NEURONAL STRUCTURE
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.
CLINICAL POINT
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.
