Nerve Anatomy

1. Overview

2. Connective Tissue Layers (Inside Out)

• The innermost layer; loose connective tissue (reticular fibers, sparse fibroblasts, capillaries) that surrounds each individual nerve fiber (axon + Schwann cell)
• Collagen fibrils run both parallel to and circumferentially around the fiber; most are secreted by Schwann cells
• Contains mast cells and macrophages for immune surveillance and repair
• Sparse capillaries here form part of the blood-nerve barrier
• Histology A Text and Atlas With Correlated Cell and Molecular Biology, p. 981-982

• Groups of nerve fibers + endoneurium are bundled into fascicles by the perineurium
• 2-6 layers of flat, specialized fibrocyte-like cells with edges sealed by tight junctions - these create the main blood-nerve barrier that regulates the fascicular microenvironment
• Cells have actin microfilaments, pinocytotic vesicles, and cytoplasmic densities, resembling smooth muscle cells ultrastructurally
• Perineurial cell processes can extend inward to surround sub-groups of axons at branching points
• Junqueira's Basic Histology, 17e, p. 469-470

• Dense irregular fibrous coat that surrounds the entire nerve trunk and fills the spaces between fascicles
• Has a superficial dense region and a deeper, looser region containing larger blood vessels
• Extends as interfascicular epineurium between fascicle groups
• Current Surgical Therapy 14e, p. 980

• Loose connective tissue extending from the epineurium to surrounding tissues
• Allows nerve gliding during range of motion and carries the segmental (extrinsic) blood supply to the nerve
• Campbell's Operative Orthopaedics 15th Ed, p. 3840

3. The Individual Nerve Fiber

Myelinated Fibers

• A single Schwann cell rotates around one axon many times, laying down multiple layers of its membrane (containing sphingomyelin) - this is the myelin sheath
• Myelin is an electrical insulator that reduces ion flow ~5000-fold
• The segment of fiber covered by one Schwann cell = internode (0.1-1.8 mm; longer internodes on more heavily myelinated fibers)
• The gap between successive Schwann cells, where the axon membrane is exposed = node of Ranvier (2-3 µm long)
• Action potentials jump from node to node = saltatory conduction - this increases velocity 5-50 fold and is energy efficient
• Guyton & Hall Medical Physiology, p. (Ch. 5)

Unmyelinated Fibers

• Several axons (~10-20) are embedded within the cytoplasmic processes of a single Schwann cell (Remak cell) but are not wrapped
• Conduct impulses slowly by continuous conduction along the membrane
• Unmyelinated fibers outnumber myelinated ~2:1 in a typical nerve trunk
• Sensory and motor nerves have unmyelinated:myelinated ratio of ~4:1

4. Nerve Fiber Classification

Erlanger-Gasser Classification (Ganong's Review of Medical Physiology, 26e)

Lloyd-Hunt Classification (Sensory Only - Costanzo Physiology, 7e)

5. Fascicular Topography

• Proximally, sensory and motor axons intermingle significantly within nerve trunks
• More distally, they segregate into distinct motor and sensory fascicles
• This has direct surgical implications: proximal nerve repairs (e.g. facial nerve within the temporal bone) risk synkinesis/dyskinesis due to poorly differentiated fascicles; distal repairs have better outcomes because the fascicular architecture is more defined
• Current Surgical Therapy 14e, p. 980

6. Blood Supply

• Extrinsic (segmental) supply: enters through the mesoneurium at multiple points along the nerve
• Intrinsic (longitudinal) supply: runs within the epineurium and perineurium; branches penetrate into the endoneurium
• Endoneurial capillaries have tightly joined endothelium - a second component of the blood-nerve barrier
• This dual supply means peripheral nerves can tolerate moderate stretching or positional changes without ischemia

Summary: Structural Hierarchy

Nerve trunk
 └── Epineurium (dense irregular CT)
      └── Fascicles (grouped by interfascicular epineurium)
           └── Perineurium (tight-junction sealed fibrocytes)
                └── Individual nerve fibers
                     └── Endoneurium (loose CT)
                          └── Axon + Schwann cell (± myelin)