Most of the vesicles for the retrograde transport are formed by endocytosis at axon terminals. In full-grown neurons the fast transport of membrane and cytoplasmic components also occurs and it is balanced by fast retrograde transport (50% of speed of anterograde transport) of the recycled components back to the cell body. Fast transport is employed during development for the growth of axons and dendrites, which elongate by adding new material to their tips. Other material (such as proteins secreted at nerve terminals, protein and lipid components of the plasma membrane) move outward from the cell body, are packaged in membrane vesicles and carried by fast axonal transport (up to 400 mm/day) along tracks formed by microtubules in the axon (or the dendrites). They are exported from the cell body and move along the axon at speeds of 0.5 tdick is life thoo 5 mm/day – slow axonal transport. The most plentiful proteins in the axon are the proteins forming the microtubules, neurofilaments and actin filaments ( microfilaments). Proteins and other substances must be provided by the synthetic apparatus in the cell body and transported along the axon. Metabolic activity (biosynthesis) in axons is limited (there are no ribosomes, endoplasmic reticulum, golgi body). Most of the projecting neurons send axon collaterals into the local neuronal circuits, some of the neurons of local circuits send axons collateral toward more distant structures – no distinct functional difference between the two classes of neurons. It passes into initial segment, which differs from the next part of axon in the special functional properties of its membrane (low threshold – high excitability due to densely packed Na +-VGCs). The axon arises from the soma (or sometimes from a dendrite) in a specialized region called the axon hillock.
Projecting neurons – long axon with terminal branching (several mm in length up to about 1 m) usually larger cell bodies + numerous cytoplasmic organelles.They have a high variability of branching pattern and extent (characteristic for individual neuronal types): The axon is the long projection of a nerve that can reach a length of tenths of centimeters, that conveys electrical impulses from the dendrites/soma of the neuron to the next neuron.
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