Biomedical Engineering Reference
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(photoreceptor nuclei), with a 76.9% reduction in the number of cells. Ganglion
cells were reduced by 30.7%. However, cell count in the INL (bipolar, horizontal,
and amacrine cells) was not significantly different from the control group. In
another study, morphometric analysis of six eyes with disciform AMD showed
a similar pattern [31].
In RP, cell loss occurs in all layers [32-34]. In both the macula and the
extramacular regions, cell loss is most profound in the ONL, followed by the
ganglion cell layer (GCL) and then the INL. More cells in the INL and GCL
are preserved in the macula, when compared with extramacular regions. In the
macula, 78-88% of the inner nuclear cells and 30-48% of ganglion cells are
retained, whereas in the extramacular regions, 40% of the inner nuclear cells
and 20-30% of ganglion cells are retained. In addition to the reduction of viable
cells, retinal degeneration is also associated with a progressive retinal remod-
eling which involves rewiring and anomalous circuitry within the retinal layers.
In the final phase of remodeling, retina displays hypertrophy of Muller cells,
formation of microneuromas (tangles of cell processes from different layers), and
cellular translocation (migration of amacrine cells into the GCL and movement
of ganglion cells into the INL) [35-38].
Electrical Stimulation of the Nerve
Nerve cells are excitable, which means that they can change their transmem-
brane potential in response to a stimulus. This is accomplished normally through
either a physical perturbation for sensory receptor neurons (e.g. photoreceptors)
or synaptic transmission where one neuron signals another at a connection
called a synapse. In either case, a neuron is excited and the conductance of the
cell membrane changes, resulting in ions crossing the membrane at different
rates different from equilibrium. This movement of charged particles results
in a transient change in the membrane potential, which serves as a signaling
mechanism. The signal propagates along the nerve membrane to the synapse,
where it can serve to either excite or inhibit activity in other nerve cells.
Nerve cells can be artificially activated by electric fields that depolarize
the cell membrane. Electrical stimulation from an extracellular microelectrode
has been extensively used in both experimental and clinical settings. Early
experiments suffered from an inability to precisely control electrical energy. With
the emergence of microelectronic technology, more precise stimulus currents
could be delivered to neurons. Concomitant advances in medicine have enabled
increasing access to neural structures. The combined efforts in both engineering
and medicine have yielded electrically active, microelectronic implants that can
stimulate nerves to restore lost sensory function or modulate neural function.
Visual Response to Electrical Stimulation of the Retina in Outer
Retinal Disease
Many studies have documented that electrical stimulation of neurons in the
visual pathway produces the perception of light. Fewer have looked at electrical
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