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Figure 2-1. A myelinated neuron. [Source: OpenStax]
A neuron consists of three parts: cell body (soma), dendrites and axon. Its cell body is similar to other cell types with a nucleus and several organelles. The axon extends from the axon hillock in the soma. Its initial segment, known as the axon initial segment (AIS), is the initiation site for action potentials. Some axons are covered by myelin sheath, others do not. The neuron shown in Figure 2-1 contains myelin sheath.
Each neuron has only one axon, although the axon terminal may have several branches, connecting to other neurons. The connecting point between two neurons is called a "synapse". Most synapses are formed between an axon terminal and another neuron's dendrites, but there are also axon-soma and axon-axon synapses. A neuron may have one to a thousand dendrites. Hence a single neuron could be connected with thousands of neurons.
Figure 2-2. Connection between neurons. [Source: NIH]
Neurons have a variety of shapes, three examples are given here: pyramidal neuron, medium spiny neuron and Purkinje neurons. The pyramidal neuron is characterized by its triangular shaped soma (Figure 2-3). They are distributed in the cerebral cortex, hippocampus and amygdala, and also the major excitatory neurons in the prefrontal cortex (PFC). Medium spiny neurons (Figure 2-4) are the main neurons in the striatum, accounting for 95%. The striatum plays a key role in the action selection between "Go" and "NoGo" (Chapter 10). Purkinje neurons (Figure 2-5) is located in the cerebellum, named after its discoverer Jan Evangelista Purkyne.
Figure 2-3. Pyramidal neurons. [Source: Wikipedia]
Figure 2-4. Medium spiny neurons. [Source: Wikipedia]
Figure 2-5. Purkinje neurons. [Source: Wikipedia]