The fundamental element of the neural network is called a neuron. As shown in figure 2.2, a neuron mainly consists of three parts: dendrites, soma, and axon. Dentrites are the tree-like structure that receives the signal from surrounding neurons, where each line is connected to one neuron. Axon is a thin cylinder that transmits the signal from one neuron to others. At the end of axon, the contact to the dendrites is made through a synapse. The inter-neuronal signal at the synapse is usually chemical diffusion but sometimes electrical impulses. A neuron fires an electrical impulse only if certain condition is met [Zur92].
The incoming impulse signal from each synapse to the neuron is either excitatory or inhibitory, which means helping or hindering firing. The condition of causing firing is that the excitatory signal should exceed the inhibitory signal by a certain amount in a short period of time, called the period of latent summation. As we assign a weight to each incoming impulse signal, the excitatory signal has positive weight and the inhibitory signal has negative weight. This way, we can say, ``A neuron fires only if the total weight of the synapses that receive impulses in the period of latent summation exceeds the threshold." [Arb87].