ABSTRACT

Neurons are distinguished from other cells by neurites, long cylindrical processes that come in two varieties, dendrites and axons. Dendrites are highly branched extensions of the cell body, may be up to 1 mm in length and account for up to 90% of the surface area of many neurons. Nerve cells usually have only one axon which arises typically from the cell body but may emerge from a proximal dendrite. Axons tend to be long, untapered, less highly branched, never spiny and may have a myelin sheath, whereas dendrites are shorter, tapered, highly branched and may bear spines. Axon terminals are often rich in mitochondria which indicates their high requirement for metabolic energy. Thus axonal microtubules can transport mitochondria out of the cell body into the axon but cannot transport other organelles. Because microtubules are orientated in both directions in dendrites, all organelles are transported into these neurites.