Tracts neuroanatomy made ridiculously simple
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The nerve cells that furnish the paleospinothalamic tract are multireceptive or wide dynamic range nociceptors. The second-order neurons also receive input from mechanoreceptors and thermoreceptors. The majority of the first-order nociceptive neurons make synaptic connections in Rexed layer II (substantia gelatinosa) and the second-order neurons make synaptic connections in laminae IV-VIII. The paleospinothalamic tract is phylogenetically old.
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The purple line represents the sensory trigeminal tract. The interrupted blue line represents the spinal trigeminal tract. The solid blue line represents the spinalthamic tract. The sensory and spinal trigeminal tracts. This pathway is responsible for the immediate awareness of a painful sensation and for awareness of the exact location of the painful stimulus. 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). The PF-CM complex is located within the intralaminar thalamus and are known also as intralaminar (IL) nuclei. 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). 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 first-order nociceptive neurons from the head, face and intraoral structures have somata in the trigeminal ganglion (Figure 7.2). The VPL sends axons to the primary somatosensory cortex (SCI). The VPL is thought to mainly be concerned with discriminatory functions. 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. 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. The first-order nociceptive neurons (in the DRG) make synaptic connections in Rexed layer I neurons (marginal zone). The neospinothalamic tract has few synapses and constitutes the classical lateral spinothalamic tract (LST) (Figure 7.1). Each pain tract originates in different spinal cord regions and ascends to terminate in different areas in the CNS. The first-order neurons are located in the dorsal root ganglion (DRG) for all three pathways. The ascending pathways that mediate pain consist of three different tracts: the neospinothalamic tract, the paleospinothalamic tract and the archispinothalamic tract. Pain travels through “redundant” pathways, ensuring to inform the subject to "Get out of this situation immediately." 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 somatosensory transmission are essential to understanding this somatosensory modality. Without these attributes, the organism has no means to prevent or minimize tissue injury. Sensitivity and reactivity to noxious stimuli are essential to the well-being and survival of an organism.