Neurons and Neuroglia

The cells of the brain are a virtual menagerie of cell types, and within each type, there is substantial variation in properties such as size and shape. At the molecular level, the variation is even more profound. This variation applies to neurons as well as to nonneuronal, or neuroglial, cells. This chapter provides an overview of this diversity. We begin with the neuron and its major subcellular compartments—the dendrite, cell body, axon, and synapse. Each of these subdomains has its own specialized function and unique physiology. The dendrite serves as the antenna of the cell; it is the primary site for receiving input from upstream neurons. It responds to these inputs with small changes in membrane potential that sum as they propagate along the neuronal membrane. The cell body is the site where these potential changes meet. If the total potential change is sufficient, the neuron passes a signal to the next cell by means of an action potential. The axon is the conduit along which the axon potential travels to reach the downstream target. Like the dendrite, it has a highly specialized composition of membrane channels whose proper functioning allows a strong membrane depolarization to propagate over great distances without diminishing in strength. The synapse is a pairing of the axon terminus and the downstream target (most often the dendrite of another neuron). At this specialized structure the action potential triggers the release of a small aliquot of neurotransmitter that interacts with target receptors, causing a depolarization, and the process is thus repeated. Although the neuron is the primary cell responsible for transmitting information over long distances, it is not alone in the brain. Several nonneuronal cell types known as neuroglia are intermingled with the neurons and are indispensable for the brain’s proper functioning. Astrocytes form an anastomosing network that surrounds the neurons and the vasculature of the brain. They are heavily involved in maintaining the ionic and nutritional environment in which the neurons function, in particular at the synapse where they also serve to clear neurotransmitters such as glutamate. Radial glia serve a guidance role during development and serve as a source of precursors during development and beyond. Oligodendrocytes are responsible for creating the myelin of the central nervous system, the fatty coating of the neuronal axon that allows for rapid, efficient conduction of the action potential. In the peripheral nervous system, this same function is carried out by a neural crest-derived cell known as a Schwann cell. Microglial cells are the mediators of the innate immune system of the brain and function is ways very similar to macrophages in the periphery. In the aggregate these elements and their basic interactions are the cellular “toolkit” the brain uses to achieve the functions described in later chapters.