Medical Physiology, 3rd Edition

CHAPTER 10. Organization of the Nervous System

Bruce R. Ransom

The human brain is the most complex tissue in the body. It mediates behavior ranging from simple movements and sensory perception to learning, memory, and consciousness. It is the organ of the mind and accounts for the human capacity for invention, discovery, and language. Many of the brain's functions are poorly understood. In fact, the most prominent function of the human brain, its capacity to think, is hardly understood at all. Its capacity to reflect upon itself is a philosophical paradox. Our lack of knowledge about fundamental aspects of brain function stands in marked contrast to the level of comprehension that we have about the primary functions of other organ systems such as the heart, lungs, and kidneys. Nevertheless, tremendous strides have been made in the past few decades.

In this part of the book, we present the physiology of the nervous system in a manner that is intended to be complementary to texts on neurobiology and neuroanatomy. In this chapter, we review the basic cellular, developmental, and gross anatomy of the nervous system. In Chapter 11, we discuss the fluid environment of the neurons in the brain, how this environment interacts with the rest of the extracellular fluid of the body, and the role of glial cells. Chapters 12 and 13 focus on the broad physiological principles that underlie how the brain's cellular elements operate. Another major goal of this section is to provide more detailed information on those parts of the nervous system that play key roles in the physiology of other systems in the body. Thus, in Chapter 14, we discuss the autonomic nervous system, which controls viscera such as the heart, lungs, and gastrointestinal tract. Finally, in Chapters 15 and 16, we discuss the special senses and simple neuronal circuits.

The nervous system can be divided into central, peripheral, and autonomic nervous systems

The manner in which the nervous system is subdivided is somewhat arbitrary. All elements of the nervous system work closely together in a way that has no clear boundaries. Nevertheless, the traditional definitions of the subdivisions provide a useful framework for talking about the brain and its connections, and are important if only for that reason.

The central nervous system (CNS) consists of the brain and spinal cord (Table 10-1). It is covered by three “membranes”—the meninges. The outer membrane is the dura mater; the middle is the arachnoid; and the delicate inner membrane is called the pia mater. Within the CNS, some neurons that share similar functions are grouped into aggregations called nuclei. The CNS can also be divided into gray matter, which contains neuron cell bodies, and white matter, which is rich in myelin (see pp. 199–201).

TABLE 10-1

Subdivisions of the Nervous System

SUBDIVISION

COMPONENTS

SPECIAL FEATURES

Central

Brain (including CN II and retina) and spinal cord

Oligodendrocytes provide myelin
Axons cannot regenerate

Peripheral

Peripheral ganglia (including cell bodies); sensory receptors; peripheral portions of spinal and cranial nerves (except CN II), both afferent and efferent

Schwann cells provide myelin
Axons can regenerate

Autonomic

Selected portions of the CNS and PNS

Functionally distinct system

CN, cranial nerve.

The peripheral nervous system (PNS) consists of those parts of the nervous system that lie outside the dura mater (see Table 10-1). These elements include sensory receptors for various kinds of stimuli, the peripheral portions of spinal and cranial nerves, and all the peripheral portions of the autonomic nervous system. The sensory nerves that carry messages from the periphery to the CNS are termed afferent nerves (from the Latin ad + ferens [carrying toward]). Conversely, the peripheral motor nerves that carry messages from the CNS to peripheral tissues are called efferent nerves (from the Latin ex + ferens [carrying away]). Peripheral ganglia are groups of nerve cells concentrated into small knots or clumps that are located outside the CNS.

The autonomic nervous system (ANS) is that portion of the nervous system that regulates and controls visceral functions, including heart rate, blood pressure, digestion, temperature regulation, and reproductive function. Although the ANS is a functionally distinct system, it is anatomically composed of parts of the CNS and PNS (see Table 10-1). Visceral control is achieved by reflex arcs that consist of visceral afferent (i.e., sensory) neurons that send messages from the periphery to the CNS, control centers in the CNS that receive this input, and visceral motor output. Moreover, visceral afferent fibers typically travel together with visceral efferent fibers.

Each area of the nervous system has unique nerve cells and a different function

Nervous tissue is composed of neurons and neuroglial cells. Neurons vary greatly in their structure throughout the nervous system, but they all share certain features that tailor them for the unique purpose of electrical communication (see Chapter 12). Neuroglial cells, often simply called glia, are not primary signaling cells and have variable structures that are suited for their diverse functions (see Chapter 11).

The human brain contains ~1011 neurons and slightly more glial cells. Each of these neurons may interact with thousands of other neurons, which helps explain the awesome complexity of the nervous system.

Few, if any, of the receptors, ion channels, or cells in the human brain are unique to humans. The unparalleled capabilities of the human brain are presumed to result from its unique patterns of connectivity and its large size.

The brain's diverse functions are the result of tremendous regional specialization. Different brain areas are composed of neurons that have special shapes, physiological properties, and connections. One part of the brain, therefore, cannot substitute functionally for another part that has failed. Any compensation of neural function by a patient with a brain lesion (e.g., a stroke) reflects enhancement of existing circuits or recruitment of latent circuits. A corollary is that damage to a specific part of the brain causes predictable symptoms that enable a clinician to establish the anatomical location of the problem, a key step in diagnosis of neurological diseases.

Cells of the Nervous System

Development of Neurons and Glial Cells

Subdivisions of the Nervous System

References