Neurons

A neuron is the fundamental signaling cell of the nervous system. Each neuron has three main regions: dendrites, branched extensions that receive messages from other cells; the cell body (soma), which contains the nucleus and integrates incoming signals; and the axon, a single fiber that carries the neuron's output signal away, sometimes over long distances. Many axons are wrapped in myelin, a fatty insulating sheath that dramatically speeds transmission.

Neurons communicate through an electrochemical process. When the inputs arriving at a neuron push it past its threshold, it fires an action potential — a brief, all-or-nothing electrical impulse that travels down the axon. At the axon's terminal branches, the impulse triggers release of chemical messengers called neurotransmitters into the synapse, the tiny gap between cells. Neurotransmitters bind receptors on the next cell, either exciting it toward firing or inhibiting it.

Functionally, neurons fall into three classes: sensory (afferent) neurons carry information from the sense organs toward the brain and spinal cord; motor (efferent) neurons carry commands from the central nervous system to muscles and glands; and interneurons, the most numerous type, connect neurons within the central nervous system and handle the processing in between.

Neurons anchor the biological-bases content on several exams. AP Psychology tests neural structure, the action potential, and neurotransmission as the foundation of the biology of behavior; the USMLE Step 1 builds on neuronal signaling in neurophysiology and neuropathology; and the CCMA touches neuron basics within its nervous system unit. Know the parts, the firing sequence, and the three functional classes.