Nociceptive Reception and Transmission

Nociceptive pain occurs as a result of the activation of the peripheral nervous system by noxious stimuli, such as inflammation caused by peripheral injury or disease.³⁷Mersky H, Bogduk N. Part III: Pain Terms, A Current List with Definitions and Notes on Usage. IASP Task Force on Taxonomy. 2017. p8.

The four steps of pain signaling and processing

The neurophysiologic underpinnings of pain can be divided into four stages: transduction, transmission, pain modulation, and perception.³⁸National Pharmaceutical Council and Joint Commission on Accreditation of Healthcare Organizations. Pain: Current Understanding of Assessment, Management, and Treatments. Reston, VA: National Pharmaceutical Council; 2001. p10 At every point in the process, the intensity and propagation of pain signals can be either inhibited or facilitated by neural pathways. This capacity to modulate signaling may account, in part, for wide variations in pain perception between different individuals who sustain the same injury.³⁹ Ossipov MH. The Perception and Endogenous Modulation of Pain. Scientifica. 2012. p1-2.

Transduction

Transduction occurs when a stimulus, such as pressure, thermal energy, or chemical irritation, is converted into a nerve signal or action potential. This occurs at the ends of sensory nerve cells known as nociceptors whose terminals are sensitive to this type of activation.⁴⁰National Pharmaceutical Council and Joint Commission on Accreditation of Healthcare Organizations. Pain: Current Understanding of Assessment, Management, and Treatments. Reston, VA: National Pharmaceutical Council; 2001. p4.

Transmission

Transmission is the process of transferring pain information from the peripheral to the central nervous system.⁴¹National Pharmaceutical Council and Joint Commission on Accreditation of Healthcare Organizations. Pain: Current Understanding of Assessment, Management, and Treatments. Reston, VA: National Pharmaceutical Council; 2001. P6 Signals are transmitted along the axons of nociceptors. Primary nociceptive sensory nerve fibers, synapse with second-order neurons in the dorsal horn of the spinal cord. From here, neurons project to the brainstem, thalamus, and hypothalamus, as well as to reflex arcs to mediate an avoidance response.⁴²Buschmann H et al. Analgesics. Wiley-VCH. 2002. p5

Modulation

Pain modulation refers to up or down regulation of pain signals throughout the spinal cord and the brain.⁴³Hudspith MJ, Siddall PJ, Munglani R. Physiology of pain. In: Hemming HC, Hopkins PM, eds. Foundations of Anesthesia. 2nd ed. London, UK: Mosby; 2006. p277 Many of these signals never reach consciousness because they are dampened by intrinsic modulatory activity within the central nervous system. The gate control theory, advanced by Melzack and Wall, focused on descending pathways from the brain to the spinal cord that inhibited pain signaling.⁴⁴Tobaldini G. Pain Inhibits Pain: an Ascending-Descending Pain Modulation Pathway Linking Mesolimbic and Classical Descending Mechanisms. Molecular Neurobiology. 2019. p1-2 The current view is that signals originating in the brain can both inhibit and facilitate pain signal transmission.⁴⁵Wall and Melzack’s Textbook of Pain, 5th Edition. 2006. p130-135.

Perception

Perception of pain is the awareness—typically an uncomfortable awareness—associated with a specific area of the body.⁴⁷NPC and JCAHO. Pain: Current Understanding of Assessment, Management, and Treatments. 2001. p7. It depends on the transmission of pain signals through the thalamus to the cortex and limbic system.⁴⁶Hudspith MJ, Siddall PJ, Munglani R. Physiology of pain. In: Hemming HC, Hopkins PM, eds. Foundations of Anesthesia. 2nd ed. London, UK: Mosby; 2006. p281At this point in pain processing, perception of the pain experience is influenced by social and environmental cues, as well as by cultural conditioning and past personal experiences.⁴⁷NPC and JCAHO. Pain: Current Understanding of Assessment, Management, and Treatments. 2001. p7.