24: Pain

Introduction

• Noxious Insults Activate Nociceptors

• Signals from Nociceptors Are Conveyed to Neurons in the Dorsal Horn of the Spinal Cord

• Hyperalgesia Has Both Peripheral and Central Origins

• Nociceptive Information Is Transmitted from the Spinal Cord to the Thalamus

  • Five Major Ascending Pathways Convey Nociceptive Information

  • Several Thalamic Nuclei Relay Nociceptive Information to the Cerebral Cortex

• Pain Is Controlled by Cortical Mechanisms

  • Cingulate and Insular Areas Are Active During the Perception of Pain

  • Pain Perception Is Regulated by a Balance of Activity in Nociceptive and Non-Nociceptive Afferent Fibers

  • Electrical Stimulation of the Brain Produces Analgesia

• Opioid Peptides Contribute to Endogenous Pain Control

  • Endogenous Opioid Peptides and Their Receptors Are Distributed in Pain-Modulatory Systems

  • Morphine Controls Pain by Activating Opioid Receptors

  • Tolerance and Addiction to Opioids Are Distinct Phenomena

• An Overall View

Pain describes the unpleasant sensory and emotional experiences associated with actual or potential tissue damage. Pricking, burning, aching, stinging, and soreness are among the most distinctive of all the sensory modalities. As with the other somatic sensory modalities—touch, pressure, and position sense—pain serves an important protective function, alerting us to injuries that require evasion or treatment. In children born with insensitivity to pain, severe injuries often go unnoticed and can lead to permanent tissue damage. Yet pain is unlike other somatic sensory modalities, or vision, hearing, and smell in that it has an urgent and primitive quality, possessing both affective and emotional components.

The perception of pain is subjective and is influenced by many factors. An identical sensory stimulus can elicit quite distinct responses in the same individual under different conditions. Many wounded soldiers, for example, do not feel pain until they have been removed from the battlefield; injured athletes are often not aware of pain until a game is over. Simply put, there are no purely "painful" stimuli, sensory stimuli that invariably elicit the perception of pain in all individuals. The variability of the perception of pain is yet another example of a principle that we have encountered in earlier chapters: Pain is not the direct expression of a sensory event but rather the product of elaborate processing by the brain of a variety of neural signals.

When pain is experienced it can be acute, persistent, or in extreme cases chronic. Persistent pain characterizes many clinical conditions and is usually the reason that patients seek medical attention. In contrast, chronic pain appears to have no useful purpose; it only makes patients miserable. Pain's highly individual and subjective nature is one of the factors that make it so difficult to define objectively and to treat clinically.

In this chapter we discuss the neural processes that underlie the perception of pain in normal individuals and explain the origins of some of the abnormal pain states that are encountered clinically.

Noxious Insults Activate Nociceptors

Many organs in the periphery, including skin and subcutaneous structures such as ...