The conduction pathways for the somatesthetic senses—a term that includes sensations from cutaneous receptors and proprioceptors—are shown in chapter 8 (fig. 8.20). These pathways involve three orders of neurons in series. Sensory information from proprioceptors and pressure receptors is first carried by large, myelinated nerve fibers that ascend in the dorsal columns of the spinal cord on the same (ipsilateral) side. These fibers do not synapse until they reach the medulla oblongata of the brain stem; hence, fibers that carry these sensations from the feet are remarkably long. After the fibers synapse in the medulla with other second-order sensory neurons, information in the latter neurons crosses over to the contralateral side as it ascends via a fiber tract, called the medial lemniscus, to the thalamus (chapter 8, fig. 8.20). Third-order sensory neurons in the thalamus that receive this input in turn project to the postcentral gyrus (the sensory cortex, fig. 8.7).
Sensations of heat, cold, and pain are carried into the spinal cord mostly by thin, unmyelinated sensory neurons. Within the spinal cord, these neurons synapse with second-order association neurons that cross over to the contralateral side and ascend to the brain in the lateral spinothalamic tract. Fibers that mediate touch and pressure ascend in the anterior spinothalamic tract. Fibers of both spinothalamic tracts
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