Axonal transport

Classification of axonal transport

To fully understand neuron physiology, substance transport is classified according to two main criteria: direction and movement velocity.

By direction, two types of flow are distinguished:

• Anterograde (forward): substances move from the cell body (perikaryon) to the peripheral terminals of the axon or dendrite.
• Retrograde (backward): cargo moves from the process terminals back to the nerve cell body. It is important to note that this type of transport is characteristic of both axons and dendrites.

By velocity and localization, transport has distinct quantitative characteristics:

1. Slow axonal flow (always anterograde) is only 1–3 mm/day.
2. Fast axonal flow (also anterograde) proceeds at a velocity of 100–1000 mm/day.
3. Dendritic flow (forward) proceeds at an average velocity of about 75 mm/day.

Characteristics of transported substances

The composition of the transported "cargo" directly depends on the direction of transport. Substances can move in dissolved form or be packaged in specialized vesicles.

Within anterograde transport (toward nerve terminals), the following are moved:

• Various metabolites acting as precursors for the synthesis of neurotransmitters and meeting the energy needs of the terminal.
• Oxygen, which is transported exclusively within mitochondria necessary to maintain local oxidative processes.
• Protein structures, including various enzymes.
• Neurohormones (this item is strictly specific to axons of specialized neurosecretory cells).

During retrograde transport (back to the neuron soma), predominantly end products of intracellular metabolism are carried. The perikaryon uses them for subsequent utilization or metabolic processing.

Molecular mechanisms of cargo movement

The choice of the molecular mechanism by which transport is carried out directly depends on the physical state of the transported substance.

Transport of dissolved substances (fast) is not simple diffusion, as might be assumed. Its mechanism is based on a directed fluid flow under the action of hydrodynamic pressure. This flow passes through the intertubular space located along neurotubules.

Transport of formed elements (vesicles, granules, organelles) is more complex and requires mandatory ATP energy expenditure. It is implemented according to the so-called "monorail" principle: a specialized carrier protein attaches firmly with one part to the cargo being moved and with the other part to the neurotubule. Then the protein performs rhythmic "stepping" movements along the tubule.

The key motor proteins in this process are:

• Kinesin: a molecular motor that provides exclusively forward (anterograde) transport.
• Dynein: a motor protein specialized in backward (retrograde) transport of cargo to the cell body.

Analysis of illustrative histological material

When working with microslides in practical classes, it is important to pay attention to the type of neurons and their staining technique. Let us examine the main examples.

Sensory ganglion (spinal ganglion): Pseudounipolar neurons are localized here (not to be confused with multipolar ones, which are often discussed in general texts). For their visualization at low and high magnification, classical hematoxylin and eosin staining is standardly used.

Spinal cord: Multipolar neurons are present in the spinal cord. 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 shade.
• Nigrosin staining: gives the opposite visual picture — the dark nucleus stands out clearly against the general gray background of the cytoplasm.

Briefly: answers to topical questions

What else to study on this topic

• Structure and functions of neurotubules as guides for transport
• Specifics of neurosecretory cells and synthesis of neurohormones
• Differences between pseudounipolar and multipolar neurons
• Physicochemical bases of hydrodynamic pressure in the intertubular space
• The role of ATP in conformational changes of kinesin and dynein molecules

Other topics in the section "Nervous tissue: neurocytes, gliocytes, nerve fibers"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.