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No. 25 Telencephalon Ⅳ. The Telencephalon The telencephalon consists of right and left cerebral hemispheres, which together are referred to as the cerebrum. The cerebrum has an outer surface of gray matter and an inner white matter. The gray matter is composed primarily of nerve cell bodies and unmyelinated nerve fibers. This surface layer is called the cerebral cortex. The white matter is called medullary substance, which is composed of tracts of myelinated nerve fibers. Within the hemispheres, there are several masses of gray matter known as the basal nuclei. A fluid-filled cavity called a lateral ventricle is located within each cerebral hemisphere. Ⅰ) External Features and lobes of telencephalon Two hemispheres of cerebrum are almost completely separated by the cerebral longitudinal fissure. At the bottom of this fissure a large bundle of transverse fibers, the corpus callosum, crosses between the two hemispheres. The cerebral transverse fissure intervenes between the hemispheres and the cerebellum. 1. The sulci, fissures, gyri and the lobes of the cerebrum 1) The sulci, fissures, gyri The surface of the cerebrum has many rounded ridges called gyri (singular: gyrus). Separating the gyri are furrows. The deeper furrows are called fissures; the shallower ones are sulci (singular: sulcus). The folding of the cortex that produces the gyri and sulci makes the surface area of the cerebral cortex much greater than it would be if the brain’s surface were smooth. As it is, a significant percentage of the cerebral cortex is located in the fissures and sulci and is not visible from the surface. There are three constant sulci (or fissures) for demarcation on the surface of the hemispheres. ① The lateral sulcus begins as a deep furrow on the inferior surface of the hemisphere and runs posteriorly and upward onto the dorsolateral surface. ② The central sulcus runs from the lateral sulcus to the longitudinal fissure at right angle. Each hemisphere is further divided into a frontal lobe and a parietal lobe by the central sulcus. ③ The parietooccipital sulcus lies on the medial surface and separates the parietal and occipital lobes as its name indicates. 2) The lobes of the cerebrum There are five lobes of each hemisphere. The hemisphere can be divided by these three sulci and an imaginary line into five lobes. The imaginary line is drawn from the upper end of the parietooccipital sulcus to the preoccipital notch which lies about 4 cm anterior to the occipital pole on the inferolateral border. They are frontal, parietal, occipital, temporal lobes and insula. ① The frontal lobe The frontal lobe is the area in front of the central sulcus and above the lateral sulcus. ② The parietal lobe The parietal lobe is the area bounded by central sulcus, the imaginary line and the lateral sulcus. ③ The occipital lobe The occipital lobe is the area posterior to the imaginary line and the parietooccipital sulcus. ④ The temporal lobe The temporal lobe occupies the area inferior to the lateral sulcus and in front of the imaginary line. ⑤ The insula The insula lies deeply in the floor of the lateral sulcus, and is overlapped by portions of the frontal, parietal and temporal lobes. 2. The gyri and sulci on the dorsolateral surface of the hemisphere 1) In the frontal lobe Precentral sulcus and precentral gyrus Superior, middle, and inferior frontal gyri Superior, middle and inferior frontal sulci 2) In the parietal lobe Postcentral sulcus and postcentral gyrus Intraparietal (or interparietal) sulcus Superior and inferior parietal lobules Supramarginal gyrus and angular gyrus 3) In the temporal lobe Superior and inferior temporal sulci Superior, middle and inferior temporal gyri Transverse temporal gyri 4) In the occipital lobe Not constant. 3. The gyri and sulci on the medial and inferior surfaces of the hemisphere 1) The gyri and sulci on the medial surface Paracentral lobule, Corpus callosum, Callosal sulcus, Cingulate gyrus and cingulate sulcus, Paracentral sulcus and Marginal ramus, Calcarine sulcus, Cuneus and Lingual gyrus, 2) The gyri and sulci on the inferior surface ① The inferior surface of frontal lobe Olfactory bulb, olfactory tract and olfactory trigone, Anterior perforated substance. ② The inferior surface of temporal lobe Lateral and medial occipitotemporal gyri, Occipitotemporal sulcus, Collateral sulcus, Parahippocampal gyrus and uncus, Hippocampal sulcus, Dentate gyrus, Hippocampus, Hippocampal formation: It includes hippocampus and dentate gyrus. Limbic lobe: On the medial surface of the cerebral hemisphere, a large arcuate convolution formed primarily by the cingulate, parahippocampus gyri and the hippocampus and dentate gyrus surrounding the upper brain stem, constitute the limbic lobe. Ⅱ) Functional Localization of the Cerebral Cortex On the basis of the effects of electrical stimulation of specific areas of the cerebral cortex in humans, from observing the clinical manifestations of brain disease or damage in humans, and from the results obtained from detailed experiments on other mammals, it has been determined that certain areas of the cortex are related to specific functions. Some of these areas have been precisely mapped and numbered in a system called the Brodmann classification, but for our purposes it is sufficient to consider only the general locations of the major functional areas. 1. First (Primary) somatic motor area, or motor center (1) Location It is located in the precentral gyrus and the anterior part of the paracentral lobule. It is equivalent to the 4 and 6 areas of the Brodmann classification. (2) Function The neurons of this area control the conscious and precise voluntary contractions of skeletal muscles. The axons of the neurons of this area form the pyramidal tract. (3) Characteristics ① Inverted image The projection of all parts of body in this area is inverted image, but the projection of head and face is the right image. The cortex of the upper 1/3 part of precentral gyrus and the anterior part of paracentral lobule control the muscles of inferior limb. The middle 1/3 part of precentral gyrus manipulates the muscles of upper limb. The inferior 1/3 part of precentral gyrus manipulates the muscles of head and face. ② Crossed controlling The first somatic motor area controls the movement of contralateral limb. ③ Projection area in the motor center The projection areas of all parts of the body are related to the functions and meticulous level of the corresponding controlled region. 2. First (Primary) somatic sensory area (general sensory area) (1) Location It occupies the postcentral gyrus and the posterior part of the paracentral lobule. It consists of areas 3, 1 and 2 of Brodmann’ map. (2) Function Within this area are the terminations of the sensory pathways that carry general sensory information concerning temperature, touch, pressure, pain, and proprioception from the body to the cortex of the brain. (3) Characteristics The projecting images in the primary sensory area of all the parts of body are similar to that in the primary somatic motor area. ① Inverted image The projection of all parts of body in this area is inverted image, but the projection of head and face is the right image. The inverted image of human body. The right image of head and face. ② Crossed controlling. The contralateral half of the body is represented as inverted. ③ Projection area The size of the cortical area for a particular part of the body is determined by the functional importance of the part and its need for sensitivity. Thus the area for the face, especially the lips, is disproportionately large, and a large area is signed to the hand, particularly the thumb and index fingers. 3. The visual area It surrounds the calcarine sulcus on the medial surface of the occipital lobe, corresponding to area 17 of Brodmann’s map. This area contributes to the interpretation of visual experience. The chief source of afferent fibers to area 17 area is the lateral geniculate nucleus of thalamus by way of the geniculocalcarine tract. 4. The auditory area (acoustic area) It is located in the transverse temporal gyri corresponding to areas 41 and 42 of Brodmann’s map. This area contributes to the interpretation of acoustic experience. The medial geniculate nucleus of the posterior thalamus is the principal source of fibers ending in the acoustic cortex; these fibers constitute the acoustic radiation in the medullary center. 5. Vestibular area It is located in front of superior temporal gyrus. 6. Olfactory area In the medial part of uncus of parahippocampal gyrus. 7. Gustatory area 8. Visceral activation area In the limbic lobe. 9. The language areas Four language cortical areas of special importance in language have been demonstrated. They are situated in the left hemisphere, with few exceptions, and this hemisphere is therefore the dominant hemisphere as a rule with respect to language. 1) The motor speech area (Broca’s area) It occupies the posterior 1/3 portion of the inferior frontal gyrus (to Brodmann’s map 44 and 45 ). Motor aphasia, caused by a lesion in Broca’s area. 2) The writing area The posterior portion of the middle frontal gyrus (Brodmann’s map 8) is the written word area. Written aphasia is caused by a lesion in the written word area. 3) The auditory speech area (sensory language area) This area occupies the posterior part of the superior temporal gyrus (Brodmann’ map 22). Sensory aphasia, in which comprehension of language, naming of objects, and repetition of a sentence spoken by the examiner are all defective, is caused by a lesion in the sensory language area, more specifically in sensory language area. 4) The visual speech area (reading area) It locates in the angular gyrus (39 area if Brodmann). Visual aphasia is caused by a lesion in the visual speech area. Ⅲ) Internal Structures Each cerebral hemisphere includes a large volume of white matter constituting the medullary center, the basal ganglia and lateral ventricle. 1. The basal nuclei Located deep within each cerebral hemisphere are several masses of gray matter known collectively as the basal nuclei. ① Caudate nucleus It is a long, arching nucleus, remaining in the wall of the lateral ventricle and grows around with it in C-shaped course. It consists of an anterior portion or head, which tapers into a slender tail extending backward and then forward into the temporal lobe and terminates at the amygdaloid nucleus. ② Lentiform nucleus It is subdivided into the putamen and the globus pallidus; The caudate nucleus and the lentiform nucleus are sometimes referred to as the corpus striatum. The caudate nucleus and putamen are new structures phylogenetically, which have the similar connections, so both together are called the neostriatum, while the globus pallidus is referred to as the paleostriatum. ③ Claustrum It is a thin layer of gray matter just deep to the cortex of the insula; between the insula cortex and the putamen of the lentiform nucleus, from which it is separated by the fibers of the external capsule. ④ Amygdaloid nucleus It is located at the tip of the tail of the caudate nucleus. The band of white matter located between the basal nuclei and the thalamus is called the internal capsule. 2. The medullary center It composed of three kinds of fibers depending on the nature of their connections. 1) The association fibers They are confined to a hemisphere connecting one cortical area with another. Superior longitudinal fasciculus, Inferior longitudinal fasciculus, Cingulum. Uncinate fasciculus. Arcuate fibers. 2) The commissural fibers They connect the gyri in one hemisphere to the gyri in the other hemisphere. Corpus callosum: From forward back, it can be divided into 4 parts: the rostrum, genu, body, splenium. Anterior and posterior commissures. 3) The projection fibers They are connections between the cortex and the subcortical structures. The fibers fan out as the corona radiata in the medullary center. The projection fibers are concentrated in the internal capsule. The internal capsule It is between the thalamus, caudate nucleus and lentiform nucleus. Constitution: It consists of an anterior limb, a genu and a posterior limb, which have topographic relationships with adjacent gray messes. ① Anterior limb The anterior limb is bounded by the lenticular nucleus and the head of the caudate nucleus. Projection fibers: Anterior thalamic radiation. Frontopontine tract. ② Genu The genu is medial to the apex of the lentiform nucleus. Projection fibers: Corticonuclear tract. ③ Posterior limb The posterior limb is bounded by the lenticular nucleus and the dorsal thalamus. Projection fibers: Central thalamic radiation (Thalamocortical tract). Corticospinal tract. Corticorubral tract. Parieto-occiptto-temporo-pontine tract (corticopontine tract). Optic radiation. Acoustic radiation. A lesion on one side of the posterior limb of the internal capsule may results in: “three hemiplegia syndorm”. a. the abnormalities of voluntary motion of the contralateral trunk and limbs b. the abnormalities of sensation of the contralateral half body. c. the blindness in contralateral halves of both visual fields. 3. The lateral ventricles They are roughly C-shaped cavities lined by ependymal epithelium one in each cerebral hemisphere, and filled with cerebrospinal fluid. Each lateral ventricle consists of a body in the region of the parietal lobe from which anterior, posterior and inferior horns extend into the frontal, occipital and temporal lobes, respectively. The lateral ventricles communicate with the third ventricle through the interventricular foramen. Portions of lateral ventricle contain choroid plexus, formed by an invagination of pia matter covering layer of ependyma on the medial surface of the cerebral hemisphere. This plexus presents in the body and the inferior horn of the lateral ventricle and is the main structure for production of cerebral spinal fluid (CSF). Ⅳ) The Limbic System The limbic system is used to include the limbic lobe as well as associated subcortical structures, such as the amygdaloid complex, hypothalamus, epithalamus, anterior thalamic nuclei. Functions: The limbic system is concerned with: ① emotional and genesis, together with visceral response to the emotions ② survival of individual and species ③ processes involved in memory The limbic system is also known as the visceral brain because of its substantial influence on visceral functions through the autonomic nervous system.
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