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Figure 7-1. A. Spinal cord in transverse section illustrating the course of the afferent fibers and the major ascending pathways. Fast-conducting pain fibers are not confined to the spinothalamic tract but are scattered diffusely in the anterolateral funiculus (see also Fig. 7-3). (Adapted from Martin JH: Neuroanatomy: Text and Atlas. New York, McGraw-Hill, 2003, with permission.) B. Transverse section through a cervical segment of the spinal cord illustrating the subdivision of the gray matter into laminae according to Rexed and the entry and termination of the main sensory fibers. (Adapted from Fields HL: Pain. New York, McGraw-Hill, 1987, with permission.)
Figure 7-2. The spinothalamic and trigeminothalamic tracts (pain, thermal sense) are shown. In the bottom section, the fibers that form the spinothalamic tract cross over two or three segment rostral to their entry into the cord, not at the same level as depicted. Offshoots from the ascending anterolateral fasciculus (spinothalamic tract) to nuclei in the medulla, pons, and mesencephalon and nuclear terminations of the tract are indicated. The cortical representation of sensation is shown grossly; it is shown more explicitly in Fig. 8-5 and discussed in Chap. 8. The lemniscal (posterior column) system is shown in Fig. 8-4.
Figure 7-3. Spinal cord showing the segmental and laminated arrangement of nerve fibers within major tracts. On the left side are indicated the “sensory modalities” that appear to be mediated by the two main ascending pathways. C, cervical; T, thoracic; L, lumbar; S, sacral. (Adapted by permission from Brodal A: Neurological Anatomy, 3rd ed. New York, Oxford University Press, 1981.)
Figure 7-4. Mechanism of action of enkephalin (endorphin) and morphine in the transmission of pain impulses from the periphery to the CNS. Spinal interneurons containing enkephalin synapse with the terminals of pain fibers and inhibit the release of the presumptive transmitter, substance P. As a result, the receptor neuron in the dorsal horn receives less excitatory (pain) impulses and transmits fewer pain impulses to the brain. Morphine binds to unoccupied enkephalin receptors, mimicking the pain-suppressing effects of the endogenous opiate enkephalin.