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The brain’s primary function is to produce behavior, or movement. To produce behavior as we search, explore, and manipulate our environment, the brain must absorb information about the world—about the objects around us: their size, shape, and location. Without stimuli, the brain cannot orient the body and direct it to produce an appropriate response.

The nervous system’s organs are designed to admit information from the world and to convert this information into biological activity that produces perception, subjective experiences of reality. The brain thus produces what we believe is reality so that we can move. This subjective reality is essential to carrying out any complex task.

When you answer the telephone, for example, your brain directs your body to reach for it as the nervous system responds to vibrating air molecules by producing the subjective experience of a ringtone. We perceive this stimulus as sound and react to it as if it actually exists, when in fact the sound is merely a fabrication of the brain. That fabrication is produced by a chain reaction that takes place when vibrating air molecules hit the eardrum. Without the nervous system, especially the brain, sound does not exist—only the movement of air molecules.

But there is more to hearing a phone’s ringtone than vibrating air molecules. Our mental construct of reality is based not only on the sensory information we receive but also on the cognitive processes we might use to interact with that incoming information. Hearing a ringtone when we are expecting a call has a meaning vastly different from its ringing at three o’clock in the morning, when we are not expecting a call.

The subjective reality the brain constructs can be better understood by comparing the sensory realities of two different kinds of animals. You are probably aware that dogs perceive sounds that humans do not. This difference in perception does not mean that a dog’s nervous system is better than ours or that our hearing is poorer. Rather, the perceptual world constructed by a dog brain simply differs from that of a human brain. Neither experience is “correct.” The difference in subjective experience is due merely to two differently evolved systems for processing sensory stimuli.

When it comes to visual perception, our world is rich with color, whereas dogs see very little color. Human brains and dog brains construct different realities. Subjective differences in brains exist for good reason: they allow different animals to exploit different features in their environments. Dogs use their hearing to detect the movements of prey, such as mice in the grass; early humans probably used color vision for identifying ripe fruit in trees. Evolution, then, fosters adaptability, equipping each species with a view of the world that helps it survive.

The brain is plastic: neural tissue has the capacity to adapt to the world by changing how its functions are organized. Just as the brain of the young man profiled in Research Focus 2-1 adapted to cerebellar agenesis, a person blind from birth has enhanced auditory capacities because some of the brain’s visual regions have been co-opted for hearing. The brain is also plastic in the sense that connections among neurons in a given functional system are constantly changing in response to experience.

For us to learn anything new, neural circuits must change to represent and store this knowledge. As we learn to play a musical instrument or speak a new language, the cortical regions taking part can actually increase in size to accommodate the learning. An important aspect of human learning and brain plasticity is related to the development of language and to the expansion of the brain regions related to language. We have learned to read, to calculate, to compose and play music, and to develop the sciences. Clearly, the human nervous system evolved long before we mastered these achievements.

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In turn, culture now plays a dominant role in shaping our behavior. Because we drive cars and communicate electronically, we—and our nervous system—must differ from those of our ancestors who did not engage in these activities. The basis for change in the nervous system is neuroplasticity, the nervous system’s fundamental potential for physical or chemical change that enhances its adaptability to environmental change and its ability to compensate for injury.

Although it is tempting to see neuroplasticity as a trait unique to animals’ nervous systems, it is really part of a larger biological capacity called phenotypic plasticity, the individual’s capacity to develop into more than one phenotype—the characteristics we can see or measure. (See Gilbert & Epel, 2009, for a wonderful discussion of biological plasticity.) Stated simply, an individual’s genotype (genetic makeup) interacts with the environment to elicit a specific phenotype. This phenotype emerges from a large genetic repertoire of possibilities, a phenomenon that in turn results from epigenetic influences.

Epigenetic factors do not change genes but rather influence how genes inherited from parents express specific traits. The two mice pictured in Figure 2-1 appear very different: one is fat, one thin; one has dark fur, the other is light-colored. Yet these mice essentially are clones, genetically identical. They appear so different because their mothers were fed different diets while pregnant. The diet supplements added chemical markers, or epigenetic tags, on specific genes. The tags determine whether the gene is available to influence cells, including neurons, leading to differences in body structure and eating behavior.

From an anatomical standpoint, the brain and spinal cord together make up the central nervous system. The nerve fibers radiating out beyond the brain and spinal cord, as well as all the neurons outside the brain and spinal cord, form the peripheral nervous system. Figure 2-2A charts this anatomical organization. PNS nerves carry sensory information into the CNS and carry motor instructions from the CNS to the body’s muscles and tissues, including those that perform such functions as blood circulation and digestion.

In a functional organization, little changes, but the focus turns to how the parts of the system work together. Neurons in the somatic division of the PNS connect through the cranial and spinal nerves to receptors on the body’s surface and on its muscles. Somatic neurons gather sensory information for the CNS and convey information from the CNS to move muscles of the face, body, and limbs. Similarly, the autonomic division of the PNS enables the CNS to govern the workings of your body’s internal organs—your heartbeat, urination, pupillary response, and the diaphragm movements that inflate and deflate your lungs. The enteric nervous system, which is sometimes considered part of the ANS, controls digestion and stomach contractions.

And so, from a functional standpoint (Figure 2-2B), the major PNS divisions step up to constitute, along with the CNS, an interacting four-part system:

The directional flow of neural information is important. Afferent (incoming) information is sensory, coming into the CNS or one of its parts, whereas efferent (outgoing) information is leaving the CNS or one of its parts. When you step on a tack, the afferent sensory signals are transmitted from the body into the brain and felt as pain. Efferent signals from the brain trigger a motor response: you lift your foot (Figure 2-3).

When buying a new car, people like to look under the hood and examine the engine, the part of the car responsible for most of its behavior—and misbehavior. All most of us can do is gaze at the maze of tubes, wires, boxes, and fluid reservoirs. What we see makes no sense except in the most general way. We know that the engine somehow generates power to make the car move and to run the sound system, lights, and wipers. But knowing this tells us nothing about what all the many engine parts do.

When it comes to our behavior, the brain is the engine. In many ways, examining a brain for the first time is similar to looking under the car hood. We have a vague sense of what the brain does, but most of us have no sense of how its parts accomplish these tasks. We may not even be able to identify those parts. If you are familiar with the anatomical terms and orientations used in drawings and images of brains, read on. If you prefer to review this terminology before you continue, consult The Basics: Finding Your Way Around the Brain on pages 38–39.

Protecting the Nervous System

We start our functional overview by opening the hood and observing the brain snug in its home in the skull. The first thing you encounter is not the brain but rather a tough triple-layered protective covering, the meninges (sing. meninx) (Figure 2-4). The outer dura mater (from Latin, meaning hard mother) is a tough double layer of fibrous tissue that encloses the brain and spinal cord in a kind of loose sac. In the middle is the arachnoid layer (from Greek, meaning like a spider’s web), an ultrathin sheet of delicate connective tissue that follows the brain’s contours. The inner layer, or pia mater (from Latin, meaning soft mother), is a moderately tough membrane of connective tissue fibers that cling to the brain’s surface.

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Between the arachnoid layer and the pia mater flows cerebrospinal fluid (CSF), a colorless solution of sodium chloride and other salts. CSF cushions the brain so that it can move or expand slightly without pressing on the skull. The symptoms of meningitis, an infection of the meninges and CSF, are described in Clinical Focus 2-2, Meningitis and Encephalitis, on page 42.

Cerebral Geography

After removing the meninges, we can examine the brain’s surface features, most prominently its two nearly symmetrical left and right hemispheres. Figure 2-5 diagrams the left hemisphere of a typical human forebrain oriented in the upright human skull. The outer forebrain consists of a thin, folded film of nerve tissue, the cerebral cortex, detailed in the frontal view in Figure 2-5. The word cortex, Latin for tree bark, is apt, considering the cortex’s heavily folded surface and its location, covering most of the rest of the brain. Unlike the bark on a tree, however, the brain’s folds are not random but rather demarcate its functional cortical zones.

Make a fist with your right hand and hold it up, as shown on the right in Figure 2-5, to represent the positions of the forebrain’s broad divisions, or lobes, in the skull. Each lobe is named for the skull bone it lies beneath.

Examining the Brain’s Surface from All Angles

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As we look at the dorsal view in Figure 2-6A, the brain’s wrinkled left and right hemispheres resemble a walnut meat taken whole from its shell. These hemispheres constitute the cerebrum, the major forebrain structure and most recently evolved feature of the CNS. Visible from the opposite ventral view in Figure 2-6B are the brainstem, including the wrinkly hemispheres of the smaller cerebellum (Latin for little brain). Both the cerebrum and the brainstem are visible in the lateral and medial views in Figure 2-6C and D.

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Much of the crinkled-up cerebral cortex is invisible from the brain’s surface. All we can see are bumps, or gyri (sing. gyrus), and cracks, or sulci (sing. sulcus). Some sulci are so deep that they are called fissures. The longitudinal fissure runs between the cerebral hemispheres and the lateral fissure along the sides of the brain. Both are shown in various views in Figure 2-6, along with the central sulcus that runs from the lateral fissures across the top of the cerebrum.

Looking at the bottom of the brain, the ventral view in Figure 2-6B, we see in the midst of the wrinkled cerebrum and ventral to the cerebellum a smooth, whitish structure with little tubes attached. This central set of structures is the brainstem, the area responsible for most unconscious behavior. The tubes mark out the cranial nerves that run to and from the brain as part of the SNS.

Cerebral Circulation

The brain’s surface appears to be covered with blood vessels. As with the rest of the body, the arteries feed blood to the brain and send it back through veins to the kidneys and lungs for cleaning and oxygenation. The cerebral arteries emerge from the neck to wrap around the outside of the brainstem, cerebrum, and cerebellum, finally penetrating the brain’s surface to nourish its inner regions.

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Three major arteries send blood to the cerebrum—the anterior, middle, and posterior cerebral arteries, shown in Figure 2-7. Because the brain is highly sensitive to blood loss, a blockage or break in a cerebral artery is likely to lead to the death of the affected region. This condition, known as stroke, is the sudden appearance of neurological symptoms as a result of severely interrupted blood flow. Because the three cerebral arteries supply different parts of the brain, strokes disrupt different brain functions, depending on the artery affected.

Because the brain’s connections are crossed, stroke in the left hemisphere affects sensation and movement on the right side of the body. The opposite is true for those with strokes in the right hemisphere. Clinical Focus 2-3, Stroke, on page 45, describes some disruptions that stroke causes, both to the person who has it and to those who care for stroke victims.

The simplest way to examine the inside of something is to cut it in half. Of course, the orientation of the cut affects what we see. Consider slicing through a pear. If we cut from side to side, we cut across the core, providing a dorsal view; if we cut from top to bottom, we cut parallel to the core, providing a medial view. Our impression of the inside of a pear is clearly influenced by how we slice it. The same is true of the brain.

Macro View

We can reveal the brain’s inner features by slicing it parallel to the front of the body, downward through the middle in a coronal section (Figure 2-8A). The resulting frontal view, shown in Figure 2-8B, makes immediately apparent that the brain’s interior is not homogeneous. Both dark and light regions of tissue are visible, and though these regions may not be as distinctive as car engine parts, they nevertheless represent different brain components.

The darker regions are gray matter, largely composed of cell bodies and capillary blood vessels. Gray matter neurons either collect and modify information or support this activity. The lighter regions are white matter, mostly nerve fibers with fatty coverings that produce the white appearance, much as fat droplets in milk make it appear white. White matter fibers form connections between and among the brain’s cells.

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A second feature, apparent at the center of our frontal view in Figure 2-8B, are the ventricles—two wing-shaped cavities that contain cerebrospinal fluid. The brain’s four ventricles, shown in place in Figure 2-9, are filled with CSF made by the cells lining the ventricles. All four ventricles are connected, so CSF flows from the two lateral ventricles to the third and fourth ventricles, which lie on the brain’s midline, and into the cerebral aqueduct, a canal that runs down the length of the spinal cord. CSF is also found in the space between the lower layers of the meninges that wrap around the brain and spinal cord (see Figure 2-4).

Although the ventricles’ functions are not well understood, researchers think that they play an important role in maintaining brain metabolism. CSF may allow certain compounds access to the brain, and it probably helps the brain excrete metabolic wastes. In the event of trauma to the brain or spinal cord, CSF cushions the blow.

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Cutting through the brain vertically from front to back produces a sagittal section (Figure 2-10A). If we make our cut down the brain’s midline, that is, in the midsagittal plane, we divide the cerebrum into its two hemispheres, revealing several distinctive structures in the resulting medial view (Figure 2-10B). One feature is a long band of white matter that runs much of the length of the cerebral hemispheres. This band, the corpus callosum, contains about 200 million nerve fibers that join the two hemispheres and allow them to communicate.

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Figure 2-10B clearly shows that the cortex covers the cerebral hemispheres above the corpus callosum; below it are various internal subcortical regions. The brainstem is a subcortical structure that generally controls basic physiological functions. But many subcortical regions are forebrain structures intimately related to the cortical areas that process motor, sensory, perceptual, and cognitive functions. This relation between the cortex and the subcortex alerts us to the concept that redundant functions exist at many levels of nervous system organization.

If you were to compare medial views of the left and right hemispheres, you would be struck by their symmetry. The brain, in fact, has two of nearly every structure, one on each side. The few one-of-a-kind structures, such as the third and fourth ventricles, lie along the brain’s midline (see Figure 2-9B). Another one-of-a-kind structure is the pineal gland, which Descartes declared the seat of the mind in his dualistic theory of how the brain works.

Microscopic Inspection: Cells and Fibers

The brain’s fundamental units—its cells—are so small that they can be viewed only with the aid of a microscope. A microscope quickly reveals that the brain has two main types of cells, illustrated in Figure 2-11. Neurons carry out the brain’s major functions, whereas glial cells aid and modulate the neurons’ activities—for example, by insulating them. Both neurons and glia come in many forms, each marked by the work that they do.

We can see the brain’s internal structures in even greater detail by dyeing their cells with special stains (Figure 2-12). For example, if we use a dye that selectively stains cell bodies, we can see that the neurons in the cortical gray matter lie in layers, revealed by the bands of tissue in Figure 2-12A and C. Each layer contains cells that stain characteristically. Figure 2-12A and B shows that stained subcortical regions are composed of clusters, or nuclei, of similar cells.

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Although layers and nuclei appear very different, both form functional units in the brain. Whether a particular brain region has layers or nuclei is largely an accident of evolution. By using a stain that selectively dyes neuronal fibers, as shown in Figure 2-12B and D, we can see the borders of the subcortical nuclei more clearly. In addition, we can see that the stained cell bodies lie in regions adjacent to those with most of the fibers.

A key feature of neurons is that they are connected to one another by fibers known as axons. When axons run along together, much like the wires that run from a car engine to the dashboard, they form a nerve or tract (Figure 2-13). By convention, a tract is a collection of nerve fibers in the brain and spinal cord, whereas bundles of fibers outside of the CNS are typically called nerves. Thus, the pathway from the eye to the brain is the optic nerve, whereas the pathway from the cerebral cortex to the spinal cord is the corticospinal tract.

Overview of Brain Function and Structure

Before you continue, check your understanding.

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Answers appear in the Self Test section of the book.