This pathway carries the modalities of pain and temperature and a form of touch sensation called crude or light touch. The sensations of itch and tickle, and other forms of sensation (e.g., "sexual") are likely carried in this system. In the periphery the receptors are usually simply free nerve endings, without any specialization.
These incoming fibers (sometimes called the first order neuron) enter the spinal cord and synapse in the dorsal horn (see Figure 4 and Figure 32). There are many collaterals within the spinal cord that are the basis of several protective reflexes (see Figure 44). The number of synapses formed is variable, but eventually a neuron is reached that will project its axon up the spinal cord (sometimes referred to as the second order neuron). This axon will cross the midline, decussate, in the ventral (anterior) white commissure, usually within two to three segments above the level of entry of the peripheral fibers (see Figure 4 and Figure 32).
These axons now form the anterolateral tract, located in that portion of the white matter of the spinal cord. It was traditional to speak of two pathways — one for pain and temperature, the lateral spino-thalamic tract, and another for light (crude) touch, the anterior (ventral) spino-thalamic tract. Both are now considered together under one name.
The tract ascends in the same position through the spinal cord (see Figure 68 and Figure 69). As fibers are added from the upper regions of the body, they are positioned medially, pushing the fibers from the lower body more laterally. Thus, there is a topographic organization to this pathway in the spinal cord. The axons of this pathway are either unmyelinated or thinly myelinated. In the brainstem, collaterals are given off to the reticular formation, which are thought to be quite significant functionally. Some of the ascending fibers terminate in the ventral posterolateral (VPL) nucleus of the thalamus (sometimes referred to as the third order neuron in a sensory pathway), and some in the nonspecific intralaminar nuclei (see Figure 12 and Figure 63).
There is a general consensus that pain sensation has two functional components. The older (also called the paleospinothalamic) pathway involves the reported sensation of an ache, or diffuse pain that is poorly localized. The fibers underlying this pain system are likely unmy-elinated both peripherally and centrally, and the central connections are probably very diffuse; most likely these fibers terminate in the nonspecific thalamic nuclei and influence the cortex widely. The newer pathway, sometimes called the neospinothalamic system, involves thinly myelinated fibers in the PNS and CNS, and likely ascends to the VPL nucleus of the thalamus and from there is relayed to the postcentral (sensory) gyrus. Therefore, the sensory information in this pathway can be well localized. The common example for these different pathways is a paper cut — immediately one knows exactly where the cut has occurred; this is followed several seconds later by a diffuse poorly localized aching sensation.
The cross-sectional levels for this pathway include the lumbar and cervical spinal cord levels, and the brainstem levels mid-medulla, mid-pons, and upper midbrain.
In the spinal cord, this pathway is found among the various pathways in the anterolateral region of the white matter (see Figure 32, Figure 68, and Figure 69), hence its name. Its two parts cannot be distinguished from each other or from the other pathways in that region. In the brainstem, the tract is small and cannot usually be seen as a distinct bundle of fibers. In the medulla, it is situated dorsal to the inferior olivary nucleus; in the uppermost pons and certainly in the midbrain, the fibers join the medial lemniscus (see Figure 40).
Lesions of the anterolateral pathway from the point of crossing in the spinal cord upward will result in a loss of the modalities of pain and temperature and crude touch on the opposite side of the body. The exact level of the lesion can be quite accurately ascertained, as the sensation of pain can be quite simply tested at the bedside by using the end of a pin. (The tester should be aware that this is quite uncomfortable or unpleasant for the patient being tested.)
FIGURE 34: Anterolateral System — Pain, Temperature, and Crude Touch
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