I– II together constitute the superficial dorsal horn, which receives information from nociceptors (see Chapter 5). Lamina II—the substantia gelatinosa—is a pale- staining area through which a few myelinated fibres pass; it is also subdivided into two regions. The outer region— IIo—is involved in the processing of nociception from unmyelinated afferents, whilst the inner region—Iii— receives input from unmyelinated low threshold mecha­noreceptors and thinly myelinated low threshold or itch responsive afferents. Laminae III–VI receive input from low- threshold cutaneous receptors. The ventral horn consists of laminae VII–IX and contains the cell bodies of motor neurons and interneurons and the terminals of pri­mary afferents from muscle and joints. α- Motor neuron cell bodies are the most prominent cell structures visible in this region. Different muscles have their motor neu­rons located in discrete cell groups in the ventral horn. The motor neuron populations supplying different mus­cles are differentially organized within the ventral horn (Fig. 4.4B). Distal muscles are represented laterally, while proximal muscles are located more medially. Extensor muscle groups are located more ventrally within the ventral horn, compared to flexor muscle motor neurons. Visceral afferents, which primarily mediate unconscious sensory input from visceral organs and blood vessels, ter­minate in laminae I, IIo and V. They encode information in a way which is different from that of cutaneous and skeletal muscle afferents (see Chapter 5).

input (from slowly and rapidly adapting receptors), and area 3a receives input from muscle proprioceptors. Area 2 mostly receives input from joints and deep tissues (thought to be important for stereognosis), and area 1 receives infor­mation from rapidly adapting cutaneous mechanorecep­tors. This leads to multiple representations of the body in