Biological Neural Nets

The human nervous system consists of small cellular units, called neurons. These neurons when connected in tandem form nerve fiber. A biological neural net is a distributed collection of these nerve fibers.

A neuron receives electrical signals from its neighboring neurons, processes those signals and generates signals for other neighboring neurons attached to it. The operation of a biological neuron, which decides the nature of output signal as a function of its input signals is not clearly known to date.

However, most biologists are of the opinion that a neuron, after receiving signals, estimates the weighted average of the input signals and limits the resulting amplitude of the processed signal by a non-linear inhibiting function [6]. The reason for the non-linearity as evident from current literature [26] is due to the concentration gradient of the Potassium ions within a neuronal cell with respect to the Sodium-ion concentration outside the cell membrane. This ionic concentration gradient causes an electrical potential difference between the inner and outer portion of the neuronal cell membrane, which ultimately results in a flow of current from outside to inside the cell. A neuron, thus, can receive signals from its neighboring neurons. The variation of weights of the input signals of a neuron is due to the differences in potential gradient between a neuron and its surrounding cells. After the received signals are processed in a nerve cell, an invasion in diffusion current occurs due to the synaptic inhibiting behavior of the neuron. Thus, the processed signal can propagate down to other neighboring neurons.

A neuron has four main structural components [1]-[2]: the dendrites, the cell body, the axon and the synapse. The dendrites act as receptors, thereby receiving signals from several neighborhood neurons and passing these on to a little thick fiber, called dendron. In other words, dendrites are the free terminals of dendrons. The received signals collected at different dendrons are processed within the cell body and the resulting signal is transferred through a long fiber named axon. At the other end of the axon, there exists an inhibiting unit called synapse. This unit controls the flow of neuronal current from the originating neuron to receiving dendrites of neighborhood neurons. A schematic diagram, depicting the above concept, is presented in fig.14.1.

Dendrites

Fig. 14.1: A biological neuron.

Dendrites

Fig. 14.1: A biological neuron.

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