Retinal ganglion cells arrayed across the retina are illuminated by a transgenic marker (junctional adhesion molecule B [JAM-B]). Whereas the axons of all ganglion cells converge at the back of the eye and exit the retina as the optic nerve en route to the brain, the dendrites of this particular set of neurons also share the same orientation, branching downward from the cell bodies. Their common form reflects a common function: detection of upward motion in the visual field. (Reproduced, with permission, from In-Jung Kim and Joshua Sanes.)
In all biological systems, from the most primitive to the most advanced, the basic building block is the cell. Cells are often organized into functional modules that are repeated in complex biological systems. The vertebrate brain is the most complex example of a modular system. Complex biological systems have another structural feature: They are architectonic—that is, their anatomy, fine structure, and biochemistry all reflect a specific physiological function. Thus, the construction of the brain and the cell biology, biophysics, and biochemistry of its component neurons reflect its fundamental function, which is to mediate behavior.
The great diversity of nerve cells—the fundamental units from which the modules of the nervous systems are assembled—is derived from one basic cell plan. Three features of this plan give nerve cells the unique ability to communicate with one another precisely and rapidly over long distances. First, the neuron is polarized, possessing receptive dendrites on one end and communicating axons with synaptic terminals at the other. This polarization of functional properties is commonly used to restrict the flow of impulses to one direction. Second, the neuron is electrically and chemically excitable. Its cell membrane contains specialized proteins—ion channels and receptors—that permit the influx and efflux of specific inorganic ions, thus creating electrical currents that alter the voltage across the membrane. Third, the neuron contains proteins and organelles that endow it with specialized secretory properties that allow it to release neurotransmitters at synapses.
In this part of the book, we shall be concerned with the properties of the neuron that give it the ability to generate electrical signals in the form of synaptic and action potentials (see Chapter 4). The initiation of a signal depends on ion channels in the cell membrane that open in response to changes in membrane voltage and to neurotransmitters released by other nerve cells. In Chapter 5, we consider general properties of ion channels. Neurons use three major classes of channels for signaling: (1) resting channels generate the resting potential and underlie the passive properties of neurons that determine the time course of synaptic potentials, their spread along dendrites, and the threshold for firing an action potential, as discussed in Chapter 6; (2) voltage-gated channels are responsible for the active currents that generate the action potential, which is discussed in Chapter 7; and (3) ligand-gated channels open in response to neurotransmitters to generate synaptic potentials. In this section, we focus mainly on resting and voltage-gated channels. In the next section, we consider in more detail ligand-gated channels.