Neurosurgery notes/Anatomy/Peripheral nerves anatomy/Nerve biology/Axonal transport

Axonal transport

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Nerve biology

Structure

  • Microtubules (made of tubulin) run along the length of the axon and provide the main cytoskeletal "tracks" for transportation.
  • Motor proteins
    • Kinesin:
      • Move cargoes in the anterograde (forwards from the soma to the axon tip)
    • Dynein
      • Moves cargo in the retrograde (backwards to the soma (cell body) directions
  • Cargoes including
    • Mitochondria
    • Cytoskeletal polymers
    • Autophagosomes
    • Synaptic vesicles containing neurotransmitters.

Function

  • Axonal transport is responsible for moving essential materials between the neuronal cell body (soma) and the axon terminal.
  • It transports organelles, proteins, lipids, neurotransmitter-containing vesicles, and other cellular components.
  • Enables delivery of newly synthesized substances (such as enzymes or synaptic vesicle precursors) from the cell body to the synapse (anterograde transport).
  • Carries worn-out organelles, endocytic vesicles, and signaling molecules back from the axon terminal to the cell body for recycling or degradation (retrograde transport).
  • Maintains synaptic function and axonal health by constantly supplying the molecules and organelles needed for neuronal signaling and proper function.
  • Supports neuronal growth, regeneration, and response to injury or signaling cues.

Types of axonal transport

notion image
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I. Fast Anterograde Axonal Transport
I. Fast Anterograde Axonal Transport
  • 100-400 mm/day in a saltatory fashion (start-stop-start)
  • Continuous: like long distance train
  • Cargo includes:
    • Synaptic vesicles and synaptic vesicle precursors
    • Mitochondria and other membrane organelles
    • Integral membrane proteins
    • Secretory polypeptides
    • Neurotransmitters
    • Elements of smooth endoplasmic reticulum
 
II. Fast Retrograde Axonal Transport
II. Fast Retrograde Axonal Transport
  • 200-270 mm/day
  • Cargo includes:
    • Endosomes
    • Damaged mitochondria and other organelles
    • Elements of smooth endoplasmic reticulum
    • Regulatory signals (growth factors and neurotrophins)
    • Viruses and toxins (e.g., tetanus, herpes simplex, rabies, polio)
III. Slow Axonal Transport (Anterograde Only)
III. Slow Axonal Transport (Anterograde Only)
  • A lot of pauses/stop and go: Like local MRT
  • Different substances move at two different speeds:
    • Slow Component a (SCa)
      • 0.2-2.5 mm/day (rate of neurite elongation)
      • Microtubules
      • Neurofilaments
      • Cytoskeletal proteins(e.g., actin and tubulin)
    • Slow Component b (SCb)
      • 5.0-6.0 mm/day
      • Cytosolic proteins
      • Clathrin
      • Calmodulin
      • Soluble enzymes and other proteins

Pathology

  • Diseases that uses it to transport microorganism
    • Rabies virus:
      • From the site of bite,
      • Retrograde axoplasmic flow.
    • Polio virus:
      • From the GI tract
      • Retrograde axoplasmic flow.
    • Tetanus bacteria:
      • From site of infection to the brain along the endoneurium of nerve fibres
    • HSV
      • During reactivation from latency, the herpes simplex virus (HSV) enters its lytic cycle, and uses anterograde transport mechanisms to migrate from dorsal root ganglia neurons to the skin or mucosa that it subsequently affects
  • APO E gene
    • Apoprotein E is produced mainly in astrocytes and is responsible for transportation of lipids within the brain.
    • The protein mediates neuronal protection, interactions with estrogens, and modulation of synaptic proteins.
    • Possession of the APO E4 allele has been shown to result in greater propensity to develop age-related cognitive impairment, a decrease in the synapse/neuron ratio, and increased susceptibility to exogenous neurotoxins