Figure 1.

Features of excitatory neurons in the brain. The cell body (soma) of a typical excitatory pyramidal neuron is ∼10 μm in diameter and is located in one of several sheets of tightly packed somas that define the layers of the neocortex and hippocampus. Apical and basal dendrites extend from the soma, reaching into adjacent areas that are referred to as neuropil. Postsynaptic structures are located in tiny membrane protuberances called spines that can be seen along the dendrites. Each soma gives rise to one axon, which has a smaller diameter than the numerous dendrites. The axon can extend for millimeters from the soma and branches to form thousands of presynaptic terminals where transmitter is released onto the postsynaptic sites of other neurons. The axon hillock is located at the base of the axon. Action potentials are usually initiated at this site; they travel along the axon (arrows) to presynaptic terminals keeping a uniform amplitude of depolarization. Back-propagating action potentials travel in the opposite direction through the soma and into the dendrites. The size of their depolarization decreases as they travel and is regulated by the composition of dendritic ion channels. (From Peters and Kaiserman-Abramof 1970; modified, with permission, © Wiley.)