Neuronal structure reflects the functional characteristics of the individual neuron. Incoming information is projected to a neuron mainly through axonal terminations on the cell body and dendrites. These synapses are isolated and are protected by astrocytic processes. The dendrites usually make up the greatest surface area of the neuron. Some protrusions from dendritic branches (dendritic spines) are sites of specific axodendritic synapses.
Each specific neuronal type has a characteristic dendritic branching pattern called the dendritic tree, or dendritic arborizations. The neuronal cell body varies from a few micrometers (µm) in diameter to more than 100 µm. The neuronal cytoplasm contains extensive rough endoplasmic reticulum (rough ER), reflecting the massive amount of protein synthesis necessary to maintain the neuron and its processes. The Golgi apparatus is involved in packaging potential signal molecules for transport and release. Large numbers of mitochondria are necessary to meet the huge energy demands of neurons, particularly those related to the maintenance of ion pumps and membrane potentials. Each neuron has a single (or occasionally no) axon, usually emerging from the cell body or occasionally from a dendrite (e.g., some hip- pocampal CA neurons). The cell body tapers to the axon at the axon hillock, followed by the initial segment of the axon, which contains the Na+ channels, the first site where action potentials are initiated. The axon extends for a variable distance from the cell body (up to 1 m or more). An axon larger than 1 to 2 µm in diameter is insulated by a sheath of myelin provided by oligodendroglia in the central nervous system (CNS) or Schwann cells in the peripheral nervous system (PNS). An axon may branch into more than 500,000 axon terminals, and may terminate in a highly localized and circumscribed zone (e.g., primary somatosensory axon projections used for fine discriminative touch) or may branch to many disparate regions of the brain (e.g., noradrenergic axonal projections of the locus coeruleus). A neuron whose axon terminates at a distance from its cell body and dendritic tree is called a macroneuron or a Golgi type I neuron; a neuron whose axon terminates locally, close to its cell body and dendritic tree, is called a microneuron, a Golgi type II neuron, a local circuit neuron, or an interneuron. There is no typical neuron because each type of neuron has its own specialization. However, pyramidal cells and lower motor neurons are commonly used to portray a so-called typical neuron.
Neurons require extraordinary metabolic resources to sustain their functional integrity, particularly that related to the maintenance of membrane potentials for the initiation and propagation of action potentials. Neurons require aerobic metabolism for the generation of adenosine triphosphate (ATP) and have virtually no ATP reserve, so they require continuous delivery of glucose and oxygen, generally in the range of 15% to 20% of the body’s resources, which is a dispro- portionate consumption of resources. During starvation, when glucose availability is limited, the brain can shift gradually to using beta-hydroxybutyrate and acetoacetate as energy sources for neuronal metabolism; however, this is not an instant process and is not available to buffer acute hypoglycemic episodes. An ischemic episode of even 5 minutes, resulting from a heart attack or an ischemic stroke, can lead to permanent damage in some neuronal populations such as pyramidal cells in the CA1 region of the hippocampus. In cases of longer ischemia, widespread neuronal death can occur. Because neurons are postmitotic cells, except for a small subset of interneurons, dead neurons are not replaced. One additional consequence of the postmitotic state of most neurons is that they are not sources of tumor formation. Brain tumors derive mainly from glial cells, ependymal cells, and meningeal cells.