Post-synaptic potentials and action potentials: Membrane potentials

The complex shape of a typical neuron tells a story about communication. Many elements of a neuron - its cell body and dendrites and dendritic spines - serve the function of receiving information. Integration of that information takes place at a neuron’s axon hillock and initial segment, and conduction of the integrated signal occurs down its axon. And at the hundreds of terminals formed by its axon, a neuron transmits information to its targets. This division of labor among the several parts of a neuron is referred to dynamic polarization. For a typical neuron, communication is a matter of receiving information from other neurons, sometimes as many as a few thousand, and transmitting information to a few thousand other neurons. All of this takes place at synapses. By its structure and its function, a synapse is a pivot point; it is a place where a fundamental transformation takes place in how information is transmitted and processed. Communication from one neuron to another (intercellular) is predominantly chemical, whereas communication from one part of a neuron to another (intracellular) is predominantly electrical. This is a vital part of every neuron’s ability to communicate. Each neuron uses electrical currents to gather information at its synapses, carry information from its receptive surfaces, conduct information down its axon, and initiate events in axon terminals to produce release of chemical messengers. Along all of those surfaces, electrical currents are carried by ions, principally cations and most frequently Na⁺. Those are the most basic elements to every neuron’s function, and they are the topics we will discuss below.