the presence of Alzheimer's disease plaques. Retrograde transport Retrograde transport shuttles molecules/organelles away from axon termini toward the cell body. Retrograde axonal transport is mediated by cytoplasmic dynein, and is used for example to send chemical messages and endocytosis products headed to endolysosomes from the axon back to the cell. Operating at average in vivo speeds of approximately 2 μm/sec, fast retrograde transport can cover 10-20 centimeters per day. Fast retrograde transport returns used synaptic vesicles and other materials to the soma and informs the soma of conditions at the axon terminals. Retrograde transport carries survival signals from the synapse back to the cell body, such as the TRK, the nerve growth factor receptor. Some pathogens exploit this process to invade the nervous system. They enter the distal tips on an axon and travel to the soma by retrograde transport. Examples include tetanus toxin and the herpes simplex, rabies, and polio viruses. In such infections, the delay between infection and the onset of symptoms corresponds to the time needed for the pathogens to reach the somata. Herpes simplex virus travels both ways in axons depending on its life cycle, with retrograde transport dominating polarity for incoming capsids. Consequences of interruption Whenever axonal transport is inhibited or interrupted, normal physiology becomes pathophysiology, and an accumulation of axoplasm, called an axonal spheroid, may result. Because axonal transport can be disrupted in a multitude of ways, axonal spheroids can be seen in many different classes of diseases, including genetic, traumatic, ischemic, infectious, toxic, degenerative and specific white matter diseases called leukoencephalopathies. Several rare neurodegenerative diseases are linked to genetic mutations in the motor proteins, kinesin and dynein, and in those cases, it is likely that axonal transport is a key player in mediating pathology. Dysfunctional axonal transport is also linked to sporadic (common) forms of neurodegenerative diseases such as Alzheimer's and Parkinson's. This is mainly due to numerous observations that large axonal accumulations are invariably seen in affected neurons, and that genes known to play a role in the familial forms of these diseases also have purported roles in normal axonal transport. However, there is little direct evidence for involvement of axonal transport in the latter diseases, and other mechanisms (such as direct synaptotoxicity) may be more relevant. Arrest of axoplasmic flow at the edge of ischemic areas in vascular retinopathies leads to swelling of nerve fibres, which give rise to soft exudates