Neurons are the main type of cell in the nervous system that generate and transmit electrochemical signals. They primarily communicate with each other using neurotransmitters at specific junctions called synapses. Neurons come in many shapes that often relate to their function, but most share three main structures: an axon and dendrites that extend out from a cell body.
The neuronal cell body—the soma— houses the nucleus and organelles vital to cellular function. Extending from the cell body are thin structures that are specialized for receiving and sending signals. Dendrites typically receive signals while the axon passes on the signals to other cells, such as other neurons or muscle cells. The point at which a neuron makes a connection to another cell is called a synapse.
Neurons receive inputs primarily at postsynaptic terminals, which are frequently located on spines—small bumps protruding from the dendrites. These specialized structures contain receptors for neurotransmitters and other chemical signals. Dendrites are often highly branched, allowing some neurons to receive tens of thousands of inputs. Neurons most commonly receive signals at their dendrites, but they can also have synapses in other areas, such as the cell body.
The signal received at the synapses travels down the dendrite to the soma, where the cell can process it and determine whether it should send the message forward or not. The action potential is the main electrical signal generated by neurons. It carries the information forward onto the next cell. It is first generated at the axon hillock—the junction between the soma and the axon.
Axons vary in length but can be quite long. For example, some extend from the spinal cord all the way to the foot. Longer axons are usually wrapped in a fatty myelin sheath that insulates the axon, helping to maintain the electrical signal. The myelin sheath is created by glia—another type of cell in the nervous system. In myelinated axons, the action potential is regenerated at each node of Ranvier—repeated gaps in the myelin—until it reaches the terminal at the end of the axon, or presynaptic terminal.
The presynaptic terminal has vesicles that contain pools of neurotransmitters. Action potentials trigger the vesicles to undergo exocytosis by fusing to the cell membrane and releasing neurotransmitter into the synaptic cleft—the gap between cells at a synapse. Different neurotransmitters can have varying effects on the postsynaptic cell. An excitatory synapse increases the chances of initiating an action potential on the postsynaptic cell, while an inhibitory synapse decreases the chances of an action potential.
The overall shape of neurons—their morphology—can vary dramatically and often relates to their function. Some neurons have few dendritic processes and a single axon, others have very convoluted dendritic arbors, while others have axons that can span the length of the organism. The diverse morphologies are often used to define the type of neuron. The number of inputs—synaptic connections—can influence how a cell responds to signals. Therefore, the morphology of the dendrites, and the number of synapses they contain, is an important feature that can determine the type of neuron. In the peripheral nervous system, the dendrites can also define the receptive field of a cell—the physical space on the body that they are sensitive to.
The Spanish anatomist Santiago Ramon y Cajal, working in the late 19th and early 20th century, pioneered the tracing of individual neurons and provided fundamental insights into their very nature. He produced stunning depictions of cells that still offer a considerable amount of detail. Using the staining technique developed and named after the Italian biologist Camillo Golgi, he was able to trace the structure of many different kinds of cells in the brain. He also sketched some of the basic connections of neuronal circuits—networks of neurons that are activated together to process specific information.
