In this section we examine the structure of neurons and how they propagate electrical and chemical signals. Neurons are distinguished by their elongated, asymmetric shape, by their highly localized proteins and organelles, and most of all by a set of proteins that controls the flow of ions across the plasma membrane. Because one neuron can respond to the inputs from multiple neurons, generate electrical signals, and transmit the signals to multiple neurons, a nervous system has considerable powers of signal analysis. For example, a neuron might transmit a signal only if it receives five simultaneous activating signals from input neurons. The receiving neuron measures both the amount of incoming signal and whether the five signals are roughly synchronous. Fast synaptic input from one neuron to another can be either excitatory—combining with other signals to trigger electrical transduction in the receiving cell—or inhibitory, discouraging such transmission. In addition to excitatory and inhibitory synapses, neurons receive slower neuromodulatory inputs such as norepinephrine, dopamine, serotonin, and acetylcholine, which activate G protein–coupled receptors (see Chapter 15) to change the threshold for excitation or inhibition. Thus the properties and connections of individual neurons set the stage for integration and refinement of information. The output of a nervous system is the result of its circuit properties, that is, the wiring or interconnections between neurons, and the strength of these interconnections. We will begin by looking at how signals are received and sent, and in subsequent parts of the chapter we will look at the molecular details of the machinery involved.