Cytology and General Histology: Atlas

Nerve tissue

These are neuronal processes (axis cylinders), surrounded by a coat of chain-forming glial cells (in the peripheral nervous system) or of their processes (in the central nervous system)
Depending on the glial coat architecture, one may classify the fibers into unmyelinated and myelinated nerve fibers
Glial cells of the peripheral nervous system that participate in the nerve fiber formation are called Schwann cells

Unmyelinated nerve fibers

are cords covered by a basement membrane. Their thickness is 1-4 μm. They contain several (5-20) thin neuronal processes (mainly dendrites), which are immersed partially or entirely into pits on Schwann cell surfaces
provide for low (1-2 m/sec) speed of excitation conduction

Myelinated nerve fibers

are also cords covered by a basement membrane, though the thickness is 8-20 μm. They contain one neuronal process (usually axon), which is immersed into Schwann cell cytoplasm along the entire nerve length. The fiber is surrounded by a helix of myelin layers folded in numerous coils. These are folds of double-layered plasma membrane in Schwann cells (mesaxon)
Some cell portions (at the boundaries between adjacent Schwann cells) have no myelin coat and called nodes of Ranvier
A depolarization wave propagates in a saltatory way from one node of Ranvier to another. This ensures a high (5-120 m/sec) speed of nerve impulse conduction

Myelinated nerve fibers

All the surface of neuronal processes is surrounded by concentric spiral stratified coats, which are formed by thin flattened processes of glial cells (oligodendroglia of the central nervous system)
A glial cell body gives rise to 40-50 cell processes that take part in the coat formation for several fibers at the same time

Unmyelinated neuronal processes

have no specialized glial coats
are free to "float" among neurons and glia; the processes are separated from one another by astrocyte processes

are terminal parts of sensory dendrites and motor axons
depending on their functions, they may be classified into receptive (afferent) and effector (efferent) endings

Receptive (afferent) nerve endings are terminal dendritic branches of sensory neurons:

free endings (no glial envelope around the process)
unencapsulated endings (the processes are covered by glial elements)
encapsulated endings (the processes are covered by a glial coat and both of them are ensheathed by a connective tissue capsule)

Effector (efferent) nerve endings

are terminal parts of motor axons
form chemical synapses where the postsynaptic region is represented by specialized plasma membrane portions of functioning organ tissue elements (muscles, glandular cells)
neurovascular synapses provide for neurohormonal traffic from terminals of efferent neurosecretory cell processes into the capillary blood stream