Notes
Peripheral Nerve Anatomy
Sections
Peripheral Nerve Histology
Summary
Nerve vs Nerve fibers (axons)
- A peripheral nerve encompasses bundles of nerve fibers (axons) and has various connective tissue coverings (superficial epineurium, perineurium, and endoneurium).
Nerve Coverings
Outside to Inside:
Superficial epineurium
- Superficial epineurium is the outer covering of the nerve.
- It is a supporting coat: a cylindrical, dense connective tissue sheath.
Perineurium
- Perineurium encases separate nerve fascicles.
- It is a mechanically strong sheath that is dense and forms a protective barrier around the nerve fascicle: a blood-nerve barrier.
- Perineurium comprises a flattened form of epithelial cells that are joined by special junctions, which helps it withstand tremendous pressure.
Endoneurium
- Endoneurium is a loose connective tissue; it comprises collagenous fibers.
Additional, less often addressed terminology:
Deep epineurium
- Deep epineurium accounts for the connective tissue sandwiched between the nerve fascicles.
- We find vasculature in this region.
Perineurial septa
- Perineurial septa pass through the nerve fascicles and carry vasculature to the nerve fibers.
Nerve fibers (Axons)
- The nerve fiber is the nerve cell axon.
- Myelin sheath surrounds myelinated axons.
- Schwann cells: each myelinates at most one axon internode.
Myelin
Unmyelinated cell
In an unmyelinated cell, we see:
- The Schwann cell nucleus intermixed with axons within Schwann cell cytoplasm.
- The mesaxon is the zone of apposition for the Schwann cell membrane and the axon.
- Multiple axons can pass through a Schwann cell, when the nerve fiber is unmyelinated.
Myelinated cell
In a myelinated cell, we see:
- The Schwann cell nucleus lies off to the side of the axon, which is enveloped in concentric circles of myelin: a myelinated sheath.
Schwann cells vs oligodendrocytes
- Unlike oligodendrocytes, which can myelinate up to 50 individual axons, a Schwann cell only myelinates one axon, as shown here (specifically only one peripheral nervous system internode).
Dorsal root ganglion
We highlight the following:
- Ganglion cell
- Cell nucleus
- Surrounding satellite cells (which support the ganglionic neurons).
- Satellite cells are derived from neural crest cells in the same way that Schwann cells are.
Peripheral nerve in longitudinal view
We highlight the following:
- Schwann cells
- Myelin sheath
- The myelin sheath is vacuolated because its high lipid content creates this artifact upon paraffin embedding.
- Deep stained nerve fibers (axons).