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Neurons arise from roughly spherical neuroblast precursors. Newly born neurons can migrate long distances before growing into dramatically elongated cells. Fully differentiated neurons take many forms, but generally share certain key features (see Figure 22-1). The nucleus is found in a rounded part of the cell called the cell body. Branching cell processes called dendrites (from the Greek for “treelike”) are found at one end, and are the main structures where signals are received from other neurons via synapses. Incoming signals are also received at synapses that form on neuronal cell bodies. Neurons often have extremely long dendrites with complex branches, particularly in the central nervous system (i.e., the brain and spinal cord). This allows them to form synapses with, and receive signals from, a large number of other neurons—
When a neuron is first differentiating, the end of the cell opposite the dendrites undergoes dramatic outgrowth to form a long extended arm called the axon, which is essentially a transmission wire. The growth of axons must be controlled so that proper connections are formed, through a complex process called axon guidance that involves dynamic changes to the cytoskeleton and is discussed in Section 18.8. The diameters of axons vary from just a micrometer in certain neurons of the human brain to a millimeter in the giant fiber of the squid. Axons can be meters in length (e.g., in giraffe necks), and are often partly covered with electrical insulation called the myelin sheath (see Figure 22-1b), which is made by specific classes of glial cells, oligodendrocytes (in the central nervous system) and Schwann cells (in the peripheral nervous system). The insulation speeds electrical transmission and prevents short circuits. The short, branched ends of the axon at the opposite end of the neuron from the dendrites are called the axon termini. This is where signals are passed along to the next neuron or to another type of cell such as a muscle or hormone-