Neurons in the CNS are located in the gray matter, while in the peripheral nervous system (PNS) they are located in the ganglia. They can be broken into three types:

In the CNS, neurons have the greatest variety of size (4-135μm in diameter) and shape (e.g. spherical and pyramidal). Components of the neuron including the following:

Neurons are classified on the basis of the number of processes. These include:

Elements of the Chemical Synapses:

Peripheral neuroglia:

Schwann cells have a neural crest origin and support myelinated and unmyelinated nerve cells in the PNS as their main function by producing a lipid rich layer (myelin sheath, or the neurilemma). Neurilemma insulates axons from the extracellular compartment, allowing rapid conduction of impulses. The axon hillock and terminal arborizations are free of myelin. The myelin sheath is segmented, formed by numerous Schwann cells. Junctions between adjacent Schwann cells are called nodes of Ranvier (also myelin free). Myelin associates with small amounts of cytoplasm, including the following:

Satellite cells have a neural crest origin and surround ganglionic neuronal cell bodies. They are cuboidal in shape, and form complete layers around the cell body (with only the nuclei visible). They are found in paravertebral and peripheral ganglia, where dendrites penetrate the satellite cell layer to form synapses. Sensory ganglia have NO synapses. They are analogous to Schwann cells, providing insulation and nutrition.

Central neuroglia:

Oligodendrocytes form myelin in the CNS by concentric layers of the oligodendrocyte plasma membrane. They appear as small cells with scant cytoplasm that appear as a rim around the nucleus. The cells align in rows between axons. Each oligodendrocyte gives off several (3-50) tongue-like processes, each of which around around an axon forming and internodal segment.

Astrocytes provide physical and metabolic support for CNS neurons. There are two kinds seen:

Both have prominent bundles of intermediate filaments, consisting of glial fibrillary acidic protein (GFAP). They have elaborate processes extending between vessels and neurons. The ends of the processes form end feet that cover large areas of the outer surface of blood vessels or axolemma. They play a role in movement of metabolites and waste to and from neurons, and regulate ionic concentrations in intracellular compartments by taking up K+.They may also have a role in regulating tight junctions of the blood brain barrier.

Microglia are phagocytic cells of the CNS, part of MPS. They are present in small numbers, and proliferate during injury/disease. They originate in the bone marrow and remove debris of cells. They are the smallest of the glia, with an elongated nucleus and short, twisted processes covered with ruffled spikes.

Ependymal cells line brain ventricles and central canal of the spinal cord and are responsible for secreting cerebrospinal fluid (CSF). They are made of simple cuboidal or low columnar epithelium that rest on basal lamina. They have basal folds and their free surfaces are covered with cilia and microvilli.