TLL 15x Peripheral Nerves

Peripheral Nerves

Peripheral nerves are covered by Schwann cells, which are specialized glial cells that provide support, insulation, and facilitate nerve signal transmission. The way Schwann cells cover axons differs significantly between unmyelinated and myelinated nerves, affecting their function and conduction properties.

• Schwann Cells (green)
• Unmyelinated Axons (left; yellow)
• Myelinated Axons (right; yellow)

Schwann cells form a continuous chain along the nerve, with each cell covering a segment of an axon.

Subcellular Structures

• Nucleus (blue) / Nucleolus (yellow) / Nuclear Envelope (purple)
• Mitochondria (red): Generate ATP to support metabolic and nerve impulse transmission
• Endoplasmic Reticulum (cyan)
• Lysosomes (orange): Involved in cellular maintenance and waste removal
• Synaptic Vesicles (purple): Transport neurotransmitters to axon terminals
• Cytoskeleton: Microtubules (blue) and neurofilaments (red)

Each Schwann cell is surrounded by a basal lamina (purple).

Courtesy of Thomas L. Lentz, Department of Cell Biology, Yale University School of Medicine, New Haven, Connecticut.

Unmyelinated Nerves

Unmyelinated axons are small-diameter fibers (0.2-1.5 µμm) that are bundled together within invaginations of a single Schwann cell. These axons conduct signals slowly via continuous conduction, and primarily carry pain, temperature, and autonomic signals.

Structure and Organization:

• Schwann Cell (green): Encloses multiple unmyelinated axons (typically 5-20) within invaginations of a single cell
  • Mitochondria (red): Generate ATP to remove the influx of ions after an action potential passes
  • Endoplasmic Reticulum (ER, cyan): Contains mostly smooth ER
  • Synaptic Vesicles (purple): Transported to axon terminals for neurotransmitter release at synapses
  • Cytoskeleton: Arranged along the long axis of the axons
    • Microtubules (blue): Tube-like structures (25 nm diameter) that maintain axonal transport and structural integrity
    • Neurofilaments (red): Thinner filaments (10 nm diameter) that provide mechanical strength and structural integrity

Functional Characteristics:

• Conduction Type: Continuous conduction with action potentials traveling along the entire length of the axon
• Conduction Velocity: Slow (0.5-2 meters/second)
• Metabolic Support: Maintains ionic environment and provides metabolic support axons
• Fiber Types: pain, temperature, and autonomic postganglionic fibers

Myelinated Nerve

Myelinated axons are larger-diameter fibers (1-20 µm) and have Schwann cells wrapping around a single axon segment many times to form a myelin sheath. This sheath enables saltatory conduction, allowing for rapid signal transmission. These fibers typically carry motor, touch, proprioceptive, and some pain signals.

Structure and Organization:

• Schwann Cell (green): Encloses a single axon segment
  • Nucleus (blue): Flattened and located at the periphery
  • Myelin Sheath: cell membrane wraps around the axon multiple times (20-100 layers), creating a thick insulating layer
    • Lipid-rich: Approximately 70-80% lipids and 20-30% proteins
    • Tight Junctions: Seal adjacent membrane layers
  • Specialized Regions:
    • Internodal Regions (shown): Fully myelinated segment between nodes
    • Paranodal Regions: Transition zones with more cytoplasm
    • Nodes of Ranvier: Gaps between adjacent Schwann cells with a high concentration of voltage-gated sodium channels
• Myelinated Axon (yellow):
  • Mitochondria (red): Generate ATP for active ion transport as action potentials pass
  • Endoplasmic Reticulum (ER, cyan): Contains mostly smooth ER
  • Synaptic Vesicles (purple): Transport neurotransmitter to axon terminals for release at synapses
  • Cytoskeleton:
    • Microtubules (blue): Tube-like structures (25 nm diameter) that maintain axonal transport and structural integrity
    • Neurofilaments (red): Thin filaments (10 nm diameter) that provide mechanical strength and support

Functional Characteristics:

• Saltatory Conduction: Action potential “jumps” between nodes
• Conduction Velocity: Much faster (50-120 meters/second) than unmyelinated axons
• Metabolic Support: Maintains the ionic environment around the axon and provides metabolic support
• Fiber Types: motor, touch, proprioception, and some pain pathways