Chapter 025. Numbness, Tingling, and Sensory Loss (Part 2)

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Anatomy of Sensation Cutaneous afferent innervation is conveyed by a rich variety of receptors, both naked nerve endings (nociceptors and thermoreceptors) and encapsulated terminals (mechanoreceptors). Each type of receptor has its own set of sensitivities to specific stimuli, size and distinctness of receptive fields, and adaptational qualities. Much of the knowledge about these receptors has come from the development of techniques to study single intact nerve fibers intraneurally in awake, unanesthetized human subjects. It is possible not only to record from but also to stimulate single fibers in isolation. A single impulse, whether elicited by a natural stimulus or evoked by...

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Chapter 025. Numbness, Tingling, and Sensory Loss (Part 2) Anatomy of Sensation Cutaneous afferent innervation is conveyed by a rich variety of receptors, both naked nerve endings (nociceptors and thermoreceptors) and encapsulated terminals (mechanoreceptors). Each type of receptor has its own set of sensitivities to specific stimuli, size and distinctness of receptive fields, and adaptational qualities. Much of the knowledge about these receptors has come from the development of techniques to study single intact nerve fibers intraneurally in awake, unanesthetized human subjects.
It is possible not only to record from but also to stimulate single fibers in isolation. A single impulse, whether elicited by a natural stimulus or evoked by electrical microstimulation in a large myelinated afferent fiber may be both perceived and localized. Afferent fibers of all sizes in peripheral nerve trunks traverse the dorsal roots and enter the dorsal horn of the spinal cord (Fig. 25-1). From there the smaller fibers take a different route to the parietal cortex than the larger fibers. The polysynaptic projections of the smaller fibers (unmyelinated and small myelinated), which subserve mainly nociception, temperature sensibility, and touch, cross and ascend in the opposite anterior and lateral columns of the spinal cord, through the brainstem, to the ventral posterolateral (VPL) nucleus of the thalamus, and ultimately project to the postcentral gyrus of the parietal cortex (Chap. 12). This is the spinothalamic pathway or anterolateral system. The larger fibers, which subserve tactile and position sense and kinesthesia, project rostrally in the posterior column on the same side of the spinal cord and make their first synapse in the gracile or cuneate nucleus of the lower medulla. Axons of the second-order neuron decussate and ascend in the medial lemniscus located medially in the medulla and in the tegmentum of the pons and
midbrain and synapse in the VPL nucleus; the third-order neurons project to parietal cortex. This large-fiber system is referred to as the posterior column–medial lemniscal pathway (lemniscal, for short). Note that although the lemniscal and the anterolateral pathways both project up the spinal cord to the thalamus, it is the (crossed) anterolateral pathway that is referred to as the spinothalamic tract, by convention. Figure 25-1
The main somatosensory pathways. The spinothalamic tract (pain, thermal sense) and the posterior column– lemniscal system (touch, pressure, joint position) are shown. Offshoots from the ascending anterolateral fasciculus (spinothalamic tract) to nuclei in the medulla,
pons, and mesencephalon and nuclear terminations of the tract are indicated. (From AH Ropper, RH Brown, in Adams and Victor's Principles of Neurology, 8th ed. New York, McGraw-Hill, 2007.)Although the fiber types and functions that make up the spinothalamic and lemniscal systems are relatively well known, many other fibers, particularly those associated with touch, pressure, and position sense, ascend in a diffusely distributed pattern both ipsilaterally and contralaterally in the anterolateral quadrants of the spinal cord. This explains why a complete lesion of the posterior columns of the spinal cord may be associated with little sensory deficit on examination.