 The paired cerebral hemispheres are mirror-image duplicates composed of a gray cellular mantle called the cerebral cortex, an underlying mass of white matter composed of myelinated nerve fibers, and collections of subcortical neuronal masses known as the basal ganglia. Each hemisphere receives impulses conveying the senses of touch and vision largely from the contralateral half (that is, the opposite side) of the body, while auditory input comes from both sides. Pathways conveying the senses of smell and taste to the cerebral cortex are ipsilateral (that is, they do not cross to the opposite hemisphere). In turn, each cerebral hemisphere supplies motor function to the opposite side of the body, the side from which it receives sensory input.
In spite of this arrangement, the cerebral hemispheres are not functionally equal. In each individual, one hemisphere is dominant, the dominant hemisphere being concerned with language, mathematical and analytical functions, and handedness. The nondominant hemisphere is concerned with simple spatial concepts, recognition of faces, some aspects of music, and emotion. (For further discussion of cerebral dominance, see Functions of the Human Nervous System: Higher cerebral functions.)
The hemispheres are partially separated from each other by the longitudinal fissure, but in central regions this fissure extends only as deep as a broad interhemispheric commissure called the corpus callosum. It is through the corpus callosum that corresponding regions of the cerebral hemispheres are connected by various nerve projections.
 Branching nerve fibers form a dense network in the brain. The large object (to the left) is a cell body with fibers leading out from it. All around are outgrowths from other brain cells.
Each nerve cell in the brain can fire a very small electrical pulse, and does so according to inputs received from other nerve cells. Individual pulses are weak -- the entire brain only creates about 20 watts of power -- but each firing has to be sufficient to energize the next cell, or group of cells. Signal transmission from one cell to another is achieved chemically rather than electrically, with a chemical known as a neurotransmitter released at each nerve ending. The transmitter crosses the synapse (gap) to the next neuron and becomes lodged in a specific receptor site. This then activates a chain of events in the second neuron which causes it to fire and thus pass the impulse along. The passing of an impulse enables us to remember faces, think consciously and behave with intelligence.
 The cortical mantle is highly convoluted; the crest of a single convolution is known as a gyrus, while the fissure between two gyri is known as a sulcus. Sulci and gyri form a more or less constant pattern, on the basis of which each cerebral hemisphere is divided into six so-called lobes: (1) frontal, (2) parietal, (3) temporal, (4) occipital, (5) central (or insular), and (6) limbic. Two important sulci located on the lateral aspect (that is, the side surface) of each hemisphere help to distinguish these lobes. The central sulcus separates the frontal and parietal lobes, and the deeper lateral sulcus forms the boundary between the temporal lobe and the frontal and parietal lobes.
 The frontal lobe, largest of all the lobes of the brain, lies rostral to the central sulcus (that is, toward the nose from the sulcus). The precentral gyrus, located rostral to the central sulcus, constitutes the primary motor region of the brain; when parts of this gyrus are given electrical stimulation in conscious patients (operated upon under local anesthesia), they produce localized movements on the opposite side of the body that are interpreted by the patients as voluntary. Injury to parts of this gyrus results in paralysis on the contralateral (opposite) half of the body.
 Parts of the inferior frontal lobe (close to the lateral sulcus) constitute Broca's area, a region concerned with neural mechanisms that convert thoughts into speech (see Functions of the human nervous system).
The parietal lobe, posterior to the central sulcus, is divided into three parts: (1) a postcentral gyrus, (2) a superior parietal lobule, and (3) an inferior parietal lobule. The postcentral gyrus receives sensory input, both superficial and deep, from the contralateral half of the body. The sequential representation is the same as in the primary motor area, with sensations from the head area being represented in inferior parts of the gyrus and impulses from the lower extremities represented above. Lesions in the postcentral gyrus result in impaired sensation from cutaneous (surface) and deep parts of the contralateral half of the body. The superior parietal lobule, located caudal to the postcentral gyrus, lies superior to the interparietal sulcus. This lobule is regarded as an association cortex, part of which may be concerned with motor function. The inferior parietal lobule (composed of the angular and supramarginal gyri) is a cortical region concerned with the integration of multiple sensory signals. Lesions in this lobule produce various syndromes of a devastating nature.
 In the parietal and frontal lobes, each primary sensory or motor area is close to, or surrounded by, a smaller secondary area (see figure). The primary sensory area receives input only from relay nuclei in the thalamus, while the secondary sensory area receives input from the thalamus, the primary sensory area, or both. The motor regions receive input from the thalamus as well as the sensory areas of the cerebral cortex.
The temporal lobe, inferior to the lateral sulcus, fills the middle fossa of the skull. Near the margin of the lateral sulcus, two transverse temporal gyri constitute the primary auditory area of the brain. Audition is represented here in a tonotopic fashion—that is, with different frequencies represented on different parts of the area. The transverse gyri are surrounded by a less finely tuned secondary auditory area.
A medial, or inner, protrusion near the ventral surface of the temporal lobe, known as the uncus, constitutes a large part of the primary olfactory area. The outer surface of this lobe is an association area made up of the superior, middle, and inferior temporal gyri.
The occipital lobe lies caudal to (that is, below and behind) the parieto-occipital sulcus. As seen on the medial aspect of the hemisphere, this sulcus joins the calcarine sulcus in a Y-shaped formation. Cortex on both banks of the calcarine sulcus constitutes the primary visual area, which receives input from the contralateral visual field via the optic radiation. The visual field is represented near the calcarine sulcus in a retinotopic fashion—that is, with upper quadrants of the visual field laid out along the inferior bank of the sulcus and lower quadrants of the visual field represented on the upper bank. Central vision is represented most caudally and peripheral vision rostrally. Lesions in the calcarine cortex—or in the optic radiation, which projects to it—produce blindness in the contralateral (on the opposite) visual field.
The insular, or central, lobe is an invaginated triangular area on the medial surface of the lateral sulcus; it can be seen in the intact brain only by separating the frontal and parietal lobes from the temporal lobe. Branches of the middle cerebral artery cover the surface of the insula.
The limbic lobe is a synthetic lobe on the medial margin (or limbus) of the hemisphere. Composed of adjacent portions of the frontal, parietal, and temporal lobes that surround the corpus callosum, it is concerned with visceral, autonomic, and related somatic behavioral activities. This region of the cerebral cortex receives inputs from thalamic nuclei that are connected with parts of the hypothalamus and with the hippocampal formation, a primitive cortical structure within the inferior horn of the lateral ventricle.
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