Concept 34.1: Nervous Systems Are Composed of Neurons and Glial Cells

Animals need a way to transmit signals at high speeds from place to place within their bodies. Suppose, for example, that you are crowded with other people in the kitchen during a party and, being distracted, you put your hand behind you and lean on a hot stove. You respond immediately—pulling your hand away—because of nerve cells, also called neurons. Mammalian neurons routinely transmit signals at 20–100 meters per second! Thus a danger signal travels the length of your arm to reach your spinal column in less than 0.02 seconds, the first step in allowing you to respond. The speed of signal transmission by neurons is also evident in more ordinary actions. During banjo plucking, each pluck of a string takes place because of neuronal signals that originate by way of interactions among many neurons in the brain and travel to the player’s fingers (FIGURE 34.1). These signals must move at enormous speeds for notes to burst from the instrument as fast as they do.

Figure 34.1: Banjo Plucking Dramatizes the Speed of Neuronal Signaling For each finger pluck, nerve impulses are generated in the brain and travel down the spinal cord and out to the finger that plucks the string. The entire process occurs several times per second.

A neuron is a type of excitable cell, meaning its cell membrane can generate and conduct signals called impulses, or action potentials. The excitable nature of neurons is one of their key specializations. Most types of cells in an animal’s body are not excitable. The two principal types that are excitable are neurons and muscle cells (see Concept 33.1). What do we mean by an impulse, or action potential? Cell membranes ordinarily have an electrical polarity in which the outside of the membrane is more positive than the inside. An impulse, or action potential, is a state of reversed polarity. In a region of the cell membrane where an impulse or action potential is present, the cell membrane is said to be depolarized because its electrical polarity is outside-negative instead of outside-positive. In an excitable cell, by definition, after an action potential is generated at one point in the cell membrane, it propagates over the whole membrane. During this process, the region of depolarization moves along the cell membrane, and the membrane is said to “conduct” the impulse:

This conduction, or propagation, is what happens when a neuron carries a signal from the brain to the fingers, telling the fingers to pull away from danger or to pluck a banjo string. We will discuss action potentials in more detail in Concept 34.2.

Nervous systems are composed of two types of cells, neurons and glial cells. The neurons are excitable, as we have said, but the glial cells typically are not.

Neurons are specialized to produce electric signals

A neuron, or nerve cell, is a cell that is specially adapted to generate electric signals, typically in the form of action potentials. Neurons are very diverse in structure. Here we describe just their most common features. Neurons often are highly elongated. Because of this, they can act like telephone wires (“land lines”), carrying signals (action potentials) rapidly over long distances.

Another way in which neurons resemble telephone wires is that their signals travel to specific destinations—that is, to anatomically defined destinations. A neuron typically must make contact with a target cell for signals in the neuron to affect the target cell directly. Thus, in most cases, cells must be contacted by a neuron to receive the neuron’s signals. The signals, in essence, are addressed: they are received only at defined locations. In short, signal transmission by neurons is fast and addressed.

Places where neurons make functionally relevant contact with other cells are called synapses. More specifically, a synapse is a cell-to-cell contact point that is specialized for signal transmission from one cell to another. One of the two cells at a synapse is a neuron. The other can be another neuron, or it can be some other type of cell such as a sensory cell or muscle cell. Signal transmission at a synapse is usually one-way. A signal arrives at the synapse by way of one cell, called the presynaptic cell, and leaves by way of another, the postsynaptic cell.

Most neurons have four anatomical regions:

• a set of dendrites
• a cell body
• an axon
• a set of presynaptic axon terminals

We can also think of a neuron as having four functional regions, which generally correspond to the anatomical regions (FIGURE 34.2).

Figure 34.2: A Generalized Neuron Four functional regions of the cell are recognized and correspond approximately to the four major anatomical regions. The drawing shows the features typical of most neurons. The form of these features, including axon length and the density and branching pattern of dendrites, can vary greatly among the many different types of neurons.

The dendrites (from the Greek dendron, “tree”) are relatively short cell processes (extensions) that tend to branch from the cell body like twigs on a shrub. They are typically the principal sites where incoming signals arrive from other cells. Hundreds or thousands of other cells—neurons or sensory cells—may bring signals to a single neuron. These cells typically make synaptic contact with the dendrites or cell body.

The cell body contains the nucleus and most of a neuron’s other organelles. The most important function of the cell body is to combine and integrate incoming signals. Earlier we stressed that neurons achieve fast and addressed signal transmission. Of equal importance, neurons integrate signals. This means that a neuron can receive many inputs and combine them to produce a single output, and it can do this in ways that are adaptive and help promote harmonious function of the animal.

In most neurons, one cell process—the axon—is much longer than the others. The axon may extend a long distance. For instance, single axons extend from your spinal cord to your fingers. The axon is the part of a neuron that is anatomically specialized for long-distance signal conduction. Usually the signals (action potentials) conducted by the axon originate in or near the cell body and travel outward from the cell body.

Where the axon ends, it branches, and each of these branches terminates with a small swelling called a presynaptic axon terminal. These terminals make synaptic contact with other cells, enabling signals generated by the neuron to initiate signals in other neurons or in muscle cells. A neuron is said to innervate the cells with which its presynaptic axon terminals make synaptic contact.

A neuron, as we have stressed, is a single cell. Often the axons of many neurons travel together in a bundle from place to place, like a telephone cable composed of many wires. These bundles are called nerves. Individual neurons are far too small to be seen with the naked eye, but nerves consisting of many axons are often easily visible. Biologists use the term “nerve” only for axon bundles outside the brain and spinal cord. A bundle of axons in the brain or spinal cord is called a tract (or sometimes a commissure or connective).

Glial cells support, nourish, and insulate neurons

Glial cells, also called glia or neuroglia, are the second major type of cell in the nervous system. Unlike neurons, they typically are not excitable and do not conduct action potentials. Nonetheless, they are critically important. They are particularly numerous in mammals. Half the volume of the mammalian brain is glial cells, which are smaller than neurons and ten times more numerous than neurons.

Several distinct types of glial cells are present in the nervous system and play distinct roles. Certain glial cells help orient developing neurons toward their target cells during embryonic development. In adult vertebrates, glial cells in the brain and spinal cord provide metabolic support for neurons, help regulate the composition of the extracellular fluids bathing the neurons, and perform immune functions. Sometimes glial cells also assist signal transmission across synapses.

In vertebrates, but generally not in invertebrates, certain glial cells insulate axons electrically by wrapping around them, covering them with concentric layers of lipid-rich cell membrane. In the brain and spinal cord, the cell membranes of glial cells called oligodendrocytes wrap around axons. In neurons outside the brain and spinal cord, glial cells of a different type, called Schwann cells, perform this function (FIGURE 34.3). This multilayered wrap of cell membranes on axons forms a lipid-rich, electrically nonconductive sheath called myelin. Parts of the nervous system that consist mostly of myelinated axons have a glistening white appearance and are sometimes called white matter. Not all vertebrate axons are myelinated, but those that are can conduct action potentials more rapidly than unmyelinated axons, as we will discuss in Concept 34.2. We’ve now seen that nervous systems are composed of neurons and glial cells. In the next concept we will focus on how the neurons generate and transmit action potentials.

Figure 34.3: Electrically Insulating an Axon (A) Schwann cells wrap around the axon, enclosing the axon in many layers of lipid-rich Schwann cell membrane, called myelin, without intervening cytoplasm. Myelin provides electrical insulation to the axon. At the intervals between Schwann cells—the nodes of Ranvier—the axon is exposed. (B) A myelinated axon, seen in cross section at high magnification by use of an electron microscope.