Pain Doesn’t Only Travel Up

For many years, pain was primarily viewed as a one-way biological process. Tissue injury activated peripheral nociceptors, sensory information ascended through the spinal cord, and the brain generated the conscious experience of pain. Although this model remains an essential foundation of pain physiology, it provides only a partial explanation of how the nervous system processes nociceptive information. Accumulating evidence over the past several decades has demonstrated that communication within the central nervous system is fundamentally bidirectional. Rather than functioning as a passive recipient of ascending sensory input, the brain continuously communicates with the spinal cord through descending pathways that modify nociceptive transmission according to behavioural context, previous experience, attention, emotional state, and expectation.¹–³ This evolving understanding has transformed contemporary pain neuroscience by demonstrating that pain is actively regulated throughout the nervous system rather than simply transmitted from injured tissues to the brain.

Descending Modulation: An Active Regulatory System The spinal cord has traditionally been portrayed as a relay station transmitting sensory information from the body to higher centres of the nervous system. Contemporary neurophysiology, however, demonstrates that spinal processing is highly dynamic and continuously influenced by descending projections arising from the brainstem and higher cortical regions.¹³ These descending pathways interact with interneuronal networks within the dorsal horn to regulate incoming nociceptive information before it ascends to supraspinal structures. Their activity reflects the integration of multiple physiological influences, including expectation, attention, emotional state, motivation, previous experience, and behavioural priorities. Rather than functioning as a passive communication pathway, the spinal cord operates as an active processing centre within an integrated regulatory network. This concept represents one of the most important advances in modern pain neuroscience because it challenges the traditional view that pain is determined solely by the intensity of peripheral nociceptive input. Evidence for Descending Modulation Among the landmark investigations supporting this concept is the work of Stroman and colleagues (2016)³, who used simultaneous functional magnetic resonance imaging of the spinal cord and brainstem to examine nociceptive processing in humans. Their findings demonstrated that simply anticipating a painful stimulus altered patterns of neural activity throughout the spinal cord and brainstem before the stimulus was delivered. These observations provided compelling evidence that descending regulatory systems continuously influence spinal nociceptive processing and that the nervous system begins preparing for potential threat before conscious pain is experienced. The significance of this work extends beyond demonstrating descending modulation itself. It illustrates that pain processing reflects ongoing interaction between ascending sensory information and descending regulatory influences, rather than a simple linear transmission of nociceptive signals from peripheral tissues to the brain.

Recent advances in human neuroimaging and systems neuroscience have continued to refine our understanding of descending pain modulation while reinforcing the fundamental concepts described in the original literature. Contemporary investigations demonstrate that descending regulation involves coordinated activity across an extensive network including the periaqueductal grey, rostroventromedial medulla, anterior cingulate cortex, anterior insula, prefrontal cortex, amygdala, hypothalamus, and spinal dorsal horn. These interconnected regions continuously integrate sensory input with expectation, emotional state, behavioural context, and autonomic regulation before nociceptive information contributes to conscious pain perception.⁴⁻⁶ Equally important, recent reviews emphasize that descending modulation is not exclusively inhibitory. Rather, pain regulation reflects a dynamic balance between facilitatory and inhibitory influences that continually shape sensory processing according to the physiological and behavioural needs of the individual. This evolving systems-based perspective further strengthens one of the central themes of this paper: pain should not be understood as a simple ascending pathway, but as the product of continuous interaction throughout an adaptive nervous system.

Recognition of descending modulation has fundamentally changed contemporary approaches to pain neuroscience. Rather than asking only what information is arising from injured tissues, clinicians are increasingly encouraged to consider how that information is being regulated throughout the nervous system. This broader perspective acknowledges that nociceptive input represents only one component of the pain experience and that spinal processing is continuously influenced by multiple interacting biological, cognitive, emotional, and behavioural factors. Importantly, this understanding does not diminish the significance of tissue pathology. Instead, it places tissue injury within the broader context of an adaptive nervous system whose primary function is to regulate behaviour in response to changing internal and external demands. Appreciating this distinction encourages more comprehensive clinical reasoning and supports one of the central principles that underlies the HPHR approach: better clinical decisions begin with better scientific questions.