Parts of a Neuron Each neuron has a cell body, or soma, which, like other cells, houses a nucleus, smooth and
rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. In
addition, neurons have special structures for receiving and transmitting the electrical signals
necessary for neuronal communication, as shown in Figure 146.
Dendrites are tree-like structures
that extend away from the cell body to receive messages from other neurons at specialized junctions
called
synapses . Dendrites are a feature of some types of neurons, though some neurons have none
at all. Small protrusions on dendrites known as dendritic spines increase the surface area for potential
synaptic connections.
A signal that has been picked up by the dendrite then moves passively to the cell body. The
cell body contains a specialized structure, the
axon hillock that integrates signals from multiple
synapses and serves as a junction between the cell body and an
axon . An axon is a tube-like structure
that propagates the integrated signal to specialized endings called
axon terminals . These terminals
connect with additional neurons, muscles, or target organs in turn. Signals can reach these additional
cells thanks to chemicals released at axon terminals. Axons are typically found on one or two neurons,
but some neurons, such as amacrine cells in the retina, lack them entirely. Some axons are covered
with
myelin , which acts as an insulator to minimize dissipation of the electrical signal as it travels
down the axon, greatly increasing the speed of conduction. This insulation is crucial because a human
motor neuron's axon, which extends from the spine's base to the toes, can be up to a meter long.
Actually, the myelin sheath is not a portion of the neuron. Glial cells produce the myelin.
