate the mechanisms by which SWNTs
might affect the electrical activity of neuronal networks. Electrotonic
potential is a non-propagated local potential induced by a local change in ionic
conductance. It represents changes to the neuron membrane potential that
do not lead to the generation of new current by action potentials. Electrotonic
potential is conducted faster than action potential, but it attenuates rapidly
and is therefore unsuitable for long-distance signalling.
The authors also examined if an electrotonic shortcut could occur between
the soma and the dendrite. The results showed the absence of changes brought
about by SWNTs in the dendritic passive time constants. Other hypotheses
were drawn to tentatively explain the increased effects of ADP, such as (i)
potentiation of Ca
2+
-mediated currents occurring in neurons grown on SWNT
thin ilms and (ii) channels clustering, induced by mechanical interactions
between SWNT bundles and the cell cytoskeleton. The presence of the ADPs,
their dependence on trains of action potentials, and the detected sensitivity
to calcium channel blockers are indicative of the generation of dendritic Ca
2+
currents. The discontinuous and tight contacts between CNTs and neuronal
membranes were observed by transmission electron microscopy (TEM). The
SWNTs were able to modulate the physiology of the neurons. The intimate
interactions of the SWNTs with a small area of the neuritic membranes
favoured electrical shortcuts between the proximal and distal compartments
of the neurons, thus supporting the electrotonic hypothesis. A mathematical
model was proposed to simulate how ADP enhancement induced by SWNTs
might affect the neuronal activity. More precisely, the aim was to model
the membrane voltage input-output relationship between the soma and
dendrites. The results indicated that SWNTs may be effectively short-circuiting
the dendrites and soma. This shortcut would lead to diverting the electrical
activity through the nanotubes, explaining the enhanced ADP effect.
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