Figure 1.

Somatosensory circuitry from the periphery to the brain. Somatosensory information is first transmitted into the spinal dorsal horn by primary afferent neuron fibers, which extend from peripheral tissue into the spinal cord and synapse onto neurons within the dorsal horn. There are four main types of primary afferent fiber, separated by transduction velocity, and each type (Aα, Aβ, Aδ, C) shows some selectivity in somatosensory modality transmitted and synapses into different laminae as shown here (Roberts and Elardo, 1986; McGlone and Reilly, 2010; Koch et al., 2018). This produces selectivity in the modality processed in each lamina of the dorsal horn. Projection neurons within lamina I and V send axons up to the brain via ascending tracts including the spinothalamic tract and dorsal column–medial lemniscal tract (not shown in the figure; Willis, 1985; Niu et al., 2013). Briefly, axons decussate at the spinal level, then ascend on the contralateral side towards the thalamus. Some projections terminate within other brainstem regions including the parabrachial nucleus and periaqueductal gray. Descending tracts originate from several brain regions including the medial prefrontal cortex, hypothalamus, and amygdala, and project first to the periaqueductal gray (Gebhart, 2004; Huang et al., 2019). From here, descending projections then synapse within the rostral ventromedial medulla (RVM), and join with locus coeruleus (LC) descending projections. RVM/LC projection targets including the superficial dorsal horn and lamina V (D’Mello and Dickenson, 2008; François et al., 2017).