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Introduction

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  • The Primary Sensory Neurons of the Somatosensory System Are Clustered in the Dorsal Root Ganglia

  • Peripheral Somatosensory Nerve Fibers Conduct Action Potentials at Different Rates

  • Many Specialized Receptors Are Employed by the Somatosensory System

    • Mechanoreceptors Mediate Touch and Proprioception

    • Proprioceptors Measure Muscle Activity and Joint Positions

    • Nociceptors Mediate Pain

    • Thermal Receptors Detect Changes in Skin Temperature

    • Itch Is a Distinctive Cutaneous Sensation

    • Visceral Sensations Represent the Status of Various Internal Organs

  • Somatosensory Information Enters the Central Nervous System Through Cranial and Spinal Nerves

  • Somatosensory Information Flows from the Spinal Cord to the Thalamus Through Parallel Pathways

    • The Dorsal Column–Medial Lemniscal System Relays Tactile and Proprioceptive Information

    • The Spinothalamic System Conveys Noxious, Thermal, and Visceral Information

  • The Thalamus Has a Number of Specialized Somatosensory Regions

    • The Ventral Posterior Nucleus Relays Tactile and Proprioceptive Information

    • Noxious, Thermal, and Visceral Information Is Processed in Several Thalamic Nuclei

  • An Overall View

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We begin the study of the individual sensory systems with the somatosensory system (Greek soma, the body), the system in which sensory coding was first studied electrophysiologically. Somatic information is provided by receptors distributed throughout the body. One of the earliest investigators of the bodily senses, Charles Sherrington, noted that the somatosensory system serves three major functions: proprioception, exteroception, and interoception.

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Proprioception is the sense of oneself (Latin proprius, one's own). Receptors in skeletal muscle, joint capsules, and the skin enable us to have conscious awareness of the posture and movements of our own body, particularly the four limbs and the head. Although one can move parts of the body without sensory feedback from proprioceptors, the movements are often clumsy, poorly coordinated, and inadequately adapted to complex tasks, particularly if visual guidance is absent.

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Exteroception is the sense of direct interaction with the external world as it impacts on the body. The principal mode of exteroception is the sense of touch, which includes sensations of contact, pressure, stroking, motion, and vibration, and is used to identify objects. Some touch involves an active motor component—stroking, tapping, grasping, or pressing—whereby a part of the body is moved against another surface or organism. The sensory and motor components of touch are intimately connected anatomically in the brain and are important in guiding behavior.

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Exteroception also includes the thermal senses of heat and cold. Thermal sensations are important controllers of behavior and homeostatic mechanisms needed to maintain the body temperature near 37°C (98.6°F). Finally, exteroception includes the sense of pain, or nociception, a response to external events that damage or harm the body. Nociception is a prime motivator of actions necessary for survival, such as withdrawal or combat.

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The third component of somatic sensation, interoception, is the sense of the function of the major organ systems of the body and its internal state. Although most of the events recorded by receptors in the viscera do not become conscious sensations, the ...

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