B. Brain
3. Diencephalon (Fig. 1.5)
terminate here. The diencephalon consists of the following divisions: thalamus, hypothalamus, epithalamus, and subthalamus. The functional organization in the diencephalon is the basis for much of the function seen in the cerebrum and also contains the multimodal associations that make the cerebrum of the human such a potent analyzer (Table 1.4).
The thalamus forms the largest division of the diencephalon and it is divided into several groupings of nuclei. All of the ascending pathways terminate in tha-lamic nuclei and then their information is projected onto their respective region of the cerebral cortex. A detailed discussion of the thalamus is found in Chapter 6.
The epithalamus is a small zone on the mediodorsal portion of the diencephalon consisting of habenular nuclei, with its associative pathway, the stria medullaris, and the pineal gland; its functions are similar to the hypothalamus.
The hypothalamus is the smallest subdivision of the diencephalon and is found inferiorly in the third ventricle. However, because it controlls the pituitary gland and functions as the “head ganglion” in the autonomic nervous system, it might well be the most important portion and emotional center of the diencephalon. The hypothalamus and the autonomic nervous system are discussed in Chapter 7.
The subthalamus, found below the thalamus, consists of the subthalamic nucleus and the zona incerta with its fiber pathways. It is an important subcortical region in the basal ganglia (Chapter 10)
The functions of this region are to integrate sensory and motor information and to begin to interpret these data according to the perceptions of the emotional areas in the brain.
4. Cerebrum: Cerebral Hemispheres (Fig. 1.6)
The cerebrum, forming the bulk of the brain and thus of the central nervous system, consists of a left and a right hemisphere containing the cortical gray matter, white matter, and basal nuclei (Table 1.5). In each hemisphere, we find four lobes: the rostral frontal lobe, the middle parietal lobe, and the posterior occipital lobe with the inferiorly located temporal lobe. The cerebral cortex consists of a corrugated surface, the cortical gray matter, which is laminated and has six layers, is broken up into numerous gyri, separated by narrow spaces or grooves, the sulci (Fig. 1.6).
The 12 to 15 billion cortical neurons are found in the gray cortical mantle.
Table 1.5 Overview of cerebral functions (Figs. 1.6 and 1.7)
Frontal lobe Volitional movement of the muscles of the body, including limbs, face, and voice; judgmental center
Parietal lobe Tactile discrimination from the body, body image, taste, and speech Occipital lobe Vision (sight and interpretation)
Temporal lobe Emotions and new memories, hearing, language, olfaction, and visual recognition
Cingulate gyrus Emotions
Insular gyri Emotions and language
Students handling a preserved brain are often initially struck by the number of the gyri and sulci in the cerebral hemispheres and surprised by its weight (about 1500 g). If you have a series of brains to observe, note that there is a great variation in size. Remember that the size of the brain is related to the size of the skeleton, muscles, and viscera (but not adipose tissue), so a person of small stature has a proportionally smaller brain. Because women tend to be smaller than men, a woman’s brain is usually smaller, although this in no way reflects intelligence or abilities. One of the first clues about the different func-tions in the two cerebral hemispheres was observed in stroke victims. Because the left cerebral hemisphere is dominant for language in right-handed people (93% of the population), strokes in that hemisphere usually affect speech. In addition to language, dominant functions include initiation of movement, emo-tions, and artistic abilities.
Note that each hemisphere provides motor controls to the opposite side of the body. This is because the motor pathway from each cerebral cortex crosses in the transition between the spinal cord and medulla; consequently, the sensory fibers also cross over to the opposite side of the body. In over 90 % of the human popula-tion, movement initiates from the left hemisphere, making it dominant for initiation of movement.
Fig. 1.6 Lateral surface of the cerebrum. The sensory (precentral) and motor strips (precentral) are marked. The somatotopic organization of this region is labeled: Head, Arm, Thorax Abdomen, and Foot. The foot region of the sensory–motor strip extends onto the medial surface of the hemisphere as the paracentral lobule. (Modified from Curtis, Jacobson, and Marcus, An introduc-tion to the neurosciences, Saunders, 1972)
General Features
The cerebral cortex is divided into four lobes, the frontal, parietal, occipital, and temporal lobes (Fig. 1.6). Two other important regions in the cerebrum are the insula deep within the lateral sulcus and the cingulate gyrus on the medial surface of each hemisphere. Three of the lobes also have poles: the frontal, temporal, and occipital pole. The frontal lobe is separated from the parietal lobe by the central sulcus and from the temporal lobe by the lateral sulcus. The other lobes can be identified (Fig. 1.6) by first drawing a line from the parieto-occipital sulcus on the medial surface of the hemisphere to the preoccipital notch on the inferior lateral surface of the hemisphere.
A line is then extended posteriorly from the lateral sulcus until it intersects the first line. The parietal lobe lies above the lateral sulcus and behind the central sulcus, anterior to the occipital lobe. The temporal lobe lies below the lateral sulcus anterior to the occipital lobe. The occipital lobe is most posteriorly placed.
Functional Localization
The cerebral cortex includes motor, sensory, auditory, and visual regions. In addition, broad areas are involved with multimodal integration, which combines sensory and motor with an emotional content to determine how to respond in any situation.
These emotional or limbic areas occupy much of the temporal and frontal lobes.
(The limbic region denotes the cortical and subcortical brain areas with tracts pri-marily involved with emotions, including the cingulate gyrus, hippocampal forma-tion, parahippocampal gyrus, and nuclei in the diencephalon; see Chapter 14.) Human beings also use language extensively and much of the frontal–parietal–
occipital–temporal regions that abut the lateral sulcus in the left hemisphere under-take these functions. Similar areas of the right hemisphere are devoted to visual–spatial integration. You might wonder just how we have such detailed information on the organization and functions of the human nervous system. Much of it comes from research on primates or from defects observed in patients after strokes, trauma, tumors, or degenerative neurological diseases like Alzheimer’s disease.
Cortical White Matter
The axons entering or leaving the cerebral hemispheres form three distinctive groups of fibers: associational, commissural, and subcortical.
1. Associational Fibers. These two types of fiber (short U and long associational) provide the integrative circuitry for movement, language, memory, and emotions.
Short associational fibers (the U fibers) form the bulk of local connections within a hemisphere, whereas the long associational fibers interconnect diverse areas in a hemisphere, providing the multimodal associations essential one for cortical and cingulum.
2. Commissural Fibers. Commissural fibers interconnect areas in the contralat-eral hemispheres and permit learning and memory in one hemisphere to be shared with the other. The bulk of the frontal, parietal, occipital, and temporal lobes are interconnected by the corpus callosum, best seen in the medial surface of the hemisphere. The corpus callosum consists of a rostrum, genu, body, and splenium (Fig. 1.7). The most rostral part of the temporal lobe and the olfactory cortex of the uncus are interconnected by the anterior commissure. The hippoc-ampus, an important area for memory, is also connected by a commissure associated with the fornix.
3. Subcortical Fibers. This category of fibers includes fiber bundles reaching the cortex from subcortical areas, corticopetal (to cortex), and axons leaving the cortex and connecting to subcortical nuclei, corticofugal (from the cortex).
Corticopetal fibers are the subcortical afferents to each cerebral hemisphere and are primarily from the thalamus of the diencephalon. Corticofugal fibers from the cerebral hemisphere project onto subcortical structures, including the basal nuclei, diencephalon, brain stem, cerebellum, and spinal cord. The major sub-cortical fiber tracts leaving the cerebrum are the corticospinal, corticonuclear, corticopontine, corticomesencephalic, and the fornix.
The internal capsule contains the major grouping of corticofugal and corticopetal fibers of the cerebral cortex and consists of an anterior limb, genu, and posterior limb (Fig. 1.7). The anterior limb provides fibers to and from the frontal lobe. The
Fig. 1.7 Medial surface of a cerebral hemisphere, with brain stem removed. Gyri in frontal, pari-etal, occipital and temporal lobes identified and regions in the corpus callosum noted: (1) rostrum, (2) genu, (3) body, and (4) splenium. (Modified from Curtis, Jacobson, and Marcus, An introduc-tion to the neurosciences, Saunders, 1972)
genu provides fibers to and from the lower part of the frontal and parietal lobes (corticonuclear). The posterior limb is the largest portion of the internal capsule and includes the auditory radiations (auditory fibers to the auditory cortex), visual radi-ations, as well as projections from the sensorimotor cortex to the spinal cord and brain stem (corticospinal, corticonuclear, and corticopontine fibers)
Functions in the Lobes of the Cerebrum
In addition to all of the functions already mentioned, the cerebral hemispheres also have the memory stores, which are the foundation for most conscious and uncon-scious thought, cultural activity, sexual behavior, and all of the other positive or negative traits that make the human being so distinctive. Each cortical region has a memory store that permits the function of that region (Table 1.5). A detailed analy-sis of the functions of each cortical region is presented in Chapters 8 to 15.