The Architecture of the Nervous System

THE  ARCHITECTURE OF THE NERVOUS SYSTEM

By using the tools we’ve just described, researchers have gained a detailed understand-ing of the structure and functioning of the nervous system. No matter what part of the nervous system we consider, however, one theme quickly emerges—a theme of func-tional specialization: Each part of the brain—indeed, each part of the nervous systemthroughout the body—performs its own job, and the functioning of the whole depends on an exquisite coordination among these various elements. As a result, we need to understand the nervous system with reference to some anatomy—so that we’ll know what the various parts are, and approximately where they are. On this basis, let’s start by taking a broad tour of the nervous system.

The Central and Peripheral Nervous Systems

At the most general level, the nervous system can be divided into several parts (Figure 3.25). The central nervous system (CNS) includes the brain and spinal cord, working as an integrated unit. All nerves elsewhere in the body are part of the peripheral nervous system (PNS), and virtually all of the nerves in the peripheralnervous system connect to the CNS via the spinal cord. This is part of the reason why damage to the cord is so dangerous, and why the cord is protected by the bones and connective tissue of the spine. Of course, the brain, too, is well protected. It’s cov-ered, first, in a shell of bone (the skull) and three layers of tough membranes (the

meninges). It’s also floating in a bath of cerebrospinal fluid that (among other things) acts as a shock absorber when the head moves abruptly this way or that.

The peripheral nervous system itself has two distinguishable parts. The somaticnervous system (SNS) includes all the (efferent) nerves that control the skeletalmuscles as well as the (afferent) nerves that carry information from the sense organs to the CNS. The other division—the autonomic nervous system (ANS)—includes all the (efferent) nerves that regulate the various glands in the body as well as those that regulate the smooth muscles of the internal organs and blood vessels. (The name “smooth muscles” refers to how these muscles look when observed under a micro-scope; this is in contrast to the skeletal muscles, which look striped.) The ANS also includes (afferent) nerves that bring the CNS information about these various internal systems.

Finally, the autonomic nervous system is itself divided into two parts: the sympa-thetic branch, which tends to “rev up” bodily activities in preparation for vigorousaction, and the parasympathetic branch, which tends to restore the body’s internal activities to normal after the action has been completed (Figure 3.26).* These divisions of the ANS act reciprocally; excitation of the sympathetic branch leads to an increased heart rate, while excitation of the parasympathetic branch leads to cardiac slowing. Sympathetic activation produces a slowing down of peristalsis (rhythmic contractions of the intestines), so that we’re not using energy for digest-ing when we’re on the run; parasympathetic activation does the opposite—it speeds up peristalsis.

The Anatomy of the Brain

The peripheral nervous system is crucial. Without it, no motion of the body would be possible; no information would be received about the external world; the body would be unable to control its own digestion or blood circulation. Still, the aspect of the nervous system most interesting to psychologists is the central nervous system. It’s here that we find the complex circuitry crucial for perception, memory, and thinking. It’s the CNS that contains the mechanisms that define each person’s personality, control his or her emotional responses, and more.

The CNS, as we’ve seen, includes the spinal cord and the brain itself. The spinal cord, for most of its length, actually does look like a cord; it has separate paths for nerves carrying afferent information (i.e., information arriving in the CNS) and nerves carrying efferent commands (information exiting the CNS). Inside the head, the spinal cord becomes larger and looks something like the cone part of an ice cream cone. Structures at the very top of the cord—looking roughly like the ice cream on top of the cone—form the brain stem (Figure 3.27). The medulla is at the bottom of the brain stem; among its other roles, the medulla controls our breathing and blood circulation. It also helps us maintain our balance by controlling head orientation and limb positions in relation to gravity. Above the medulla is the pons, which is one of the most

important brain areas for controlling the brain’s overall level of attentiveness and helps govern the timing of sleep and dreaming.

Just behind the brain stem is the cerebellum (Figure 3.28A). For many years, inves-tigators believed the cerebellum’s main role was to control balance and coordinate movements—especially rapid and carefully timed movements. Recent studies confirm this role, but suggest that the cerebellum also has a diverse set of other functions. Damage to this organ can cause problems in spatial reasoning, in discriminating sounds, and in integrating the input received from various sensory systems (J. Bower & Parsons, 2003).

Sitting on top of the pons are two more structures—the midbrain and thalamus (see Figure 3.27). Both of these structures serve as relay stations directing information to the forebrain, where the information is more fully processed and interpreted. But these structures also have other roles. The midbrain, for example, helps regulate our experience of pain and plays a key role in modulating our mood as well as shaping our motivation.

On top of these structures is the forebrain—by far the largest part of the human brain. Indeed, photographs of the brain (Figure 3.28B) show little other than the forebrain because this structure is large enough in humans to surround most of the other brain parts and hide them from view. Of course, we can see only the outer sur-face of the forebrain in such pictures; this is the cerebral cortex (cortex is the Latin word for “tree bark”).

The cortex is just a thin covering on the outer surface of the brain; on average, it is a mere 3 mm thick. Nonetheless, there is a great deal of cortical tissue; by some estimates, the cortex constitutes 80% of the human brain (Kolb & Whishaw, 2009). This consider-able volume is made possible by the fact that the cortex, thin as it is, consists of a very large sheet of tissue; if stretched out flat, it would cover roughly 2.7 square feet (2,500 cm²). But the cortex isn’t stretched flat; instead, it’s crumpled up and jammed into the limited space inside the skull. This crumpling produces the brain’s most obvious visual feature—the wrinkles, or convolutions, that cover the brain’s outer surface.

Some of the “valleys” in between the wrinkles are actually deep grooves that divide the brain into different anatomical sections. The deepest groove is the longitudinal fissure, run-ning from the front of the brain to the back and dividing the brain into two halves— specifically, the left and the right cerebral hemispheres. Other fissures divide the cortex in each hemisphere into four lobes, named after the bones that cover them—bones that, as a group, make up the skull. The frontal lobes form the front of the brain, right behind the forehead. The central fissure divides the frontal lobes on each side of the brain from the parietal lobes, the brain’s topmost part. The bottom edge of the frontal lobes is marked bythe lateral fissure, and below this are the temporal lobes. Finally, at the very back of the brain—directly adjoining the parietal and temporal lobes—are the occipital lobes.

As we’ll see, the cortex—the outer surface of all four lobes—controls many func-tions. Because these functions are so important to what we think, feel, and do, we’ll address them in detail in a later section. But the structures beneath the cortex are just as important. One of these is the hypothalamus, positioned directly under-neath the thalamus and crucially involved in the control of motivated behaviors such as eating , drinking , and sexual activity . Surrounding the thal-amus and hypothalamus is a set of interconnected structures that form the limbicsystem (Figure 3.29). This system—especially one of its parts, the amygdala—plays a key role in modulating our emotional reactions and seems to serve roughly as an “evaluator ” that helps determine whether a stimulus is a threat or not, famil-iar or not, and so on. Nearby is the hippocampus, which is pivotal for learning and memory as well as for our navigation through space. (In some texts, the hippocam-pus is considered part of the limbic system, following an organization scheme laid down more than 50 years ago. More recent analyses, however, indicate that this ear-lier scheme is anatomically and functionally misleading. For the original scheme, see MacLean, 1949, 1952; for a more recent perspective, see Kotter & Meyer, 1992; LeDoux, 1996.)

Lateralization

The entire human brain is more or less symmetrical around the midline, so there’s a thalamus on the left side of the brain and another on the right. There’s also a left-side amygdala and a right-side one. Of course, the same is true for the cortex itself: There’s a temporal cortex in the left hemisphere and another in the right, a left occipital cortex and a right one, and so on. In almost all cases—cortical and subcortical—the left and right structures have roughly the same shape, the same position in their respective sides of the brain, and the same pattern of connections to other brain areas. Even so, there are some anatomical distinctions between the left-side and right-side structures. We can also document differences in function, showing that the left-hemisphere structures play a somewhat different role from the corresponding right-hemisphere structures.

The asymmetry in function between the two brain halves is called lateralization, and its manifestations influence phenomena that include language use and the per-

ception and understanding of spatial organization (Springer & Deutsch, 1998). Still, it’s important to realize that the two halves of the brain, each performing somewhat different functions, work closely together under almost all circumstances. This inte-gration is made possible by the commissures—thick bundles of fibers that carry infor-mation back and forth between the two hemispheres. The largest and probably most important commissure is the corpus callosum, but several other structures also ensure that the two brain halves work together as partners in virtually all mental tasks (Figure 3.30).

In some people, these neurological bridges between the hemispheres have been cut for medical reasons. This was, for example, a last-resort treatment for many years in cases of severe epilepsy. The idea was that the epileptic seizure would start in one hemisphere and spread to the other, and this spread could be prevented by disconnect-ing the two brain halves from each other (Bogen, Fisher, & Vogel, 1965; D. Wilson, Reeves, Gazzaniga, & Culver, 1977). The procedure has largely been abandoned by physicians, who are turning to less drastic surgeries for even the most extreme cases of epilepsy (Woiciechowsky, Vogel, Meyer, & Lehmann, 1997). Nonetheless, this medical procedure has produced a number of “split-brain patients”; this provides an extraordi-nary research opportunity by allowing us to examine how the brain halves function when they aren’t in full communication with each other.

This research makes it clear that each hemisphere has its own specialized capacities (Figure 3.31). For most people, the left hemisphere has sophisticated language skills and is capable of making sophisticated inferences. The right hemisphere, in contrast, has only limited language skills; but it outperforms the left hemisphere in a variety of spa-tial tasks such as recognizing faces and perceiving complex patterns (Gazzaniga, Ivry, & Mangun, 2002; Kingstone, Freisen, & Gazzaniga, 2000).

These differences between the left and right cerebral hemispheres are striking, but they’re clearly distinct from many of the conceptions of hemispheric function written for the general public. Some popular authors go so far as to equate left-hemisphere function with Western science and right-hemisphere function with Eastern culture and mysticism. In the same vein, others have argued that Western societies overemphasize rational and analytic “left-brain” functions at the expense of intuitive, artistic, “right-brain” functions.

These popular conceptions contain a kernel of truth because, as we’ve seen, the two hemispheres are different in several aspects of their functioning. But these often-mentioned conceptions go far beyond the available evidence and are sometimes inconsistent with it (Efron, 1990; J. Levy, 1985). Worse, these popular conceptions are entirely misleading when they imply that the two cerebral hemi-spheres, each with its own talents and strategies, endlessly compete for control of our mental life. Instead, each of us has a single brain. Each part of the brain—not just the cerebral hemispheres—is quite differentiated and so contributes its own specialized abilities to the activity of the whole. But in the end, the complex, sophis-ticated skills that we each display depend on the whole brain and on the coordinated actions of all its components. Our hemispheres are not cerebral competitors. Instead, they pool their specialized capacities to produce a seamlessly integrated, single mental self.