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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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Central Pain Pathways: The Spinothalamic Tract

The pathways that carry information about noxious stimuli to the brain, as might be expected for such an important and multifaceted system, are complex. The major pathways are summarized in Figure 10.3, which omits some of the less well understood subsidiary routes. Because projections from non-nociceptive temperature-sensitive neurons follow the same anatomical route, they are included in this description, even though they are not part of the pain system.

Figure 10.3. Major pathways for pain (and temperature) sensation.

Figure 10.3

Major pathways for pain (and temperature) sensation. (A) The spinothalamic system. (B) The trigeminal pain and temperature system, which carries information about these sensations from the face.

Like the other sensory neurons in dorsal root ganglia, the central axons of nociceptive nerve cells enter the spinal cord via the dorsal roots (Figure 10.3A). Axons carrying information from pain and temperature receptors are generally found in the most lateral division of the dorsal roots, but the cell bodies of these neurons are not discretely localized within the ganglia (although they are generally smaller than the mechanosensory nerve cells). When these centrally projecting axons reach the dorsal horn, they branch into ascending and descending collaterals, forming the dorsolateral tract of Lissauer (named after the German neurologist who first described this pathway in the late nineteenth century). Axons in Lissauer's tract run up and down for one or two spinal cord segments before they penetrate the gray matter of the dorsal horn. Once within the dorsal horn, the axons give off branches that contact neurons located in several of Rexed's laminae (these laminae are the descriptive divisions of the spinal gray matter in cross section, again named after the neuroanatomist who described these details in the 1950s). Both Aδ and C fibers send branches to innervate neurons in Rexed's lamina I (also called the marginal zone) and lamina II (called the substantia gelatinosa).

Information from Rexed's lamina II is transmitted to second-order projection neurons in laminae IV, V, and VI, the neurons of which also receive some direct innervation from the terminals of the first-order neurons. The axons of these second-order neurons in laminae IV–VI (which are collectively known as the nucleus proprius) cross the midline and ascend all the way to the brainstem and thalamus in the anterolateral (also called ventrolateral) quadrant of the contralateral half of the spinal cord. These fibers, together with axons from second-order lamina I neurons, form the spinothalamic tract, the major ascending pathway for information about pain and temperature. This overall pathway is also referred to as the anterolateral system, much as the mechanosensory pathway is referred to as the dorsal column—medial lemniscus system.

The location of the spinothalamic tract is particularly important clinically because of the characteristic sensory deficits that follow certain spinal cord injuries. Since the mechanosensory pathway ascends ipsilaterally in the cord, a unilateral spinal lesion will produce sensory loss of touch, pressure, vibration, and proprioception below the lesion on the same side. The pathways for pain and temperature, however, cross the midline to ascend on the opposite side of the cord. Therefore, diminished sensation of pain below the lesion will be observed on the side opposite the mechanosensory loss (and the lesion). This pattern is referred to as a dissociated sensory loss and (together with local dermatomal signs; see Box C in Chapter 9) helps define the level of the lesion (Figure 10.4).

Figure 10.4. Pattern of “dissociated” sensory loss following a spinal cord hemisection at the 10th thoracic level on the left side.

Figure 10.4

Pattern of “dissociated” sensory loss following a spinal cord hemisection at the 10th thoracic level on the left side. This pattern, together with motor weakness on the same side as the lesion, is sometimes referred to as the Brown-Séquard (more...)

As is the case of the mechanosensory pathway, information about noxious and thermal stimulation of the face follows a separate route to the thalamus (Figure 10.3B). First-order axons originating from the trigeminal ganglion cells and from ganglia associated with nerves VII, IX, and X carry information from facial nociceptors and thermoreceptors into the brainstem. After entering the pons, these small myelinated and unmyelinated trigeminal fibers descend to the medulla, forming the spinal trigeminal tract (or spinal tract of cranial nerve V), and terminate in two subdivisions of the spinal trigeminal complex: the pars interpolaris and pars caudalis. Axons from the second-order neurons in these two trigeminal nuclei, like their counterparts in the spinal cord, cross the midline and ascend to the contralateral thalamus in the trigeminal lemniscus (also called the trigemino-thalamic tract).

The complexity of the pain pathways (recall that several minor routes are omitted in this account) often makes the origin of a patient's complaints about pain difficult to assess (Box A). For the same reason, chronic pain is often difficult to treat. Such pain can arise from inflammation (as in neuritis), injury to nerve endings and scar formation (as in the pain that can follow surgical amputation; Box B), or nerve invasion by cancer. Injuries to the central nervous system structures that process nociceptive information can also lead to intractable pain. The common denominator of conditions that cause chronic pain is irritation of nociceptive endings, axons, or processing circuits causing abnormal activity that is interpreted as pain. Surgical interruption of a particular tract to abolish chronic pain is not usually effective; the pain, although initially alleviated, tends to return. Indeed, there is often no completely successful treatment for these unfortunate patients.

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Box A

Referred Pain. Surprisingly, there are few, if any, neurons in the dorsal horn of the spinal cord that are specialized solely for the transmission of visceral pain. Obviously, we recognize such pain, but it is conveyed centrally via dorsal horn neurons (more...)

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Box B

Phantom Limbs and Phantom Pain. Following the amputation of an extremity, nearly all patients have an illusion that the missing limb is still present. Although this illusion usually diminishes over time, it persists in some degree throughout the amputee's (more...)

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Copyright © 2001, Sinauer Associates, Inc.
Bookshelf ID: NBK10967

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