Chapter 34: Basic Anatomy and Physiology of Pain Mechanisms

INTRODUCTION

PAIN REHABILITATION CAN BE DEFINED AS THE process of helping an individual achieve the highest level of function, independence, and quality of life possible through the management of a painful condition and its impact on daily activities. Pain is a complex phenomenon that requires a functional approach taking into account physiological, psychological, and contextual domains. Multidisciplinary biopsychosocial rehabilitation is generally the model used for chronic pain with a superior effect on pain reduction and quality of life compared to disciplinary approaches or to surgery.¹ Considering its multifaceted aspect, it is not surprising that rehabilitation is often the most important approach in the treatment of pain.

Understanding the neurophysiology of pain is essential not only for the clinician, but also for the patient. In a recent study, the authors found that patients with higher knowledge in pain neurophysiology have less fear avoidance and lower perceived disability due to pain.² It is then essential for any clinician working with patients suffering from chronic pain to have a good background of pain neurophysiology to transfer this empowering knowledge to the patient.

In this chapter, we will introduce the basic anatomy and physiology of pain with an emphasis on endogenous excitatory and inhibitory mechanisms. We will discuss the mechanisms implicated in the development, persistency, and treatment of pain with an emphasis on the role of rehabilitation. The main goal is to introduce pain neurophysiology in functional terms so the clinician can appropriate and apply the concepts in a clinical environment.

The ideal goal in pain rehabilitation is to identify the cause, utilize an appropriate treatment, decrease pain perception, and improve function. Generally, the efficacy of treatment is limited by the presence or absence of an accurate diagnosis. In chronic pain, however, a clear diagnosis is rarely identified; the etiology of chronic pain is often complex and multifactorial. The combination of pharmacological and nonpharmacological approaches is used to alleviate suffering and improve function. Rehabilitation has always played a major role in the control of chronic pain.

Exercise is known to be one of the most important aspects of pain rehabilitation.³ However, fear avoidance belief leads to kinesiophobia, which is a major factor of poor outcomes in chronic pain.⁴ One way to reduce the fear avoidance belief is to educate patients about the neurophysiology of pain.² This knowledge will help make it clear that several nonpharmacological approaches have solid physiological grounds and are frequently essential in the treatment of pain.

Patient collaboration is a major element in the treatment of chronic pain. Their involvement is essential to get the best results. Understanding the neurophysiological mechanisms of pain will guide the clinician to choose the best treatment for a specific condition, but will also help educate the patient to become a collaborator in the treatment team.

FROM NOCICEPTION TO THE COMPLEX EXPERIENCE OF PAIN