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In order to interpret the history and clinical signs of patients with disorders of somatic sensation, the functional anatomy of the sensory components of the nervous system must be understood. As used here, somatic sensation refers to the sensations of touch or pressure, vibration, joint position, pain, and temperature, and to more complex functions that rely on these primary sensory modalities (eg, two-point discrimination, stereognosis, graphesthesia); it excludes special senses such as smell, vision, taste, and hearing.
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FUNCTIONAL ANATOMY OF THE SOMATIC SENSORY PATHWAYS
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The sensory pathway between peripheral tissues (eg, skin or joints) and the cerebral cortex involves three neurons and two central synapses (Figure 10-1).
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First-order sensory neurons from the limbs and trunk have cell bodies in the dorsal root ganglia. Each of these neurons sends a peripheral process that terminates in a free nerve ending or encapsulated sensory receptor and a central process that enters the spinal cord. Sensory receptors are relatively specialized for particular sensations and, in addition to free nerve endings (pain, itch), include Meissner corpuscles, Merkel corpuscles, and hair cells (touch); Krause end-bulbs (cold); and Ruffini corpuscles (heat). First-order sensory neurons synapse centrally at a site that depends on the type of sensation. Fibers mediating touch, pressure, or postural sensation in the limbs and trunk ascend in the posterior columns of the spinal cord to the medulla, where they synapse in the gracile and cuneate nuclei. Other fibers that mediate touch and those subserving pain, temperature, and itch appreciation in the limbs and trunk synapse on neurons in the posterior horns of the spinal cord, particularly in the substantia gelatinosa. First-order sensory neurons from the face, which have cell bodies in the trigeminal (gasserian) ganglion, travel in the trigeminal (V) nerve and enter the pons. Fibers mediating facial touch and pressure synapse in the main trigeminal (V) nerve sensory nucleus, whereas those conveying facial pain and temperature synapse in the spinal trigeminal (V) nerve nucleus.
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Second-order sensory neurons with cell bodies in the gracile and cuneate nuclei cross the midline and ascend in the medial lemniscus. Second-order sensory neurons that arise in the posterior horns of the spinal cord cross the midline and ascend in the anterolateral part of the cord: fibers mediating touch pass upward in the anterior spinothalamic tract, whereas pain, itch, and temperature fibers generally travel in the lateral spinothalamic tract. Second-order sensory neurons from the limbs and trunk are joined in the brainstem by fibers from the face: those that mediate facial touch and pressure sensation project ...