Biomedical Engineering Reference
In-Depth Information
At the heart of pattern formation, is the synchronous activity of population
of neurons which in turn rely on a nerve signal generated along an axon or
dendrite by the difference in the electric field between the internal environment
of the nerve fiber and its external environment. This difference in the electric
field is called an electric potential, or field potential, and is established by a
difference in the concentrations of various charged ions across in a given
membrane.
Stimulation of a single neuron is achieved by using a microelectrode to apply
a depolarizing current to a conductive medium in which the neuron resides. In
this way, it is possible to remove the charge from the membrane and set up the
initiation of a potential impulse of
B
mV along the nerve fiber. The neuron
membrane,
B
70 A
˚
ngstroms in width, is mainly composed of the phospholipid
leaflet, which in itself has a high capacity of
B
1uFcm
2
, although due to the
resistance of ion channels (of the entire membrane), the capacity is about 1000
ohm cm
2
.A
70mV potential inside relative to the outside is maintained
across the membrane through an ion concentration imbalance of 10 : 1 sodium,
14 : 1 chloride, and 1 : 30 potassium outside of the cell relative to inside of the
cell. By controlling the potential and imposing a decrease of 20 mV across the
membrane it is possible to cause a sudden change in potential, and thereby create
a nerve impulse through changes in the sodium and potassium conductance.
These action potentials, also introduced in Chapter 2, translate to extracellular
potentials both in the axons and larger potentials of 3-5mV, which are more
spatially confined and observed around the cell body. These extracellular
potentials influence neuronal interaction and can be perturbed to create signals
that pass through the nerve fiber. Once this electric signal reaches the synaptic
connection between two different nerves, the release of special chemicals,
neurotransmitters, is stimulated. The neurotransmitters then travel through the
synaptic connection to pass along this signal to the neuron on the receiving end.
With careful design it is possible to engineer an in vitro environment for
neuronal networks and spatially localized electrical stimulation to selectively
excite either excitatory or inhibitory neurotransmitters. Ultimately through this
fabricated system of various excitatory and inhibitory signal transmissions,
additional information can be gathered to better understand the connections
between different neurons and their neuronal network signaling patterns.
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3.2 Cultured Neurons and Neuro-electronic Interface
As the neuron has characteristic electrochemical properties, it can be viewed as
an electrogenic cell. This type of cell generates electronic transduction and
facilitates two-way interactions within the cell. Extracellular potentials can be
explained with established biophysical principles of neural excitability.
1,2
Sub-
threshold action potential currents flow in closed loops which enter at the
neuron's current sink. These currents then propagate through the cytosolic
medium and leave at the current source to return again to the cell through the
sink after travelling in extracellular space.
3
The measured voltages through
external electrodes, which can form a two-way interaction with neurons, give
