The brain consists of several large regions, each responsible for some of the activities vital for living. These include the brainstem, cerebellum, limbic system, diencephalon, and cerebral cortex (Figures 1 and 2).
The brainstem is the part of the brain that connects the brain and the spinal cord. It controls many basic functions, such as heart rate, breathing, eating, and sleeping. The brainstem accomplishes this by directing the spinal cord, other parts of the brain, and the body to do what is necessary to maintain these basic functions.
The cerebellum, which represents only one-eighth of the total weight of the human brain, coordinates the brain's instructions for skilled repetitive movements and for maintaining balance and posture. It is a prominent structure located above the brainstem.
On top of the brainstem and buried under the cortex, there is a set of more evolutionarily primitive brain structures called the limbic system (e.g., amygdala and hippocampus, as in Figure 2). The limbic system structures are involved in many of our emotions and motivations, particularly those that are related to survival, such as fear, anger, and sexual behavior. The limbic system is also involved in feelings of pleasure that are related to our survival, such as those experienced from eating and sex. The large limbic system structure, the hippocampus, is also involved in memory. One of the reasons that drugs of abuse can exert such powerful control over our behavior is that they act directly on the more evolutionarily primitive brainstem and limbic system structures, which can override the cortex in controlling our behavior. In effect, they eliminate the most human part of our brain from its role in controlling our behavior.
The diencephalon, which is also located beneath the cerebral hemispheres, contains the thalamus and hypothalamus (Figure 2). The thalamus is involved in sensory perception and regulation of motor functions (i.e., movement). It connects areas of the cerebral cortex that are involved in sensory perception and movement with other parts of the brain and spinal cord that also have a role in sensation and movement. The hypothalamus is a very small but important component of the diencephalon. It plays a major role in regulating feeding hormones, the pituitary gland, body temperature, the adrenal glands, and many other vital activities.
The cerebral cortex, which is divided into right and left hemispheres, encompasses about two-thirds of the human brain mass and lies over and around most of the remaining structures of the brain. It is the most highly developed part of the human brain and is responsible for thinking, perceiving, and producing and understanding language. It is also the most recent structure in the history of brain evolution. The cerebral cortex can be divided into areas that each have a specific function (Figure 3). For example, there are specific areas involved in vision, hearing, touch, movement, and smell. Other areas are critical for thinking and reasoning. Although many functions, such as touch, are found in both the right and left cerebral hemispheres, some functions are found in only one cerebral hemisphere. For example, in most people, language abilities are found in the left hemisphere.