The cerebrum is the largest portion of the brain. It performs the higher brain functions involved with sensations, voluntary actions, reasoning, planning, and problem solving.
The cerebrum consists of the left and right cerebral hemispheres, which are joined by a mass of myelinated axons called the corpus callosum. The cerebral hemispheres are separated by the longitudinal cerebral fissure, which lies along the superior midline and extends inferiorly to the corpus callosum.
The surface of the cerebrum has numerous folds or ridges, called gyri (ji ‘-re; singular, gyrus). The shallow grooves between the gyri are called sulci (sul ‘-se; singular, sulcus). The superficial layer of the cerebrum is composed of gray matter (cell bodies, dendrites, terminal arborizations, and unmyelinated axons) and is called the cerebral cortex. White matter, composed of myelinated and unmyelinated axons, lies deep to the cortex and composes most of the cerebrum. These axons transmit nerve impulses between regions within the same cerebral hemisphere, between the cerebral hemispheres via the corpus callosum, and between the cerebral cortex and lower brain centers. Several masses of gray matter, called nuclei, are embedded deep within the white matter of each cerebral hemisphere.
Each cerebral hemisphere is divided into five lobes. Four lobes are named for the cranial bones under which they lie. Locate the cerebral lobes in figures 8.13 and 8.14 .
- The frontal lobe lies anterior to the central sulcus and superior to the lateral sulcus.
- The parietal lobe lies posterior to the central sulcus, superior to the temporal lobe, and anterior to the occipital lobe.
- The temporal lobe lies inferior to the frontal and parietal lobes and anterior to the occipital lobe.
- The occipital lobe lies posterior to the parietal and temporal lobes. The boundaries between the parietal, temporal, and occipital lobes are not distinct.
The insula lies deep to the lateral sulcus. It is the lobe that cannot be viewed superficially.
The cerebrum is involved in the interpretation of sensory nerve impulses as sensations and in controlling voluntary motor responses, intellectual processes, the will, and many personality traits. The cerebrum has three major types of functional areas: sensory, motor, and association areas (figure 8.14).
Sensory areas receive nerve impulses formed by sensory receptors and interpret them as sensations. These areas occur in several cerebral lobes. For example, the sensory areas for vision are in the occipital lobes and those for hearing are found in the temporal lobes. Areas identifying sensations from skin (cutaneous) stimulation lie along the postcentral gyri (gyri just posterior to the central sulci) of the parietal lobes. Sensory areas for taste are located at the inferior end of the postcentral gyri. The sensory areas for smell are located in the inferior part of the frontal lobe and the medial aspect of the temporal lobe.
Ascending sensory axons carrying sensations from the skin cross over from one side to the other prior to reaching the thalamus. Thus, the postcentral gyrus in the left cerebral hemisphere receives nerve impulses from the skin on the right side of the body, and vice versa.
Motor areas are located in the frontal lobe. The primary motor areas that control skeletal muscles lie along the precentral gyri (gyri just anterior to the central sulci) of the frontal lobes. The region anterior to the primary motor area is the premotor area.
The premotor area is involved in complex learned activities, such as writing, tying your shoes, and driving a car. Also in the premotor area is the frontal eye field, which controls voluntary eye movements. The motor speech area (Broca area), which controls the ability to speak, is located near the inferior end of the primary motor area. It is found in only one hemisphere: the left hemisphere in about 90% of people.
Descending motor axons cross over from one side to the other in the brainstem. Thus, the left side of the cerebrum controls skeletal muscles on the right side of the body, and vice versa.
Association areas occur in each cerebral lobe, where they interrelate sensory inputs and motor outputs. They play critical roles in the interrelationships of sensations, memory, will, and the coordination of motor responses. The common integrative area is a major association area that is located at the junction of the temporal, parietal, and occipital lobes. It is involved with the interpretation of complex sensory experiences and thought processes. The posterior language area (Wernicke area), which is an association area located in the temporal and parietal lobes, is used to interpret the meaning of spoken and written language. Like the motor speech area, it is found in only one hemisphere: the left hemisphere in about 90% of people.
The prefrontal area, which is located in the anterior portion of the frontal lobe, is involved with functions such as planning, complex behaviors, conscience, generating personality, and executive functions. Executive functions include distinguishing between good and bad, understanding future consequences, social control of urges, and working towards a goal. Portions of the prefrontal area are not fully developed until a person is in his or her 20s, which is why teenagers often have issues with impulse control and poor decision-making.
The two cerebral hemispheres perform different functions in most people, although each performs basic functions of receiving sensory input and initiating voluntary motor output. In about 90% of the population, the left cerebral hemisphere controls analytical and verbal skills, such as mathematics, reading, writing, and speech. In these persons, the right hemisphere controls musical, artistic and spatial awareness, imagination, and insight. In some persons, this pattern is reversed; in a few, there seems to be no specialization. Men also have greater lateralization than women, which is why damage to a hemisphere can have greater effects in men.