Neurons and Neuroglia

Neuronal Function

While the shape of a neuron can take many forms, the functioning of the adult neuron emphasizes the common features of several cellular subdomains. The goal of this section is not to describe the biochemical and biophysical mechanisms that the cell uses to create, store, and transmit electrical signals. Rather, this section follows a “packet” of information as it moves through a typical nerve cell in the brain, with the goal of introducing the biology of the cell.

The information arrives at the nerve cell at a highly specialized structure known as a synapse ( Fig. 65.3A ), usually located on a specialized dendritic structure known as a spine ( Fig. 65.3B ). At this site, a process from the previous neuron in the circuit approaches to within a few hundred angstroms of the next cell but does not make contact with it (see Fig. 65.3C ). The gap between the cells—the synapse—remains continuous with the extracellular space. The presence of the gap requires a specialized mechanism for transferring the electrical signal from the presynaptic to the postsynaptic cell. This transfer is achieved by way of the presynaptic cell secreting a chemical transmitter substance into the gap. The transmitter is usually a small molecule, primarily glutamate or γ-aminobutyric acid (GABA), but also acetylcholine, noradrenaline, or peptides such as substance P or vasointestinal peptide. Diffusion carries this pulse of chemicals across the gap to the membrane of the postsynaptic cell, which is covered with receptor proteins. These receptors recognize the secreted chemicals and transform the information from the chemical pulse into an electrical event that can be propagated down the neuron. This mechanism is discussed in more detail later.

A synapse can occur virtually anywhere on the postsynaptic cell, but the most common location is on the neuronal dendrite. The dendrites on many neurons have spines, and it is with these structures that the presynaptic neuron forms a synapse. A portion of the spine-rich Purkinje cell dendrite is shown in Fig. 65.3B . Spines can vary in size and shape but are generally no more than a few micrometers in length, with a bulb-like shape at the end of a tapered shaft ( white arrows, enlarged area of Fig. 65.3B ). Synaptic spines usually receive input from a single axon, although examples exist of multiple axons terminating on a single spine. An electrical signal that initiates in a spine travels to the dendritic branch on which it occurs and moves down the dendrite toward the cell body. Although many neuritic processes look similar (especially in culture), a dendrite can usually be distinguished from an axon because it is tapered—decreasing in diameter as distance from the cell body increases. A nerve cell can have a single dendritic shaft emanating from its cell body (as in the Purkinje cell in Fig. 65.2A ), or it can have several. A typical pyramidal cell in the cerebral cortex, for example, has a single apical dendrite but also several basal dendrites; a cerebellar granule cell has four to six short dendrites in a star-shaped configuration around the cell body.

The nerve cell body is the most prominent feature of the neuron in most histologic preparations (e.g., hematoxylin-eosin). If one considers size alone, however, the cell soma is usually considerably smaller than both the dendrite and the axon in surface area and volume. Consider that a typical neuronal cell body is about 25 μm in diameter. If we assume it is roughly spherical, we can calculate that its volume would be around 8000 μm ³ . The axon is much thinner, typically only about 1 μm in diameter; but it can be up to 1 meter in length, meaning its volume can be as much as 3 × 10 ⁶ μm or nearly 400 times the cell body. The volume of the dendrite can be even larger. This relatively small soma-to–process volume ratio represents a logistic problem for the cell body: the genetic machinery is located exclusively in the nucleus, yet the products of this machinery must be transported in relatively large quantities to sites on the cell that are up to a meter away. The ways in which the neuron deals with this challenge is detailed in later sections of this chapter.

Returning to the movement of information along the neuron, once the electrical signal reaches the cell body from the dendrite, it travels to the point on the cell where the axon emerges. The axon is the structure that captures the summed electrical information from the dendrites and cell body and routes it to the next neuron (or target organ) in the circuit. Morphologically, the axon can be distinguished from the dendrite because it typically has a constant caliber over its entire length. Biochemically, the protein composition of the axonal membrane differs from the somatodendritic membrane as do the structural proteins that make up the cytoskeleton, in particular the microtubule-associated proteins. Transverse bands of actin can be found that offer structural support to the cylindrical structure, and longitudinally oriented arrays of microtubules inside the axon serve as transport highways moving vesicles and organelles in both anterior and posterior directions. Soon after the axon leaves the cell body, a specialized region known as the initial segment (or axon hillock ) can be identified ( Fig. 65.4 ). This part of the cell is the biochemical boundary between the cell body and the axon and is the point of initiation of the action potential. Up to this point, the information packets from the dendrites and cell body travel primarily by electrotonic spread, with different packets of electrical activity coming together and summing in a graded (analog) manner. By contrast, the axon transmits information in a strictly digital fashion. If the combined electrogenic signal reaching the axon from the cell body is sufficiently strong, the axon will fire and pass the information along; otherwise, the signal stops and proceeds no farther in the circuit. If the decision is a “go,” the axon transforms the information into a self-propagating electrical wave known as an action potential that travels undiminished down the axon to its end. The axon potential is an electrical signal that results from the coordinated functioning of sodium and potassium channels, usually in collaboration with glial cells (see later).

The synaptic terminal is the site where the information packet leaves the neuron for the next cell in the circuit. The morphology of the axon terminal is topologically complementary to the postsynaptic site (usually a spine) to which it will transfer its information. The regular caliber of the ending of the axon and its linear array of constituent microtubules swells in diameter, and a collection of vesicles and mitochondria are found. When the axon potential reaches the presynaptic terminal, a series of biochemical events is initiated that results in the secretion of a burst of neurotransmitter; this enables the information packet to pass across the acellular gap of the synapse and initiates an electrical response in the next cell. While this description applies directly to many neurons, it is inadequate to fully describe the large number of cells that function as neurons in the human body. Each neuron transmits information from one part of its cell body to another, but the nature of the information transferred is often quite different from one cell to the next. This diversity of functions is paralleled by the diversity of morphologies (see Fig. 65.2 ) and biochemistries.

Sensory Neurons

The fundamental function of the nervous system is to enable an organism to respond quickly to its environment. To this end, a wide variety of cell types have evolved that efficiently transform information about the environment into electrical impulses that can be integrated and translated into a behavioral response. These cells are neurons, but rather than receiving information from a preceding neuron in a circuit, their input comes in the form of signals in the environment. These signals can be grouped into three basic modalities: mechanical, chemical, and physical.

Mechanical Receptors

The simplest receptor cells of this type receive information about touch and pain. As might be expected, most are located in the skin and other integuments. Receptors for light touch are illustrated in Fig. 65.5 . These receptor endings have different precisions and sensitivities and are specialized to receive different types of stimuli. Each receptor represents a specialized ending of a sensory ganglion axon. These include Ruffini endings or Pacini corpuscles (for deep receptors) and Merkel cells or Meissner corpuscles (for more superficial receptors). Sometimes the neurite of a mechanoreceptor is wrapped around the interfusal muscle fibers of one of the striated muscles. The cellular deformation associated with movement of the axonal membrane activates a series of stretch-sensitive ion channels. The ionic current through these channels initiates the electrical activity that signals a sensory stimulus to the organism.

A subset of mechanical receptors has evolved to serve the auditory and vestibular systems. The principal function of these cells is the same as for light touch receptors: deformation of a hair opens a number of specialized ion channels and results in the generation of an electrical signal. The difference in the acousticovestibular system is that “hairs” are actually cilia on the basal surface of the cell and hence are part of the receptor cell itself. Indeed, the ion channels that are opened in response to movement of the hair are located at the tip of the cilium. A diagram of this cell type is shown in Fig. 65.6 . In the auditory system, the vibration of sound waves is transduced into the vibration of fluid in the cochlea. Receptor cells at different positions in the cochlear spiral respond to different auditory frequencies and transmit both pitch and volume information to the auditory system. In the vestibular system, a morphologically similar configuration of receptors is found in the semicircular canals. Movement of the head in any of the three orthogonal planes leads to movement of the fluid in the canals. This movement displaces the cilia of the vestibular receptors and initiates an electrical signal that is transmitted through cranial nerve VIII to the brain, where the vestibular system interprets the information to determine the orientation and movement of the organism in space. In each of these systems, however, the common feature of the receptor is that deflection of a hair leads to deformation of the cellular membrane, which in turn leads to the opening of a specific set of ion channels. The resulting change in conductance leads to an electrical packet of information that moves along the neuron to the rest of the brain.

Effector Neurons

By far the most common target of a neuron is another neuron. Thus most axons terminate with synapses on the dendritic spines or shafts of other neurons (see Fig. 65.3B ). Cell bodies and even presynaptic terminals also serve as common axonal targets. The networks of neurons formed by these interconnections are what allow an organism to perform complex behavioral responses to environmental stimuli. The axon of a neuron can have other targets, however. Usually, the function of these cells is to transmit the calculations of the nervous system to a non–nervous system cell or structure and effect a change in organism behavior.

Neuronal Organization

When several neurons link to form a circuit, sophisticated computational feats can be accomplished, and useful work can be performed. Because of this higher level of organization, neurons can be categorized on the basis of not only their own shape and size, but also the role they play in a circuit. For example, neurons are often divided into two classes based on the distance of the target from the cell body. Projection neurons send long axons to other neurons in distant regions of the nervous system. Thalamocortical neurons (thalamus to cerebral cortex) and dentatorubral neurons (cerebellar dentate nucleus to red nucleus) are two examples of this type of cell. Local circuit interneurons, by contrast, have short, ramified axons; they participate mainly in local computational processes.

Neurons can also be classified based on the function they perform within a circuit. In the simple descriptions of the previous sections, the firing of a neuron was assumed to stimulate the next neuron to fire as well. In fact, neurotransmitters can also inhibit the electrical activity of other neurons. The function of an inhibitory neuron, therefore, is to suppress the action of the downstream target, making it less likely to fire. The role of an inhibitory neuron is crucial to the more sophisticated functions of a neural circuit. It allows for the temporal or spatial sharpening of a response as neighboring cells are prevented from firing or the runaway tendency of a circuit is dampened. Epileptic activity and even more complex neurological disorders, such as autism, may well be due to a failure of such inhibitory action.

Neuronal Structure

Neurons, like most other cells, are pale, clear, and difficult to see in living tissue. As a consequence, many of the significant advances in the study of the nervous system were made possible by improved methods of visualizing the nerve cell and its processes. The basophilic dyes are the oldest and still most widely used method of staining nervous tissue. Common stains of this class include hematoxylin and cresyl violet. These dyes bind avidly to RNA and DNA and thus highlight the heterochromatin of the nucleus and the rough endoplasmic reticulum of the cytoplasm ( Fig. 65.7A ). Basophilic stains are most commonly used on sections with a thickness ranging from 2 to 20 μm. Examination of nervous tissue stained with such reagents results in a clear picture of the cell body and proximal dendrites; axons and distal dendrites are usually invisible, so white matter and neuropil are generally clear of stain.

During the late 1800s and early 1900s, silver salts were found to have a special avidity for nervous tissue. Owing to their binding to neuron-specific classes of intermediate filaments, a variety of protocols were developed that revealed the neuronal axon with great clarity. Among this class of stains, the Bodian and Bielschowsky stains are still commonly used. A special class of silver stain is the Golgi impregnation method. In this procedure, pieces of tissue are incubated for many weeks in heavy metal salts. During this time, a small number of cells take up the salts, filling their entire intracellular spaces. The tissue is then embedded and sectioned. When the sections are “developed” in reducing agents, an opaque black precipitate is formed that fills the impregnated cells completely. For unknown reasons, only 1% to 2% of the cells react in this fashion (seemingly at random). The details of an individual cell can be seen against a clear background (see Fig. 65.7B ). Although the technique reveals the finest details of dendritic structures, axons are more resistant to filling and are commonly invisible in Golgi preparations. The technique is used to best advantage in sections ranging from 80 to 120 μm thick.

In the second half of the 20th century, new technologies dramatically expanded our ability to visualize and analyze the nerve cells of the brain. Beginning in the 1950s and 1960s, the transmission electron microscope led to a quantum leap in the ability to resolve the details of nerve cellular structure. In this method, small pieces of tissue (typically 2 to 3 mm wide) are embedded in plastic and cut with a glass or diamond blade into sections ranging in thickness from tens to hundreds of nanometers. Before embedding, the tissues are usually stained with uranyl acetate, lead citrate, and osmium tetroxide, lipophilic dyes that reveal membrane structure with a high degree of clarity. Phosphotungstic acid is a frequently used stain that has a particular affinity for synapses. The resolution afforded by the electron microscope allows the fine structure of the cell to be revealed, and the organelles of the cell body can be analyzed. The unique advantages of using electron microscopy to view the nervous system include the ability to resolve synaptic structures (see Fig. 65.7E ), such as synaptic vesicles, details of the pre- and postsynaptic membranes, and the material of the synaptic cleft (see Fig. 65.3C ). Axon and dendrite morphology, with their unique collections and arrangements of filaments, can also be seen.

In the 1970s and 1980s, serum antibodies were developed as highly specific stains using the techniques of immunocytochemistry. Lightly fixed tissue is exposed to an antiserum or monoclonal antibody raised against a particular neuronal epitope. The antibody selectively binds to the neural structure that contains that epitope. Most frequently, this primary antibody is revealed through the application of a secondary antibody that is derivatized to carry a detection molecule—either a fluorescent compound, such as fluorescein or rhodamine, or an enzyme, such as horseradish peroxidase or alkaline phosphatase. In the former case, the location of the antibody is determined by examining the tissue in a fluorescence microscope. In the latter instance, the marker enzyme is localized though the use of a specific substrate whose action deposits an insoluble, chromogenic product. Immunocytochemistry is most commonly used to reveal proteins (e.g., tyrosine hydroxylase, MAP-2, GABA _A α ₆ receptor), but the location of carbohydrates (e.g., polysialic acid, gangliosides, chondroitin sulfate proteoglycan) can also be determined. Immunocytochemistry can be used at the electron microscopic level as well, where peroxidase or gold particles are used to reveal the location of the secondary antibody. Fig. 67.5D is an example of immunocytochemistry using two different antibodies to stain a cultured neuron from the mouse cerebral cortex. The first antibody was raised against MAP-2, one of the major cytoskeletal components of neuronal dendrite. It is revealed with a green fluorophore and shows five major dendrites emanating from a cell body, which is also labeled. The second antibody is against ankyrin G, a component of the axon initial segment. This is revealed with a red fluorophore. Note that the axonal process labeled by ankyrin G (indicated by the white arrow ) is devoid of MAP-2, confirming the identity of this process as an axon.

In the 1980s and 1990s, advances in molecular biology enabled the detection of messenger RNA (mRNA) for specific proteins through a technique known as in situ hybridization. Medium- to high-abundance mRNAs can be localized to the cell body (where they await translation into protein). Rather than detection with an antibody, a specific mRNA is labeled by hybridizing one or more nucleotide probes that are uniquely complementary (antisense) to it. The labeled probes are then detected by any of several means, most of which amplify the signal by tagging custom-designed secondary probes or through an antibody reaction against such derivatized nucleotides incorporated into the probe. Commonly used derivatives include biotin or digoxigenin added to a uracil (uridine 5′-triphosphate) to mimic a thymidine that is then incorporated during the synthesis of the probe. The tissue is treated with proteinase to remove bound proteins and then hybridized at temperatures that ensure the specificity of the probe-message interaction. A digoxigenin-labeled probe hybridized to the message for the Purkinje cell–specific RAR-related orphan nuclear receptor RORα is shown in Fig. 65.7C . An important aspect of the interpretation of such images is that the location of the message marks the mRNA, not the protein. In most cases, the in situ hybridization signal is seen in the cell body, although a small number of mRNA molecules are transported into the dendrite. The image shown in Fig. 65.7C could thus be the result of labeling the mRNA for a protein found in the nucleus, the cell body, or the synapse. The strong (purple) signal tells us that the cell is capable of synthesizing the protein (the mRNA is there), but we do not learn anything about where the protein, once synthesized, will be located.