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The neural systems responsible for somatic different pathways that cross at different sensation begin in the periphery where vari• locations along the neuraxis. Injuries at ous types of receptors give rise to the general different levels of the nervous system somatic afferent pathways. It is useful to begin produce different types of somatosensory by setting out some of the key concepts for symptoms because of the existence of physicians. these parallel pathways. An understand• ing of the organization of the different 1. The somatosensory system transmits informa• components of the somatosensory system tion from peripheral receptors to the CNS via forms the basis for localizing lesions that several parallel pathways (Figure IlL I ). The affect the somatosensory pathways. parallel nature of the somatosensory sys• tem derives from two facts: (I) different 2. Conscious perception of somatic sensation de• classes of peripheral receptor are special• pends on inputs to the cerebral cortex. Thus, ized for different submodalities of soma• interruption of the pathways for somatic tosensory information (pain, temperature, sensation anywhere along their course discriminative touch, vibration sensitiv• from the periphery to the cortex leads to ity, and position sense), and (2) different a loss of conscious sensation. An excep• receptors give rise to axons that travel tion to this generalization is pain percep• through the spinal cord and brain over tion, which persists after cortical injuries Anterolateral column Anterior white commissure
Figure 111.1. Overview of somatosensory system thalamus. Axons of thalamic relay neurons then organization. The basic route of information flow project on to the somatosensory cortex (Cx) in the somatosensory is indicated in the through the internal capsule. The component schematic. Sensory axons from one side of the conveying pain and temperature sensitivity enters body enter the cord in the dorsal roots. At this the cord and synapses on neurons in the dorsal point, the pathways related to the different horn of the spinal cord. The axons of second-order submodalities of somatic sensation diverge. The neurons decussate near the level of entry in the components conveying information about ventral white commissure and ascend in the discriminative touch, vibration sensitivity, and anterolateral column of the spinal cord on the position sense ascend ipsilaterally in the dorsal contralateral side. These fibers travel through the columns (DC). The primary afferent fibers synapse lateral brainstem in the spinothalamic tract and at the spinomedullary junction in the dorsal terminate in the thalamus. Throughout much of column nuclei. Axons from second-order neurons their course, the two components remain quite in the dorsal column nuclei then decussate in the separate. Not shown are the somatosensory lower medulla and ascend through the brainstem inputs from the face, which arrive via the in the medial lemniscus to terminate in the trigeminal nerve. 158 PART 3: THE SOMATOSENSORY SYSTEM 159
and so is thought to be mediated by sub• the spinal cord of a lesion that affects cortical systems. the somatosensory systems. 3. Pathways carrying somatosensory information 5. The pathways carrying different submodali• decussate. Hence, one side of the cortex re• ties of somatosensory information travel ceives somatosensory information from through different white matter tracts within the opposite side of the body. As a result, the spinal cord and brainstem. Information cortical injuries produce sensory symp• related to discriminative touch, vibration toms on the contralateral side. sensitivity, and position sense is carried by fibers that travel through the dorsal 4. The pathways carrying different submodali• columns and medial lemniscus. This sys• ties of somatosensory information decussate tem is thus called the dorsal column-me• at different levels. The pathways convey• dial lemniscal system. Information re• ing pain and temperature sensation de• lated to pain and temperature sensitivity cussate near their level of entry into the travels through the anterolateral col• spinal cord (at the segmental level) and umns of the spinal cord and spino thai a - travel up the spinal cord on the side mic tract in the brains tern. This system is contralateral to their point of entry. The thus called the spinothalamic system. Le• pathways that convey the sensations for sions can affect one or the other of these discriminative touch, vibration sensitiv• systems selectively. ity, and position sense travel up the spinal cord ipsilateral to their point of Through an understanding of the precise entry and decussate at the spinomed• routes taken by the various pathways con• ullary junction. These facts lead to im• veying somatosensory information, it is pos• portant diagnostic signs that allow the sible to deduce the location of lesions affect• physician to determine the level within ing the somatosensory pathways.