Foundations of Neuronal Representations - Brain-Machine Interface Engineering - page 24

Molecular
and Voltage
Gated
Signaling

Ionic
Concentrations
and Membrane
Potentials

Action potentials

Soma - Axon

Dendrite - Synapse

Membrane potentials play a pivotal role.

FIgURE 2.1: Relationship among neuron structure, electrochemistry, and generation of cell signaling.

the more likely is the neuron to fire. Most cortical neurons spend 99.9% of their lifetimes with their
membrane potentials just below threshold.
Through this signaling procedure, there are two types of electric fields produced by neurons:
waves of dendritic synaptic current that establish weak potential fields as the dendritic synaptic
current flows across the fixed extracellular cortical resistance and trains of action potentials on the
axons that can be treated as point processes. 5 The variation of the membrane potential initiated from
a presynaptic spike can be described as a postsynaptic potential as shown in Figure 2.2 . Excitatory
synapses increase the membrane potential in excitatory postsynaptic potentials, which occur after
the diffusion process of a neurotransmitter opens the channels of N -methyl-D-aspartate or non- N -
methyl-D-aspartate receptors. 6 Inhibitory postsynaptic potentials lower the rise of the membrane
potential. The electrical potentials superimpose as the sum of the individual potentials as they ap-
proach the threshold. Pulse densities are monotonically dependent on wave current densities. The
range is bounded by threshold at the low end and refractoriness at the high end, with a near-linear
small-signal range. This is similar to the sigmoidal curve in artificial neural networks, with a major
and significant difference: the nonlinearity is asymmetric with the maximal slope skewed to the
right translating the neuron's excitatory nature. As shown in chapter 1 , the LFP in upper layers of
neocortex (II-IV), ECoG, and EEG measure the electrical field produced by the dendritic currents
(also called the post synaptic potentials) of large groups of neurons. Conventionally, microelectrode

5 The review of the generation of action potentials is a brief introduction to the minimum components of the process;
for a more detailed description, see Reference 15 .
6 N -methyl-D-aspartate or glutamatergic receptors are excitatory, which are typically slower than non- N -methyl-D-
aspartate synapses.