Parallel and Serial Processing in the Somatosensory System

Abstract

The somatosensory system processes information from low threshold mechanoreceptors in the skin and deep receptors in muscles and joints to allow the active identification of objects by touch. The basic components of the somatosensory pathways from receptors through an array of cortical areas are partly known for monkeys, and processing in humans is likely to depend on comparable pathways. Information from classes of rapidly adapting (RA) and slowly adapting (SA) cutaneous receptors and muscle spindle (MS) receptors remain segregated in relays through the dorsal column-trigeminal complex and the ventroposterior thalamus. In the ventroposterior complex, we distinguish a ventroposterior nucleus that relays SA and RA information to cortical area 3b and to area 1. Muscle spindle information is relayed via the ventroposterior superior nucleus to areas 3a and 2. The ventroposterior inferior nucleus receives inputs from the spinothalamic system and relays to the second somatosensory area(S-II), the adjoining parietal ventral area (PV) and other fields. The VPI relay appears to only modulate cortical neurons and it may provide information about intense stimuli that extend into the painful range. Area 1 is dependent on 3b projections for activation even though it receives direct thalamic inputs. Area 2; S-II, and PV also depend on area 3b directly or indirectly for activation via cutaneous receptors. Activation via deep receptors remains as a result of VPS projections to area 3a and area 2, and cortical relays to S-II and PV. Further steps in a lateral stream of cortical processing involve limbic system structures that are critical in memory storage, while other projections to subdivisions of posterior parietal cortex are presumably concerned with the sensory attention and control of movements in space. The somatosensory fields also project in different patterns to two subdivisions of primary motor cortex, the supplementary motor area, premotor cortex, and motor eye fields to have presumptive roles in initiating and guiding motor behavior.KeywordsSomatosensory CortexMuscle SpindlePosterior Parietal CortexSomatosensory SystemSomatosensory AreaThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.