EXPERIMENTAL FIGURE 21-38 Different classes of sensory neurons are lost in knockout mice lacking different trophic factors or their receptors. In animals lacking nerve growth factor (NGF) or its receptor TrkA, small nociceptive (pain-sensing) neurons (light blue) that innervate the skin are missing. These neurons express the TrkA receptor and innervate NGF-producing target tissues. In animals lacking either neurotrophin-3 (NT-3) or its receptor TrkC, large proprioceptive neurons (red) innervating muscle spindles are missing. Muscle tissue produces NT-3, and the proprioceptive neurons express TrkC. Mechanoreceptors (orange; see Figure 22-32), another class of sensory neurons in the dorsal root ganglion, are unaffected in these mutants. See W. D. Snider, 1994, Cell 77:627.