The human nervous system carries out an enormous number of functions by means of many subdivisions. Indeed, the brain’s complexity has traditionally made the study of neuroanatomy a demanding task. This task can be greatly simplified by approaching the study of the nervous system from the dual perspectives of its regional and functional anatomy. Regional neuroanatomy examines the spatial relations between brain structures within a portion of the nervous system. Regional neuroanatomy defines the major brain divisions as well as local, neighborhood relationships within the divisions. In contrast, functional neuroanatomy examines those parts of the nervous system that work together to accomplish a particular task, for example, visual perception. Functional systems are formed by specific neural connections within and between regions of the nervous system; connections that form complex neural circuits. A goal of functional neuroanatomy is to develop an understanding of the neural circuitry underlying behavior. By knowing regional anatomy together with the functions of particular brain structures, the clinician can determine the location of nervous system damage in a patient who has a particular neurological or psychiatric impairment. Combined knowledge of what structures do and where they are located is essential for a complete understanding of nervous system organization. The term neuroanatomy is therefore misleading because it implies that knowledge of structure is sufficient to master this discipline. Indeed, in the study of neuroanatomy, structure and function are tightly interwoven, so much so that they should not be separated. The interrelationships between structure and function underlie functional localization, a key principle of nervous system organization.

This chapter examines the organization of the nervous system and the means to study it by developing the vocabulary to describe its regional anatomy. First, the cellular constituents of the nervous system are described briefly. Then the chapter focuses on the major regions of the nervous system and the functions of these regions. This background gives the reader insight into functional localization.

The nerve cell, or neuron, is the functional cellular unit of the nervous system. Neuroscientists strive to understand the myriad functions of the nervous system partly in terms of the interconnections between neurons. The other major cellular constituent of the nervous system is the neuroglial cell, or glia. Glia provide structural and metabolic support for neurons during development and in maturity.

All Neurons Have a Common Morphological Plan

It is estimated that there are about 100 billion neurons in the adult human brain. Although neurons come in different shapes and sizes, each has four morphologically specialized regions with particular functions: dendrites, cell body, axon, and axon terminals (Figure 1–2A). Dendrites receive information from other neurons. The cell body contains the nucleus and cellular organelles critical for the neuron’s survival and function. The cell body also receives information from other neurons and serves important integrative functions. The axon conducts information, which is encoded in the form of action potentials, to the axon terminal. Connections between two neurons in a neural circuit are made by the axon terminals of one and the dendrites and cell body of the other, at the synapse (discussed below).

Despite a wide range of morphology, we can distinguish three classes of neuron based on the configuration of their dendrites and axons: unipolar, bipolar, and multipolar (Figure 1–2B). These neurons were drawn by the distinguished Spanish neuroanatomist Santiago Ramón y Cajal at the beginning of the twentieth century. Unipolar neurons are the simplest in shape (Figure 1–2B1). They have no dendrites; the cell body of unipolar neurons receives and integrates incoming information. A single axon, which originates from the cell body, gives rise to multiple processes at the terminal. In the human nervous system, unipolar neurons are the least common. They control exocrine gland secretions and smooth muscle contractility.

Bipolar neurons have two processes that arise from opposite poles of the cell body (Figure 1–2B2). The flow of information in bipolar neurons is from one of the processes, which function like a dendrite, across the cell body to the other process, which functions like an axon. A bipolar neuron subtype is a pseudounipolar neuron (see Figure 6–2 top). During development the two processes of the embryonic bipolar neuron fuse into a single process in the pseudounipolar neuron, which bifurcates a short distance from the cell body. Many sensory neurons, such as those that transmit information about odors or touch to the brain, are bipolar and pseudounipolar neurons.

Multipolar neurons feature a complex array of dendrites on the cell body and a single axon that branches extensively (Figure 1–2B3). Most of the neurons in the brain and spinal cord are multipolar. Multipolar neurons that have long axons, with axon terminals located in distant sites, are termed projection neurons. Projection neurons mediate communication between regions of the nervous system and between the nervous system and peripheral targets, such as striated muscle cells. The neuron in Figure 1–2B3 is a particularly complex projection neuron. The terminals of this neuron are not shown because they are located far from the cell body. For this type of neuron in the human, the axon may be up to 1-m long, about 50,000 times the width of the cell body! Other multipolar neurons, commonly called interneurons, have short axons that remain in the same region of the nervous system in which the cell body is located. Interneurons help to process neuronal information within a local brain region.

Neurons Communicate With Each Other at Synapses

Information flow along a neuron is polarized. The dendrites and cell body receive and integrate incoming information, which is transmitted along the axon to the terminals. Communication of information from one neuron to another also is polarized and occurs at specialized sites of contact called synapses. The neuron that sends information is the presynaptic neuron and the one that receives the information is the postsynaptic neuron. The information carried by the presynaptic neuron is most typically transduced at the synapse into a chemical signal that is received by specialized membrane receptors on the dendrites and cell body of the postsynaptic neuron.

The synapse consists of three distinct elements: (1) the presynaptic terminal, the axon terminal of the presynaptic neuron, (2) the synaptic cleft, the narrow intercellular space between the neurons, and (3) the receptive membrane of the postsynaptic neuron. Synapses are present on dendrites, the cell body, the initial segment of the axon, or the portion of the axon closest to the cell body, and the presynaptic axon terminal. Synapses located on different sites can serve different functions.

To send a message to its postsynaptic neurons, a presynaptic neuron releases neurotransmitter, packaged into vesicles, into the synaptic cleft. Neurotransmitters are small molecular weight compounds; among these are amino acids (eg, glutamate; glycine; and γ-aminobutyric acid, or GABA), acetylcholine, and monoaminergic compounds such as norepinephrine and serotonin. Larger molecules, such as peptides (eg, enkephalin and substance P), also can function as neurotransmitters. After release into the synaptic cleft, the neurotransmitter molecules diffuse across the cleft and bind to receptors on the postsynaptic membrane. Neurotransmitters change the permeability of particular ions across the neuronal membrane. A neurotransmitter can either excite the postsynaptic neuron by depolarizing it, or inhibit the neuron by hyperpolarizing it. For example, excitation can be produced by a neurotransmitter that increases the flow of sodium ions into a neuron (ie, depolarization), and inhibition can be produced by a neurotransmitter that increases the flow of chloride ions into a neuron (ie, hyperpolarization). Glutamate and acetylcholine typically excite neurons, whereas GABA and glycine typically inhibit neurons. Many neurotransmitters, like dopamine and serotonin, have more varied actions, exciting some neurons and inhibiting others. Their action depends on a myriad of factors, such as the particular receptor subtype that the neurotransmitter engages and whether the binding of the neurotransmitter leads directly to the change in ion permeability or if the change is mediated by the actions on second messengers and other intracellular signaling pathways (eg, G protein coupled receptors). For example, the dopamine receptor subtype 1 is depolarizing, whereas the type 2 receptor is hyperpolarizing; both act through G protein coupled mechanisms. A neurotransmitter can even have opposing actions on the same neuron depending on the composition of receptor subtypes on the neuron’s membrane. Action through second messengers and other intracellular signaling pathways can have short-term effects, such as changing membrane ion permeability, or long-term effects, such as gene expression. Many small molecules that produce strong effects on neurons are not packaged into vesicles; they are thought to act through diffusion. These compounds, for example, nitric oxide, are produced in the postsynaptic neuron and are thought to act as retrograde messengers that serve important regulatory functions on pre- and postsynaptic neurons, including maintaining and modulating the strength of synaptic connections. These actions are important for learning and memory.

Although chemical synaptic transmission is the most common way of sending messages from one neuron to another, purely electrical communication can occur between neurons. At such electrical synapses, there is direct cytoplasmic continuity between the presynaptic and postsynaptic neurons.

Glial Cells Provide Structural and Metabolic Support for Neurons

Glial cells comprise the other major cellular constituent of the nervous system; they outnumber neurons by about 10 to 1. Given this high ratio, the structural and metabolic support that glial cells provide for neurons must be a formidable task! There are two major classes of glia: microglia and macroglia. Microglia subserve a phagocytic or scavenger role, responding to nervous system infection or damage. They are rapidly mobilized—they become activated—in response to different pathophysiological conditions and trauma. Activated microglia can destroy invading microorganisms, remove debris, and promote tissue repair. Interestingly, they also mediate changes in neuronal properties after nervous system damage; sometimes maladaptive changes, so they may also hinder recovery after injury. For example, neurons often become hyperexcitable after nervous system damage, and microglia can be involved in this process. Macroglia, of which there are four separate types—oligodendrocytes, Schwann cells, astrocytes, and ependymal cells—have a variety of support and nutritive functions. Schwann cells and oligodendrocytes form the myelin sheath around peripheral and central axons, respectively (Figures 1–2A and 1–3). The myelin sheath increases the velocity of action potential conduction. It is whitish in appearance because it is rich in a fatty substance called myelin, which is composed of many different kinds of myelin proteins. Schwann cells also play important roles in organizing the formation of the connective tissue sheaths surrounding peripheral nerves during development and in axon regeneration following damage in maturity. Astrocytes have important structural and metabolic functions. For example, in the developing nervous system, astrocytes act as scaffolds for growing axons and guides for migrating immature neurons. Many synapses are associated with astrocyte processes that may monitor synaptic actions and provide chemical feedback. Astrocytes also contribute to the blood-brain barrier, which protects the vulnerable environment of the brain from invasion of chemical from the periphery, which can influence neuronal firing. The last class of macroglia, ependymal cells, line fluid-filled cavities in the central nervous system (see below). They play an important role in regulating the flow of chemicals from these cavities into the brain.

Neurons and glial cells of the nervous system are organized into two anatomically separate but functionally interdependent parts: the peripheral and the central nervous systems (Figure 1–4A). The peripheral nervous system is subdivided into somatic and autonomic divisions. The somatic division contains the sensory neurons that innervate the skin, muscles, and joints. These neurons detect and, in turn, inform the central nervous system of stimuli. This division also contains the axons of motor neurons that innervate skeletal muscle, although the cell bodies of motor neurons lie within the central nervous system. These axons transmit control signals to muscle to regulate the force of muscle contraction. The autonomic division contains the neurons that innervate glands and the smooth muscle of the viscera and blood vessels (see Chapter 15). This division, with its separate sympathetic, parasympathetic, and enteric subdivisions, regulates body functions based, in part, on information about the body’s internal state.

The central nervous system consists of the spinal cord and brain (Figure 1–4B), and the brain is further subdivided into the medulla, pons, cerebellum, midbrain, diencephalon, and cerebral hemispheres (Figure 1–4C). Within each of the seven central nervous system divisions resides a component of the ventricular system, a labyrinth of fluid-filled cavities that serve various supportive functions (see Figure 1–13). Box 1–1 shows how all of the divisions of the central nervous system and the components of the ventricular system are present from very early in development, from about the first month after conception.