The somatosensory system, the sensation of the body and its movements, is not one sense but many. We can distinguish the shape of an object (discriminative touch), deep pressure, cold, warmth, pain, and the position and movement of joints. The skin is packed with a variety of somatosensory receptors; each receptor contributes in some degree to several kinds of somatosensory experience. The somatic nervous system refers to nerves that convey messages from the sense organs to the central nervous system and from the central nervous system to muscles and glands. Information from touch receptors in the head enters the central nervous system through the cranial nerves. Information from receptors below the head enters the spinal cord and passes toward the brain. The somatosensory cortex is that portion of the cerebral cortex that responds to touch and other body information. After damage to the somatosensory cortex, people generally experience an impairment of body perceptions. Because touch perception itself depends on activity in the cerebral cortex, people with an intact cortex but damage in the periphery may experience a "phantom limb," the continuing sensation of an amputated body part.
Functional organization of the somatosensory system--National Institute of Neurological Disorders and Stroke (1980-2005)
Jon Kaas, Ph.D., Principal Investigator
This research has three long-term goals. Studies include New World owl monkeys, which offer clear technical advantages, and Old World macaques, which are more closely related to humans. A major goal is to develop a better understanding of the organization of the somatosensory and motor system in primates: how these systems are divided into nuclei, areas, and modules, and how these subdivisions are interconnected to form processing networks. Planned studies combine electrophysiological microelectrode mapping procedures, histochemical architectonic methods, and multiple tracers in connection-tracing protocols in the same monkey. A major effort will be to determine and compare connections of proposed subdivisions of somatosensory cortex of the lateral sulcus, posterior parietal cortex, and premotor cortex.
Related studies of adult plasticity in primates will determine the effects on sensory representations of partial deprivations as a result of cortical lesions, nerve damage, or spinal cord lesions. We will attempt to determine the contribution of neuronal growth and neurotransmitter regulation in plasticity by using molecular markers of neuron restructuring, tracers of axon-arbors and growth, and markers of molecules associated with neuronal activity and plasticity.
Other studies will examine the effects of fetal and early postnatal deprivation and damage on subsequent development of the somatosensory system in monkey in order to understand the potential for developmental plasticity and reorganization complex sensory systems.
These three types of studies will provide a detailed and comprehensive model of sensorimotor processing that applies to humans; an understanding of the mechanisms of adult plasticity in the somatosensory system that can guide programs of therapy in humans; and an understanding of the extent and limits of developmental plasticity that can guide medical treatment and guidance of humans with prenatal defective and early postnatal injury.
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