Anatomy and Physiology: Mechanisms of Nociceptive Transmission
Nocicpetion is the measurable physiological response of specialized sensory receptors (nociceptors) to overt or potential tissue damage and is perceived in the CNS—via the spinothalamic tract, the thalamus, and finally different areas in the neocortex—as
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Anatomy and Physiology: Mechanisms of Nociceptive Transmission Daniel Vardeh and Julian F. Naranjo
Nociception is the measurable physiological response of specialized sensory receptors (nociceptors) to overt or potential tissue damage and is perceived in the CNS—via the spinothalamic tract, the thalamus, and finally different areas in the neocortex—as pain. Initially, noxious chemical, mechanical, or thermal stimuli are detected at nerve endings of primary sensory neurons with their soma located in the dorsal root ganglion (DRG) for body sensation, and in the trigeminal ganglion (gasserian ganglion) for face sensation. Specialized receptors (transducers) located at the cell membrane of sensory nerve endings translate the intensity of a given stimulus into action potential frequency, which results in the emission of glutamate and peptides in the respective area in the spinal cord dorsal horn (mostly superficial laminae I and II with some projections to lamina V). Nociceptors can be divided into different groups by means of their anatomical structure, their characteristic expression of various proteins or the distinct receptors at their terminals, as described below. D. Vardeh, MD (*) Interventional Headache and Neck Pain Management, Department of Neurology and Anesthesia, Lahey Hospital & Medical Center, 41 Mall Road, Burlington, MA 01805, USA e-mail: [email protected] J.F. Naranjo, MD Comprehensive Interventional Pain Medicine, 7000 SW 62 Street Suite 535, Miami, FL 33143, USA
Fibers Types of Nociceptors Generally, A-δ and C-fibers are the major contributors to physiological nociception, with A-β fibers contributing in pathological states of central sensitization. C-fibers are small in diameter; they are unmyelinated and conduct impulses at the slow rate of 0.5–2 m/s. C fibers have smaller receptive fields than the A-δ nociceptors and mostly terminate in lamina LII of the spinal dorsal horn. Their activation results in a more prolonged sensation of dull and burning pain. Most C-fibers are polymodal receptors and are activated by high-threshold mechanical and various chemical stimuli, as well as by heat (starting at 39–41 °C). These polymodal C-fibers are heavily influenced by both the phenomena of sensitization (enhanced response to a lasting/repetitive stimulus of same intensity) and fatigue (reduced response to a lasting/repetitive stimulus of same intensity). A-δ fibers function as thermal and high- threshold mechanical receptors. Generally, they respond with higher discharge frequencies than C-fibers and the discriminable information supplied to the CNS is greater. Most of these fibers have polymodal properties (with high heat threshold at 40–50 °C), are thinly myelinated (conduction velocity between 5 and 35 m/s) and terminate in LI and LV of the dorsal horn. Activation of A-δ fibers generally results in a short sensation of sharp, pricking pain, in contrast to the dull sensation mediated by C-fibers.
© Springer International Publishing Switzerland 2017 R.J. Yong et al. (eds.), Pain Medicine, DOI 10.100
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