Here we will discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It will be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and the cerebellum with the fourth ventricle (Fig. 8.8a). The electrical activity of the brain can be recorded in the form of an electroencephalogram (EEG). Electrodes are taped to different parts of the scalp, and an instrument records the so-called brain waves. The EEG is a diagnostic tool; for example, an irregular pattern can signify epilepsy or a brain tumor. A flat EEG signifies brain death.
Figure 8.8 The human brain. a. The cerebrum, seen here in longitudinal section, is the largest part of the brain in humans. b. Superior view of the left and right cerebral hemispheres. They are connected by the corpus callosum.
The cerebrum is the largest portion of the brain in humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain. The cerebrum carries out the higher thought processes required for learning and memory and for language and speech.
The Cerebral Hemispheres. The cerebrum has two halves called the left and right cerebral hemispheres (Fig. 8.8b). A deep groove, the longitudinal fissure, divides the left and right cerebral hemispheres. Still, the two cerebral hemispheres are connected by a bridge of white matter within the corpus callosum. Convolutions called gyri are separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 8.9). The frontal lobe is anterior to the parietal lobe, which is anterior to the occipital lobe. The temporal lobe is the lateral portion of the cerebral hemisphere.
The cerebral cortex is a thin but highly convoluted outer layer of gray matter that covers the cerebral hemispheres. The cerebral cortex contains over one billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.
Figure 8.9 The lobes of a cerebral hemisphere. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. These lobes contain centers for reasoning and movement (frontal lobe), somatic sensing including taste (parietal lobe), hearing (temporal lobe), and vision (occipital lobe). Broca’s area is only in the left lobe.
Motor and Sensory Areas of the Cortex. The primary motor area is in the frontal lobe just anterior to the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (see Fig. 8.10a).
The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (see Fig. 8.10b). A primary taste area, also in the parietal lobe, accounts for taste sensations. A primary visual area in the occipital lobe receives information from our eyes, and a primary auditory area in the temporal lobe receives information from our ears.
Association areas. Association areas are places where integration occurs. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, and then the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex so that cerebral palsy, a condition characterized by a spastic weakness of the arms and legs, develops. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area associates new visual information with previously received visual information. It might “decide”, for example, whether we have seen this face, tool, or whatever before. The auditory association area performs the same functions with regard to sounds.
Figure 8.10 Portions of the body controlled by the primary motor area and the primary somatosensory area of the cerebrum. Notice that the size of the body part in the diagram reflects the amount of cerebral cortex devoted to that body part.
Processing Centers. There are a few areas of the cortex that receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, a processing area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities to think critically and to formulate appropriate behaviors. The unique ability of humans to speak is partially dependent upon Broca’s area, a processing area in the left frontal lobe. Signals originating here pass to the premotor area before reaching the primary motor area. Damage to this area can interfere with a person’s ability to understand words (written or spoken) and to communicate with others. Wernicke’s area, also called the general interpretive area, receives information from all the other sensory association areas. Damage to this area hinders the ability to interpret written and spoken messages even though the words are understood.
Central White Matter. Much of the rest of the cerebrum beneath the cerebral cortex is composed of white matter. Tracts within the cerebrum take information between the different sensory, motor, and association areas pictured in Figure 8.9. The corpus callosum, previously mentioned, contains tracts that join the two cerebral hemispheres. Descending tracts from the primary motor area communicate with various parts of the brain, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area (Fig. 8.10).
Basal Nuclei. While the bulk of the cerebrum is composed of tracts, there are masses of gray matter located deep within the white matter. These so-called basal nuclei (formerly termed basal ganglia) integrate motor commands, ensuring that proper muscle groups are activated or inhibited. Huntington disease and Parkinson disease, which are both characterized by uncontrollable movements, are believed to be due to an imbalance of neurotransmitters in the basal nuclei.
Limbic System. The limbic system (blue in figure) lies just inferior to the cerebral cortex and contains neural pathways that connect portions of the cerebral cortex and the temporal lobes with the thalamus and the hypothalamus:
Stimulation of different areas of the limbic system causes the subject to experience rage, pain, pleasure, or sorrow. By causing pleasant or unpleasant feelings about experiences, the limbic system apparently guides the individual into behavior that is likely to increase the chance of survival. The limbic system is also involved in learning and memory. Learning requires memory, and memory is stored in the sensory regions of the cerebrum, but just what permits memory development is not definitely known. The involvement of the limbic system in memory explains why emotionally charged events result in our most vivid memories. The fact that the limbic system communicates with the sensory areas for touch, smell, vision, and so forth accounts for the ability of any particular sensory stimulus to awaken a complex memory.
The hypothalamus and the thalamus are in the diencephalon, a region that encircles the third ventricle (see Fig. 8.8a). The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis by regulating hunger, sleep, thirst, body temperature, and water balance. The hypothalamus produces the hormones secreted by the posterior pituitary gland and secretes hormones that control the anterior pituitary. Therefore, it is a link between the nervous and endocrine systems.
The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it also participates in higher mental functions such as memory and emotions. The pineal gland, which secretes the hormone melatonin and regulates our body’s daily rhythms, is located in the diencephalon.
The cerebellum is separated from the brain stem by the fourth ventricle (see Fig. 8.8a). The cerebellum has two portions that are joined by a narrow median portion. Each portion is primarily composed of white matter, which in longitudinal section has a treelike pattern. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds. The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor impulses by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all of the muscles work together to produce smooth, coordinated voluntary movements. In addition, the cerebellum assists the learning of new motor skills, such as playing the piano or hitting a baseball.
The Brain Stem
The brain stem contains the midbrain, the pons, and the medulla oblongata (see Fig. 8.8a). The midbrain acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The word pons means “bridge” in Latin, and true to its name, the pons contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate and has reflex centers concerned with head movements in response to visual and auditory stimuli.
The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction. It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers.