Anterolateral Pathway

The second major ascending sensory system is called the spinothalamic or anterolateral pathway, which carries information about pain and temperature from the body. This pathway is best understood as two subdivided tracts. The first is called the neospinothalamic tract, which carries information about sharp pain within A-delta (small myelinated) fibers. Unlike the dorsal column-lateral lemniscal pathway, the first-order neurons in this pathway synapse with neurons within the dorsal horn of the spinal cord. Projection fibers then cross the midline of the cord and form the lateral portion of the anterolateral tract as they ascend the cord to the VPL nucleus of the thalamus.

The paleospinothalamic subdivision carries information about dull, aversive pain; temperature; and crude touch within small, unmyelinated C fibers. There are two synaptic relays with cells in the dorsal horn of the spinal cord. Axons of projection neurons then cross the cord and join the anterolateral tract, terminating in the intralaminar nucleus of the thalamus. Unlike other tracts in the somatosensory tract, the paleospinothalamic tract is not somatotopically organized (Fig. 5).

Because of multiple synapses in the spinal cord, pain information can spread to adjacent neurons in the dorsal horn, causing a widespread hyperalgesia. Pain information can also be suppressed through synaptic input at the spinal cord level so that neither local spinal reflexes nor conscious centers are informed ofthe injury. As discussed in Chapter 49, the first synapse in this path can be inhibited, both pre- and postsynaptically, by endorphins, and this neurotransmitter can be mimicked by drugs such as morphine. This gating or interruption of nociceptive information allows an organism to cope with an emergency or stress without the distraction of pain.

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