Neurological Examination, clinical cases and neuropsychological ...

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23/07/54 The parietal lobe is defined by four anatomical boundaries: the central sulcus separates the parietal lobe from the frontal lobe; the parieto‐occipital sulcus separates the parietal and occipital lobes; the lateral sulcus (sylvian fissure) is the most lateral boundary separating it from the temporal lobe; and the medial longitudinal fissure divides the two hemispheres. Immediately posterior to the central sulcus, and the most anterior part of the parietal lobe, is the postcentral gyrus (Brodmann area 3), the secondary somatosensory cortical area. Dividing this and the posterior parietal cortex is the postcentral sulcus. The posterior parietal cortex can be subdivided into the superior parietal lobule (Brodmann areas 5 + 7) and the inferior parietal lobule (39 + 40), separated by the intraparietal sulcus (IPS). The intraparietal sulcus and adjacent gyri are essential in guidance of limb and eye movement, and based on cytoarchitectural and functional differences is further divided into medial (MIP), lateral (LIP), ventral (VIP), and anterior (AIP) areas Parietal lobe Function The parietal lobe plays important roles in integrating sensory information from various parts of the body, knowledge of numbers and their relations, and in the manipulation of objects. Portions of the parietal lobe are involved with visuospatial processing. Although multisensory in nature, the posterior parietal cortex is often referred to by vision scientists as the dorsal stream of vision (as opposed to the ventral stream in the temporal lobe). This dorsal stream has been called both the 'where' stream (as in spatial vision) and the 'how' stream (as in vision for action). Various studies in the 1990s found that different regions of the posterior parietal cortex in Macaques represent different parts of space. ► The lateral intraparietal (LIP) contains a map of neurons (retinotopically‐coded when the eyes are fixed) representing the saliency of spatial locations, and attention to these spatial locations. It can be used by the oculomotor system for targeting eye movements, when appropriate. ►The ventral intraparietal (VIP) area receives input from a number of senses (visual, somatosensory, auditory, and vestibular). Neurons with tactile receptive fields represented space in a head‐centered reference frame. The cells with visual receptive fields also fire with head‐centered reference frames but possibly also with eye‐centered coordinates ► The medial intraparietal (MIP) area neurons encode the location of a reach target in nose‐centered coordinates. ► The anterior intraparietal (AIP) area contains neurons responsive to shape, size, and orientation of objects to be grasped as well as for manipulation of hands themselves, both to viewed and remembered stimuli. The somatosensory system is a diverse sensory system comprising the receptors and processing centres to produce the sensory modalities such as touch, temperature, proprioception (body position), and nociception (pain). The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal organs, and the cardiovascular system. While touch (also, more formally, tactition; adjectival form: "tactile" or "somatosensory") is considered one of the five traditional senses, the impression of touch is formed from several modalities. In medicine, the colloquial term touch is usually replaced with somatic senses to better reflect the variety of mechanisms involved. The system reacts to diverse stimuli using different receptors: thermoreceptors, nociceptors, mechanoreceptors and chemoreceptors. Transmission of information from the receptors passes via sensory nerves through htracts in the spinal cord and into the brain. Processing primarily occurs in the primary somatosensory area in the parietal lobe of the cerebral cortex. The cortical homunculus was devised by Wilder Penfield. At its simplest, the system works when activity in a sensory neuron is triggered by a specific stimulus such as heat; this signal eventually passes to an area in the brain uniquely attributed to that area on the body—this allows the processed stimulus to be felt at the correct location. The point‐to‐point mapping of the body surfaces in the brain is called a homunculus and is essential in the creation of a body image. This brain‐surface ("cortical") map is not immutable, however. Dramatic shifts can occur in response to stroke or injury. Somatosensory system Dermatome Somatic sensory areas of the cortex. 34
23/07/54 The neural architecture of the somatosensory system. Postcentral gyrus The lateral postcentral gyrus is bounded by: medial longitudinal fissure medially (to the middle) central sulcus rostrally (in front) postcentral sulcus caudally (in back) lateral sulcus inferiorly (underneath) It is the location of primary somatosensory cortex, the main sensory receptive area for the sense of touch. Like other sensory areas, there is a map of sensory space called a homunculus in this location. For the primary somatosensory cortex, this is called the sensory homunculus. A somatotopic map of the body surface onto primary somatosensory cortex. Somatosensory homunculus Brodmann areas 3, 1 and 2 comprise the primary somatosensory cortex of the human brain (or S1). Because Brodmann sliced the brain somewhat obliquely, he encountered area 1 first; however, from rostral to caudal the Brodmann designations are 3, 1 and 2, respectively. Brodmann area 3 is subdivided into area 3a and 3b. Where BA 1 occupies the apex of the postcentral gyrus, the rostral border of BA 3a is in the nadir of the Central sulcus, and is caudally followed by BA 3b, then BA 1, with BA 2 following and ending in the nadir of the postcentral sulcus. BA 3b is now conceived as the primary somatosensory cortex because 1) it receives dense inputs from the NP nucleus of the thalamus; 2) its neurons are highly responsive to somatosensory stimuli, but not other stimuli; 3) lesions here impair somatic sensation; and 4) electrical stimulation evokes somatic sensory experience. BA 3a also receives dense input from the thalamus; however, this area is concerned with proprioception. Areas 1 and 2 receive dense inputs from BA 3b. The projection from 3b to 1 primarily relays texture information; the projection to area 2 emphasizes size and shape. Lesions confined to these areas produce predictable dysfunction in texture, size, and shape discrimination. Somatosensory cortex, like other neocortex, is layered. Like other sensory cortex (i.e. visual and auditory) the thalamic inputs project into layer IV, which in turn project into other layers. Also like other sensory cortices, S1 neurons are grouped together with similar inputs and responses into vertical columns that extend across cortical layers (e.g. As shown by Vernon Mountcastle, into alternating layers of slowly adapting and rapidly adapting neurons; or spatial segmentation of the vibrissae on mouse/rat cerebral cortex). This area of cortex, as shown by Wilder Penfield and others, is organized somatotopically, having the pattern of a homunculus. That is, the legs and trunk fold over the midline; the arms and hands are along the middle of the area shown here; and the face is near the bottom of the figure. While it is not well‐shown here, the lips and hands are enlarged on a proper homunculus, since a larger number of neurons in the cerebral cortex are devoted to processing information from these areas. The positions of Brodmann area's 3, 1 and 2 are ‐ from the nadir of the central sulcus towards the apex of the postcentral gyrus ‐ 3a, 3b, 1 and 2 respectfully. These areas contain cells that project to the secondary somatosensory cortex. The human secondary somatosensory cortex (S2) is a region of cerebral cortex lying mostly on the parietal operculum. Region S2 was first described by Adrian in 1940, who found that feeling in cats' feet was not only represented in the previously described primary somatosensory cortex (S1) but also in a second region adjacent to S1. In 1954, Penfield and Jasper evoked somatosensory sensations in human patients during neurosurgery using electrical stimulation in the lateral sulcus, which lies adjacent to S1, and their findings were confirmed in 1979 by Woolsey et al. using evoked potentials and electrical stimulation. Functional neuroimaging studies have found S2 activation in response to light touch, pain, visceral sensation, and tactile attention. In monkeys, apes and hominids region S2 is divided into several "areas". The area adjoining the primary somatosensory cortex is called the parietal ventral area (PV). Adjacent to PV, but towards the posterior of the parietal operculum, is area S2 ‐ which must not be confused with region S2 (which designates the entire secondary somatosensory cortex, of which area S2 is a part). Deeper in the lateral sulcus, bordering areas PV and S2, lies the ventral somatosensory area (VS). In humans, the secondary somatosensory cortex includes parts of Brodmann areas 40 and 43. Areas PV and S2 both map the body surface. Functional neuroimaging in humans has revealed that in areas PV and S2 the face is represented nearest the entrance to the lateral sulcus, and the hands and feet deeper in the fissure, nearer the border with VS. Individual neurons in PV and S2 receive input from wide areas of the body surface (they have large "receptive fields"), and respond readily to stimuli such as wiping a sponge over a large area of skin. Areas S2 in the left and right hemispheres are densely interconnected, and stimulation on one side of the body will activate area S2 in both hemispheres. Area S2 is interconnected with Brodmann area (BA) 1 and densely so with BA 3b, and projects to PV, BA 7b, insular cortex, amygdala and hippocampus. PV connects densely with BA 5 and the premotor cortex. S2 is colored green and the insular cortex brown in the top right image (coronal section) of the human brain. S1 is green in the top left, and the supplementary somatosensory area is green in the bottom left. 35
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