Sensitivity and reactivity to noxious stimuli are essential to the well-being and survival of an organism. Without these attributes, the organism has no means to prevent or minimize tissue injury. Given the importance of pain sensation, it is not surprising that the central and peripheral neuronal pathways that subserve the transmission of nociceptive information and the pathway that transmits and modulates this transmission are essential to understanding this sensory modality. Pain travels through “redundant” pathways, ensuring to inform the subject to "Get out of this situation immediately."
Pain Pathways
The ascending pathways that mediate pain consist of three different tracts: the neospinothalamic tract, the paleospinothalamic tract and the archispinothalamic tract. The first-order neurons are located in the dorsal root ganglion (DRG) for all three pathways. Each pain tract originates in different spinal cord regions and ascends to terminate in different areas in the CNS.
Neospinothalamic Tract
The neospinothalamic tract has few synapses and constitutes the classical lateral spinothalamic tract (LST) (Figure 7.1). The first-order nociceptive neurons (in the DRG) make synaptic connections in Rexed layer I neurons (marginal zone). Axons from layer I neurons decussate in the anterior white commissure, at approximately the same level they enter the cord, and ascend in the contralateral anterolateral quadrant. Most of the pain fibers from the lower extremity and the body below the neck terminate in the ventroposterolateral (VPL) nucleus and ventroposteroinferior (VPI) nucleus of the thalamus, which serves as a relay station that sends the signals to the primary cortex. The VPL is thought to mainly be concerned with discriminatory functions. The VPL sends axons to the primary somatosensory cortex (SCI).
Figure 7.1 |
The first-order nociceptive neurons from the head, face and intraoral structures have somata in the trigeminal ganglion (Figure 7.2). Trigeminal fibers enter the pons, descend to the medulla and make synaptic connections in the spinal trigeminal nucleus, cross the midline and ascend as trigeminothalamic tract (or trigeminal lemniscus, Figure 7.2). The A delta fibers terminate in the ventroposteromedial (VPM) thalamus, and the C fibers terminate in the parafasciculus (PF) and centromedian (CM) thalamus (PF-CM complex). The PF-CM complex is located within the intralaminar thalamus and are known as IL. All of the neospinothalamic fibers terminating in VPL and VPM are somatotopically oriented and from there send axons that synapse on the primary somatosensory cortex (SC I - Brodman areas 1 & 2). This pathway is responsible for the immediate awareness of a painful sensation and for awareness of the exact location of the painful stimulus.
