Classification of Axonal Transport
To fully understand neuronal physiology, substance transport is classified according to two main criteria: direction and velocity.
Based on direction, there are two types of flow:
- Anterograde (forward): movement of substances from the cell body (perikaryon) toward the peripheral axon or dendrite terminals.
- Retrograde (backward): cargo moves from the process terminals back toward the neuronal soma. Notably, this type of transport occurs in both axons and dendrites.
Based on velocity and localization, transport has distinct quantitative parameters:
- Slow axonal transport (always anterograde) is only 1–3 mm/day.
- Fast axonal transport (also anterograde) proceeds at 100–1000 mm/day.
- Dendritic transport (forward) occurs at an average speed of about 75 mm/day.
Characteristics of Transported Substances
The composition of the transported cargo depends directly on the direction of transport. Substances can move in dissolved form or packaged inside specialized vesicles.
Within anterograde transport (toward nerve terminals), the following are carried:
- Various metabolites acting as precursors for neurotransmitter synthesis and meeting the terminal's energy demands.
- Oxygen, which is transported exclusively inside mitochondria required to maintain local oxidative processes.
- Protein structures, including various enzymes.
- Neurohormones (this feature is strictly specific to specialized neurosecretory cells).
During retrograde transport (back to the neuronal soma), metabolic waste products are predominantly transferred. The perikaryon utilizes them for subsequent disposal or metabolic processing.
Molecular Mechanisms of Cargo Movement
The molecular mechanism of transport depends directly on the physical state of the transported substance.
Transport of dissolved substances (fast) is not simple diffusion, as might be assumed. Its mechanism relies on a directed fluid flow driven by hydrodynamic pressure. This flow occurs through the intertubular space located alongside neurotubules.
Transport of formed elements (vesicles, granules, organelles) is more complex and requires ATP energy expenditure. It operates on a 'monorail' principle: a specialized carrier protein attaches to the cargo with one part and to a neurotubule with the other. The protein then undergoes rhythmic 'stepping' movements along the microtubule.
Key motor proteins in this process include:
- Kinesin: A molecular motor that exclusively drives anterograde (forward) transport.
- Dynein: A motor protein specialized in retrograde (backward) transport of cargo toward the cell body.
Analysis of Histological Slides
When examining histological slides during practical sessions, it is important to note the neuronal type and staining technique. Let us examine the main examples.
Spinal ganglion (sensory ganglion): Here, pseudounipolar neurons are localized (not to be confused with multipolar neurons often discussed in general texts). Classic hematoxylin and eosin (H&E) staining is routinely used for their visualization under low and high magnification.
Spinal cord: The spinal cord contains multipolar neurons. Specific contrasting methods are used to study them:
- Silver impregnation: Provides a clear picture where the cell nucleus appears light and the cytoplasm acquires a dark tone.
- Nigrosin staining: Produces the opposite visual result—a dark nucleus stands out clearly against the lighter gray background of the cytoplasm.