Figure 1.

Anatomy of the somatosensory system. (A) Somatosensory neuron cell bodies reside outside the spinal cord in the dorsal root ganglia (DRG). They have a single process that splits, sending an afferent projection to the periphery and an efferent projection to the spinal cord. (B) Somatosensory neurons residing in the trigeminal send processes that innervate peripheral targets through the face, mouth, and dura and central targets in the brainstem. (C) Somatosensory neurons can be divided into three broad categories based on the size of their cell bodies and degree of myelination. Within these broad categories, numerous sub-specializations exist-for example small diameter C fibers are mostly nociceptors while large diameter A neurons respond to low threshold mechanical stimuli. LTMR, low-threshold mechano-receptor.