Table I. Motile behavior of axonally transported cargoes.
| Cargo structures | Overall rate (pulse labeling) |
Instantaneous rate (light microscopy) | Directionality | Duty ratio |
|---|---|---|---|---|
| Golgi-derived vesicles (fast anterograde) |
200–400 mm/da (2–5 µm/s) | 1–5 µm/sb | Anterograde | High |
| Endocytic vesicles, lysosomes, autophagosomes (fast retrograde) | 100–250 mm/da (1–3 µm/s) | 1–3 µm/sb | Retrograde | High |
| Mitochondria | <70 mm/dc (<0.8 µm/s) | 0.3–0.7 µm/sd | Bidirectional | Intermediate |
| Microfilaments, cytosolic protein complexes (slow component b) | 2–8 mm/de (0.02–0.09 µm/s) | Unknown | Unknown | Unknown |
| Microtubules, neurofilaments (slow component a) |
0.2–1 mm/de (0.002–0.01 µm/s) | 0.3–1 µm/sf | Bidirectional | Low |
Each rate component of axonal transport corresponds to a distinct group of cargo structures. The overall rate is the average or maximum rate (including movements and pauses) determined by radioisotopic pulse labeling (for technical reasons, the rates of fast axonal transport are generally quoted as maximal rates, whereas the rates of slow axonal transport are generally quoted as average rates). Note that these rates are approximate and that there is considerable variation between different cell types and different stages of development and maturation. The instantaneous rate is the actual rate of movement of the cargo structures (between pauses) determined by light microscopy. The duty ratio is the proportion of the time that the structures spend moving, inferred by comparison of the overall and instantaneous rates. Other axonally transported cargos, such as endoplasmic reticulum, mRNAs, and ribonucleoprotein particles are not included in this table because there is insufficient information about their overall rate of movement at this time. Note that actin may also move in slow component a in some neurons, and tubulin may also move in slow component b (Oblinger et al., 1987), but the significance of this is not clear.