In 1887, Santiago Ramón y Cajal, a doctor studying slides of brain tissue, first theorized that the nervous system was made up of individual cells (Ramón y Cajal, translation, 1995). Although the entire body is composed of cells, each type of cell has a special purpose and function and, therefore, a special structure. For example, skin cells are flat, but muscle cells are long and stretchy. Most cells have three things in common: a nucleus, a cell body, and a cell membrane holding it all together. The neuron is the specialized cell in the nervous system that receives and sends messages within that system. Neurons are one of the messengers of the body, and that means that they have a very special structure, which we will explore in Video Figure 2.1: The Structure of the Neuron.

The parts of the neuron that receive messages from other cells are called the dendrites. The name dendrite means “tree-like,” or “branch,” and this structure does indeed look like the branches of a tree. The dendrites are attached to the cell body, or soma, which is the part of the cell that contains the nucleus and keeps the entire cell alive and functioning. The axon (from the Greek for “axis”) is a fiber attached to the soma, and its job is to carry messages out to other cells. The end of the axon branches out into several shorter fibers that have swellings or little knobs on the ends called axon terminals (may also be called presynaptic terminals, terminal buttons, or synaptic knobs), which are responsible for communicating with other nerve cells.

Neurons make up a large part of the brain but they are not the only cells that affect our thinking, learning, memory, perception, and all of the other facets of life that make us who we are. The other primary cells are called glia, or glial cells, which serve a variety of functions. Some glia serves as a structure on which the neurons develop and work and that hold the neurons in place. For example, during early brain development, radial glial cells (extending from inner to outer areas like the spokes of a wheel) help guide migrating neurons to form the outer layers of the brain. Other glia are involved in getting nutrients to the neurons, cleaning up the remains of neurons that have died, communicating with neurons and other glial cells, and providing insulation for neurons. Glial cells affect both the functioning and structure of neurons and specific types also have properties similar to stem cells, which allow them to develop into new neurons, both during prenatal development and in adult mammals (Bullock et al., 2005; Kriegstein & Alvarez-Buylla, 2009). Glial cells are also being investigated for their possible role in a variety of psychiatric disorders, including major depressive disorder and schizophrenia. It appears in some areas of the brain, major depressive disorder is characterized by lower numbers of specific glial cells whereas in schizophrenia, parts of the brain have a greater number (Blank & Prinz, 2013). (See Learning Objectives 12.10 and 12.15.) Recent findings also implicate glial cells in learning and behavior, both by affecting synaptic connectivity during development, and in mice transplanted with human glial cells, faster learning across a variety of learning and memory tasks (Han et al., 2013; Ji et al., 2013).

Two special types of glial cells, called oligodendrocytes and Schwann cells, generate a layer of fatty substances called myelin. Oligodendrocytes produce myelin in the brain and spinal cord (the central nervous system); Schwann cells produce myelin in the neurons of the body (the peripheral nervous system). Myelin wraps around the shaft of the axons, forming an insulating and protective sheath. Bundles of myelin-coated axons travel together as “cables” in the central nervous system called tracts, and in the peripheral nervous system bundles of axons are called nerves. Myelin from Schwann cells has a unique feature that can serve as a tunnel through which damaged nerve fibers can reconnect and repair themselves. That’s why a severed toe might actually regain some function and feeling if sewn back on in time. Unfortunately, myelin from oligodendrocytes covering axons in the brain and spinal cord does not have this feature, and these axons are more likely to be permanently damaged.

The myelin sheath is a very important part of the neuron. It not only insulates the neuron, but it also offers a little protection from damage and speeds up the neural message traveling down the axon. Sections of myelin bump up next to each other on the axon, similar to the way sausages are linked together. The places where the myelin seems to bump are actually small spaces on the axon called nodes, which are not covered in myelin. Myelinated and unmyelinated sections of axons have slightly different electrical properties. There are also far more ion channels at each node. Both of these features affect the speed at which the electrical signal is conducted down the axon. When the electrical impulse that is the neural message travels down an axon coated with myelin, the electrical impulse is regenerated at each node and appears to “jump” or skip rapidly from node to node down the axon (Koester & Siegelbaum, 2013; Schwartz et al., 2013). That makes the message go much faster down the coated axon than it would down an uncoated axon of a neuron in the brain. In the disease called multiple sclerosis (MS), the myelin sheath is destroyed (possibly by the individual’s own immune system), which leads to diminished or complete loss of neural functioning in those damaged cells. Early symptoms of MS may include fatigue, changes in vision, balance problems, and numbness, tingling, or muscle weakness in the arms or legs.

Exactly how does this “electrical message” work inside the cell?