The components of the somatosensory system process information conveyed by mechanical stimuli that either impinge on the body surface (cutaneous mechanoreception) or are generated within the body itself (proprioception). Somatosensory processing is performed by neurons distributed across several brain structures that are connected by both ascending and descending pathways. Transmission of afferent mechanosensory information from the periphery to the brain begins with a variety of receptor types that initiate action potentials. This activity is then conveyed centrally via a chain of nerve cells organized into distinct gray matter structures and white matter tracts. First-order neurons in this chain are the primary sensory neurons located in the dorsal root and cranial nerve ganglia. The next set of neurons conveying ascending mechanosensory signals is located in brainstem nuclei (although there are also projection neurons located in the spinal cord that project to the brainstem). The final link in the pathway from periphery to cerebral cortex consists of neurons found in the thalamus, which in turn project to the postcentral gyrus. These pathways are topographically arranged throughout the system, with the amount of cortical and subcortical space allocated to various body parts being proportional to the density of peripheral receptors. Studies of non-human primates show that specific cortical regions correspond to each functional submodality; area 3b, for example, processes information from low-threshold cutaneous receptors, while area 3a processes inputs from proprioceptors. Thus, at least two broad criteria operate in the organization of the somatosensory system: modality and somatotopy. The end result of this complex interaction is the unified perceptual representation of the body and its ongoing interaction with the environment.