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This section deals with pain and somatic sensations, all derived mainly from afferent impulses that arise in the organs, skin, blood vessels, connective tissues, muscles, and joints. Because of its overriding clinical importance, pain has been accorded a chapter of its own. The special senses—vision, hearing, taste, and smell—are considered in the next section, and visceral sensation, most of which does not reach consciousness, is considered with the disorders of the autonomic nervous system.

Pain is an important sign of illness and it stands preeminent among all the sensory experiences by which humans judge the existence of a disease. Relatively few medical diseases do not have a painful phase. Moreover, the diagnosis of certain diseases rests heavily on a characteristic absence of pain. Headache and the pain derived from disorders of the spinal column and roots occupy a distinctive place in clinical practice and are elaborated in their own chapters.

To deal effectively with problems of pain and altered sensation requires familiarity with the anatomy of sensory pathways and the sensory innervation of body segments as well as insight into the psychological factors that influence the perception of and reaction to pain. Two major systems, designated by their tracts in the spinal cord, subserve these functions: spinothalamic pathways for pain and the posterior column-medial lemniscal pathways for touch, joint position, deep pressure, and vibration senses.

The spinothalamic system originates in free nerve endings that coalesce into lightly myelinated axons. These fibers represent the peripheral projections of neurons contained in the dorsal root ganglia. Their central projections are contained in the dorsal (posterior) roots that enter the spinal cord and synapse in the dorsal horns onto neurons. The axons of these neurons decussate and ascend as the spinothalamic tract, terminating in the thalamus. An analogous pathway called the spinal trigeminal tract subserves pain sensation in the face.

The posterior column-medial lemniscal system originates in several distinctive specialized receptors that give rise to heavily myelinated axons whose cell bodies are also contained in the dorsal root ganglia. Their central projections enter the dorsal roots and then the dorsal horn of the spinal cord, and without synapsing or decussating, ascend as the posterior columns. An analogous anterior trigeminothalamic tract subserves somatic sensation in the face.

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