Figure 8-2. The location and morphology of mechanoreceptors in hairy and hairless (glabrous) skin of the human hand. Receptors are located in the superficial skin, at the junction of the dermis and epidermis, and more deeply in the dermis and subcutaneous tissue. The receptors of the glabrous skin are Meissner’s corpuscles, located in the dermal papillae; Merkel disc receptors, located between the dermal papillae; and bare nerve endings. The receptors of the hairy skin are hair receptors, Merkel’s receptors (having a slightly different organization than their counterparts in the glabrous skin), and bare nerve endings. Subcutaneous receptors, beneath both glabrous and hairy skin, include Pacinian corpuscles and Ruffini endings. Nerve fibers that terminate in the superficial layers of the skin are branched at their distal terminals, innervating several nearby receptor organs; nerve fibers in the subcutaneous layer innervate only a single receptor organ. The structure of the receptor organ determines its physiological function. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM: Principles of Neural Science, 4th ed. New York: McGraw-Hill, 2000.)
Figure 8-3. Distribution of the sensory spinal roots on the surface of the body (dermatomes). (Reproduced by permission from Sinclair D: Mechanisms of Cutaneous Sensation. Oxford, UK, Oxford University Press, 1981.)
Figure 8-4. Dermatomes of the upper and lower extremities, outlined by the pattern of sensory loss following lesions of single nerve roots. (Reproduced by permission from Keegan and Garrett.)
Figure 8-5. The main somatosensory pathways emphasizing the posterior column–lemniscal system (thicker tract lines). See Fig. 7-2 for comparison with the spinothalamic system.
EyeThumbIndexMiddleRingLittleHandWristElbowForearmArmShoulderHeadNeckHipLegTrunkFootToesGenitalsSensory homunculusMedialLateralNoseFaceUpper lipLower lipTeeth, gums, and jawTonguePharynxIntraabdominalFingers
Figure 8-6. The “sensory homunculus,” or cortical representation of sensation in the postcentral gyrus; compare this to the distribution of body areas in the motor cortex as shown in Fig. 3-4. (Reproduced with permission from Kandel ER, Schwartz JH, Jessel TM: Principle of Neural Science, 4th ed. New York, McGraw-Hill, 2000.)
Figure 8-7. Sites of lesions of the characteristic spinal cord sensory syndromes (shaded areas indicate regions of damage). Lower right figure shows variable extent of damage to mid-axial cord but always sparing the posterior columns.
Chapter  8    Disorders of Non-Painful Somatic Sensation
Of all the painful states that afflict humans, headache is undoubtedly the most frequent and rivals backache as one of the most common reasons for seeking medical help. In fact, there are so many cases of headache that dedicated headache clinics have been established in many medical centers. In addition to its frequency in general practice, many headaches are caused by medical rather than neurologic diseases and the subject is the legitimate concern of the general physician. Yet there is always the question of intracranial disease so that it is difficult to approach the subject without knowledge of neurology.