Biomedical Engineering Reference
In-Depth Information
and Renyi in their study of graph theory [
12
]. In the biological system, each neocortex neuron com-
municates with 5000-50,000 other neurons mostly in its proximity, but distant connections are also
present, creating a random graph structure that can be modeled as a small world [
13
]. The density
of neurons in the human neocortex is so large (10,000-100,000/mm
3
[
14
]) that despite of the huge
divergent properties, each neuron only communicates with 1% of its neighbors within its dendritic
radius. Moreover, as the number of neurons in the network increases, the minimum number needed
to connect them decreases.
From a functional perspective, the principal pyramidal cells (~85%) that make up these con-
nections are primarily excitatory, and the neurons predominantly communicate by mutual excita-
tion. However, in the absence of inhibition, the activity of the population could increase in this one
direction to an unstable state. In general, excitatory activity begets excitatory activity in a positive
direction. Therefore, excitatory population dynamics must be balanced by inhibitory neurons. In-
hibitory activity is inherently more sophisticated than excitatory because it has several modes of
operation. First, in direct negative feedback of a neuron, stability is enhanced by inhibition. Sec-
ond, if the inhibitory neuron operates in a feedforward manner on another neuron, then it acts a
filtering mechanism by dampening the effects of excitation. Finally, if the inhibitory neuron acts
as a segregator (i.e., lateral inhibition) among neural assemblies, it can serve to function as a gat-
ing mechanism between the groups of neurons. The intricate relationships between the excitatory
principal cells, inhibitory neurons, and the inherent refractory periods (i.e., when a neuron fires, it
can not fire again immediately after even if fully excited) are at the root of the admirable stability of
the spontaneous cortical activity.
2.3 NEURal SIgNalINg aNd ElECTRIC FIEldS oF
ThE BRaIN
The mechanism of neural connectionism and communication involves neuronal signaling across
a synapse (i.e., between its dendritic input and the axonal output) is mediated through a series of
changes in cell membrane potential as shown in Figure
2.1
. Molecular and voltage-gated signaling
occurs at the synapses, which are chemical-electrical connectors between the axon of presynaptic
neurons and one of many dendritic inputs of a given neuron. The axon of a firing presynaptic
neuron transfers the transient current and subsequent action potential (spike) through a series of
changes in membrane potential to its many synaptic endings. At the synapse, ions are released to
the attached dendrite, locally increasing the electrical potential at the dendrite's distal portion. This
voltage is slowly integrated with contributions from other dendritic branches and propagated until
the cell body. When the electrical potential at the base of the dendritic tree crosses a threshold that
is controlled by the cell body, the neuron fires its own action potential that is transferred through its
axonal output to other postsynaptic neurons. Therefore, the closer the membrane is to the threshold,
