The cerebral hemispheres are the most highly developed portions of the human central nervous system. Each hemisphere is a distinct half, and each has four major components: cerebral cortex, hippocampal formation, amygdala, and basal ganglia. Together, these structures mediate the most sophisticated of human behaviors, and they do so through complex anatomical connections.
The Subcortical Components of the Cerebral Hemispheres Mediate Diverse Motor, Cognitive, and Emotional Functions
The hippocampal formation is important in learning and memory, whereas the amygdala not only participates in emotions but also helps to coordinate the body's response to stressful and threatening situations, such as preparing to fight (Figure 1–10A). These two structures are part of the limbic system (see Chapter 16), which includes other parts of the cerebral hemispheres, diencephalon, and midbrain. Because parts of the limbic system play a key role in mood, it is not surprising that psychiatric disorders are often associated with limbic system dysfunction.
Three-dimensional views of deep structures of the cerebral hemisphere. A. The hippocampal formation (red) and amygdala (orange). The fornix (blue) and mammillary body (purple) are structures that are anatomically and functionally related to the hippocampal formation. B. Striatum is a component of the basal ganglia with a complex three-dimensional shape. The ventricular system is also illustrated. Note the similarity in overall shapes of the striatum and the lateral ventricle.
The basal ganglia are another deeply located collection of neurons. The portion of the basal ganglia that has the most complex shape is called the striatum (Figure 1–10B). The importance of the basal ganglia in the control of movement is clearly revealed when they become damaged, as in Parkinson disease. Tremor and a slowing of movement are some of the overt signs of this disease. The basal ganglia also participate in cognition and emotions in concert with the cerebral cortex and are key brain structures involved in addiction.
The Four Lobes of the Cerebral Cortex Each Have Distinct Functions
The cerebral cortex, which is located on the surface of the brain, is highly convoluted (Figures 1–11 and 1–12). The human cerebral cortex is approximately 2,500 cm2. Convolutions are an evolutionary adaptation to fit a greater surface area within the confined space of the cranial cavity. In fact, only one quarter to one third of the cerebral cortex is exposed on the surface. The elevated convolutions on the cortical surface, called gyri, are separated by grooves called sulci or fissures (which are particularly deep sulci). The cerebral hemispheres are separated from each other by the sagittal (or interhemispheric) fissure (Figure 1–12B).
A. Lateral surface of cerebral hemisphere and brain stem and a portion of the spinal cord. The different colored regions correspond to distinct functional cortical areas. The primary motor and somatic sensory areas are located in the pre- and postcentral gyri, respectively. The primary auditory cortex lies in the superior temporal gyrus adjacent to the sensory and motor areas. Broca's area comprises most of the inferior frontal gyrus, and Wernicke's area is in the posterior part of the superior temporal gyrus. Boldface labeling indicates key structures. The inset shows the four lobes of the cerebral cortex and the insular cortex in relation to the four lobes. B. Medial surface. The primary visual cortex is located in the banks of the calcarine fissure. A small portion extends onto the lateral surface. The divisions of the brain stem and the cerebellum are also shown in A and B.
A. Ventral surface of the cerebral hemisphere and diencephalon; the midbrain is cut in cross section. The primary visual cortex is shown at the occipital pole. B. Dorsal surface of the cerebral hemisphere. The primary motor and somatic sensory cortical areas are located anterior and posterior to the central sulcus. Broca's area is in the inferior frontal gyrus, and Wernicke's area is located in the posterior temporal lobe. The primary visual cortex is shown at the occipital pole.
The four lobes of the cerebral cortex are named after the cranial bones that overlie them: frontal, parietal, occipital, and temporal (Figure 1–11, inset). The functions of the different lobes are remarkably distinct, as are the functions of individual gyri within each lobe.
The frontal lobe serves diverse behavioral functions, from thoughts to action, cognition, and emotions. The precentral gyrus contains the primary motor cortex, which participates in controlling the mechanical actions of movement, such as the direction and speed of reaching. Many projection neurons in the primary motor cortex have an axon that terminates in the spinal cord. The superior, middle, and inferior frontal gyri form most of the remaining portion of the frontal lobe. The premotor areas, which are important in motor decision making and planning movements, are adjacent to the primary motor cortex in these gyri. The inferior frontal gyrus in the left hemisphere in most people contains Broca's area, which is essential for the articulation of speech. Much of the frontal lobe is association cortex. Association cortical areas are involved in the complex processing of sensory and other information for higher brain functions, including emotions, organizing behavior, thoughts, and memories. Areas closer to the frontal pole comprise the frontal association cortex. The prefrontal association cortex is important in thought, cognition, and emotions. The cingulate gyrus (Figure 1–11B), medial frontal lobe, and most of the orbital gyri (Figure 1–12A) are important in emotions. Psychiatric disorders of thought, as in schizophrenia, and mood disorders, such as depression, are linked with abnormal functions of frontal association cortex. The basal forebrain, which is on the ventral surface of the frontal lobe (Figure 1–12A), contains a special population of neurons that uses acetylcholine to regulate cortical excitability. These neurons are examined further in Chapter 2. Although the olfactory sensory organ, the olfactory bulb, is located on the ventral surface of the frontal lobe, its connections are predominantly with the temporal lobe (Figure 1–12A).
The parietal lobe, which is separated from the frontal lobe by the central sulcus, mediates our perceptions of touch, pain, and limb position. These functions are carried out by the primary somatic sensory cortex, which is located in the postcentral gyrus. Primary sensory areas are the initial cortical processing stages for sensory information. The remaining portion of the parietal lobe on the lateral brain surface consists of the superior and inferior parietal lobules, which are separated by the intraparietal sulcus. The superior parietal lobule contains higher-order somatic sensory areas, for further processing of somatic sensory information, and other sensory areas. Together these areas are essential for a complete self-image of the body, and they mediate behavioral interactions with the world around us. A lesion in this portion of the parietal lobe in the right hemisphere, the side of the human brain that is specialized for spatial awareness, can produce bizarre neurological signs that include neglecting a portion of the body on the side opposite the lesion. For example, a patient may not dress one side of her body or comb half of her hair. The inferior parietal lobule is involved in integrating diverse sensory information for perception and language, mathematical reasoning, and visuospatial cognition.
The occipital lobe is separated from the parietal lobe on the medial brain surface by the parietooccipital sulcus (Figure 1–11B). On the lateral and inferior surfaces, there are no distinct boundaries, only an imaginary line connecting the preoccipital notch (Figure 1–11A) with the parietooccipital sulcus. The occipital lobe is the most singular in function, subserving vision. The primary visual cortex is located in the walls and depths of the calcarine fissure on the medial brain surface (Figure 1–11B). Whereas the primary visual cortex is important in the initial stages of visual processing, the surrounding higher-order visual areas play a role in elaborating the sensory message that enables us to see the form and color of objects. For example, on the ventral brain surface is a portion of the occipitotemporal gyrus in the occipital lobe (also termed the fusiform gyrus) that is important for recognizing faces (Figure 1–12A). Patients with a lesion of this area can confuse faces with inanimate objects, a condition termed prosopagnosia.
The temporal lobe, separated from the frontal and parietal lobes by the lateral sulcus (or Sylvian fissure) (Figure 1–11A), mediates a variety of sensory functions and participates in memory and emotions. The primary auditory cortex, located on the superior temporal gyrus, works with surrounding areas on the superior temporal gyrus and within the lateral sulcus and the superior temporal sulcus for perception and localization of sounds (Figure 1–11A). The superior temporal gyrus on the left side is specialized for speech. Lesion of the posterior portion of this gyrus, which is the location of Wernicke's area, impairs the understanding of speech. The middle temporal gyrus, especially the portion close to the occipital lobe, is essential for perception of visual motion. The inferior temporal gyrus mediates the perception of visual form and color (Figures 1–11A and 1–12A). The cortex located at the temporal pole (Figure 1–12A), together with adjacent portions of the medial temporal lobe and inferior and medial frontal lobes, is important for emotions.
Deep within the lateral sulcus are portions of the frontal, parietal, and temporal lobes. This territory is termed the insular cortex (Figure 1–11, inset). It becomes buried late during prenatal development (see Figure 1-14). Portions of the insular cortex are important in taste, internal body senses, pain, and balance.
Ventricular system. The lateral ventricles, third ventricle, cerebral aqueduct, and fourth ventricle are seen from the lateral brain surface (left) and the front (right). The lateral ventricle is divided into four main components: anterior (or frontal) horn, body, inferior (or temporal) horn, and posterior (or occipital) horn. The atrium of the lateral ventricle is the region of confluence of the body, inferior horn, and posterior horn. The interventricular foramen (of Monro) connects each lateral ventricle with the third ventricle. The cerebral aqueduct connects the third and fourth ventricles.
The development of the human brain is shown from the lateral surface in relation to the face and the general shape of the cranium. The lateral ventricle is colored green. The arrows drawn over the lateral ventricle show its emerging C-shape. (Courtesy Tom Prentiss, illustrator.)
The corpus callosum contains axons that interconnect the cortex on the two sides of the brain (Figure 1–11B). Tracts containing axons that interconnect the two sides of the brain are called commissures, and the corpus callosum is the largest of the brain's commissures. To integrate the functions of the two halves of the cerebral cortex, axons of the corpus callosum course through each of its four principal parts: rostrum, genu, body, and splenium (Figure 1–11B). Information between the occipital lobes travels through the splenium of the corpus callosum, whereas information from the other lobes travels through the rostrum, genu, and body.