Dendrites and Axon

The Neurons are differentiated from other cells by neurites. These are long cylindrical structures: dendrites and axons. The Dendrites are highly branched extensions of the cell body, up to 1 mm in length, and compose of up to 90% of the surface area of a neuron. A neuron may have one or many dendrites that are arranged in a cell-typical pattern known as the dendritic tree. The majority of synaptic inputs are with dendrites. The Dendrites on spiny neurons are covered with hundreds of tiny projections known as the dendritic spines on which synapses are made. The Nerve cells lacking spines are known as aspiny neurons.

The Nerve cells usually have only one axon. It generally arises from the cell body but may appear from a dendrite, where it leaves the cell body. In either situation the site of origin is known as the axon hillock. The Axons have diameters ranging from 0.2 to 20 µm in humans (but up to 1 mm in invertebrates) and differ in length from a few µm to over a meter. They may be summarized in a myelin sheath. The Axons generally have branches, termed as axon collaterals. The ends of an axon are swollen terminals and generally hold mitochondria and vesicles. Several axons have swellings along their length known as varicosities. The Axon terminals and varicosities are the presynaptic components of a chemical synapses. The Axons are less highly branched than dendrites and dissimilar dendrites do not have protein synthetic machinery. The Proteins made in cell body are moved into and along the axon by the axoplasmic transport. Both dendrites and axons have mitochondria. The Axon terminals are very rich in mitochondria.