Chapter 1: Introduction

To understand neural networks, it is necessary to understand the ways in which one neuron communicates with another through synaptic connections and the process called synaptic transmission

Synaptic transmission comes in two basic flavors: excitation and inhibition. Just a few interconnected neurons (a microcircuit) can perform sophisticated tasks such as mediate reflexes, process sensory information, generate locomotion and mediate learning and memory. More complex networks (macrocircuits) consist of multiple imbedded microcircuits. Macrocircuits mediate higher brain functions such as object recognition and cognition. So, multiple levels of networks are ubiquitous in the nervous system.

Networks are also prevalent within neurons. These nanocircuits constitute the underlying biochemical machinery for mediating key neuronal properties such as learning and memory and the genesis of neuronal rhythmicity.

Neurons are different from most other cells in the body in that they are polarized and have distinct morphological regions, each with specific functions.

The binding to the receptors leads to a change in the permeability of ion channels in the membrane and in turn a change in the membrane potential of the postsynaptic neuron known as a postsynaptic synaptic potential (PSP). So signaling among neurons is associated with changes in the electrical properties of neurons.

With the electrode outside the cell in the extracellular medium, zero potential is recorded because the extracellular medium is isopotential (having equal electric potential). If, however, the electrode penetrates the cell such that the tip of the electrode is now inside the cell, a sharp deflection is seen on the recording device.

What distinguishes nerve cells and other excitable membranes (e.g.,muscle cells) is that they are capable of changing their resting potential.

As potentials become larger, the depolarization is sufficiently large to trigger an action potential, also known as a spike or an impulse. The action potential is associated with a very rapid depolarization to achieve a peak value of about +40 mV in just 0.5 milliseconds (msec). The peak is followed by an equally rapid repolarization phase. The voltage at which the depolarization becomes sufficient to trigger an action potential is called the threshold.

First, action potentials are elicited in an all-or-nothing fashion. Either an action potential is elicited with stimuli at or above threshold, or an action potential is not elicited. Second, action potentials are very brief events of only about several milliseconds in duration.

Just as action potentials are elicited in an all-or-nothing fashion, they are also propagated in an all-or-nothing fashion. Once an action potential is initiated in one region of a neuron such as the cell body, that action potential will propagate along the axon (like a burning fuse) and ultimately invade the synapse where it can initiate the process of synaptic transmission.
That action potential in the presynaptic neuron leads to a decrease in the membrane potential of the postsynaptic cell. The membrane potential changes from its resting value of about -60 millivolts to a more depolarized state. This potential is called an excitatory postsynaptic potential (EPSP).

The consequence of action potential in the red presynaptic neuron is to produce an increase in the membrane potential of the blue postsynaptic neuron. The membrane potential is more negative than it was before (a hyperpolarization) and therefore the membrane potential is farther away from threshold. This type of potential is called an inhibitory postsynaptic potential (IPSP) because it tends to prevent the postsynaptic neuron from firing an action potential.

They are constantly adding up the excitatory and the inhibitory synaptic input in time (temporal summation) and over the area of the dendrites receiving synaptic contacts (spatial