The somatosensory system

Abstract

The somatosensory system has by far the largest number of receptor types of any of the primate sensory systems, including mechanoreceptors, chemoreceptors, nociceptors and thermoreceptors. The sensation of touch is mainly mediated by mechanoreceptors, but there are a number of other processing channels within the somatosensory system for proprioception, pain and temperature. The classic view of two independent channels for somatosensory information from the trunk and the extremities, i.e. the dorsal column-medial lemniscus system for tactile sensitivity and position sense and the anterolateral or spinothalamic system for pain and temperature sensitivity, has been modified through the discovery of additional spinal pathways for the transmission of sensory impulses to the brain and by new views on pain mechanisms. Somatosensory information from the face is transmitted via the trigeminal nerve. In this chapter, Mountcastle’s subdivision into large- and small-fibred systems is used. The large-fibred somatic afferent system deals with the discriminatory-sensory aspects of somaesthesis and the small-fibred system with the affective-vegetative components of the perceptions evoked by all but the blandest of somatic stimuli. The discriminative-sensory systems include those of the dorsal and dorsolateral columns and the trigeminal lemniscal system that deal with the mechanoreceptive aspects of somaesthesis. The affective-vegetative systems convey somatosensory information from all the spinal columns, especially from the ventral quadrants, and from the spinal trigeminothalamic tract. Functional, anatomical and imaging data suggest that pain impulses are conveyed by specific sensory channels or labelled lines that ascend in a central homeostatic afferent pathway and have specific thalamic and cortical targets. After a brief description of receptors and peripheral pathways (Sect. 4.2), the following somatosensory systems are discussed: (1) the large-fibred dorsal column-medial lemniscus system and sensory pathways in the dorsolateral funiculus (Sect. 4.3), (2) the small-fibred sensory pathways in the ventral quadrant of the spinal cord (the anterolateral or pain system; Sect. 4.4) and (3) the trigeminal somatosensory system (Fig. 4.5). For each of these systems, Clinical Cases illustrate where and how the somatosensory circuitry may be damaged. The English terms of the Terminologia Neuroanatomica are used throughout.