Pain Perception and Physiology

Learning Outcomes

• Students should be able to define pain
• Students should be able to explain the physiology of pain
• Students should be able to describe referred pain
• Students should be able to explain the gate control theory
• Students should be able to describe responses to pain

Introduction

• The ability to perceive and react to threats is a protective homeostatic process
• Pain perception is complex activities in the central nervous system
• The character and intensity of pain perceived are shaped by these activities
• The level of pain perceived depends upon:
  • Interaction between the body's analgesia system and nervous system's transmission
  • How the mind interprets the stimuli
  • Individual perception

Definition of Pain

• Pain is an unpleasant and highly personal experience
• Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage
• Pain is whatever the experiencing person says it is

Physiology of Pain

• Nociceptors are sensory receptors for pain
• They are free nerve endings in every tissue except the brain
• Nociceptors respond to painful stimuli (noxious stimuli) like:
  • Electrical
  • Mechanical
  • Thermal
  • Chemical
• Nerve cell bodies are located in the dorsal root ganglia
• Nociceptors are classified into two types of fibers:
  • C fibers: conduct nerve impulses slowly; respond to thermal, mechanical, and chemical stimuli
  • A-delta fibers: conduct nerve impulses faster

Nociception

• When nociceptors are activated, the signals need transduction and transmission to the spine and brain
• The signals are modified before they are understood as pain
• The process of pain perception is called nociception

Physiologic Processes in Nociception

• Transduction: release of substances stimulating the nociceptors
• Transmission: movement of pain sensation to the spinal cord
• Modulation: brain sending signals down the spinal cord; the process of the brain
• Perception: pain is recognized

Pain Reception and Response

• The stimulus received is sent to the CNS
• Action can occur by a "loop" involving only the sensory neuron, integrator neuron, and effector organ
• The brain is informed and can suppress pain response by secreting endogenous opioids

Physiology of Pain - Transduction

• Begins when free nerve endings (nociceptors) of C fibers and A-delta fibers respond to noxious stimuli
• Noxious stimuli trigger the release of biochemical mediators that sensitize nociceptors

Example of Chemical Mediators

• Substance P
• Prostaglandings
• Bradykinin
• Serotonin
• Histamine

Physiology of Pain - Transduction (Continued)

• For pain impulses to be generated, there is exchange of sodium and potassium ions
• De-polarisation and re-polarisation occur at the cell membranes
• This creates an action potential and generates a pain impulse

Pain Medications

• Pain medications can work during this phase by blocking production of prostaglandin or by decreasing the movement of ions across the cell membrane

Physiology of Pain - Transmission

• Includes three segments:
  • First segment: pain impulses move from peripheral nerve fibers to the spinal cord, Substance P serves as the neurotransmitter and moves impulses across the synapse from the primary afferent neuron to the secondary neuron in the dorsal horn of the spinal cord
  • Second segment: transmission of signals from the spinal cord to the brain stem and thalamus via spinothalamic tracts
  • Third segment: transmission of signals between the thalamus and somatic sensory cortex, and to the limbic system (emotional center)

Two Types of Nociceptor Fibers

• Unmyelinated C fibers: transmit dull, aching pain
• Thin A-delta fibers: transmit sharp, localized pain

Physiology of Pain - Modulation

• It is the process of amplifying the pain
• Takes place in the dorsal horn of the spinal cord
• Often described as a descending system
• Occurs when neurons in the thalamus and brain stem send signals back down to the dorsal horn of the spinal cord
• The body has built-in chemical mechanisms (endogenous opioids) to manage pain

Descending Fibers

• Release substances such as endogenous opioids, serotonin, and norepinephrine
• These substances can inhibit the ascending noxious impulses in the dorsal horn.
• Excitatory amino acids (e.g., glutamate, N-methyl-D-aspartate/NMDA) can facilitate pain signals.

Physiology of Pain - Perception

• The final process when pain is felt
• The basic pain sensation occurs in the thalamus and continues to the limbic system (emotional center) and cerebral cortex
• The psychological context and meaning of pain, based on past experiences and future hopes, shapes the behavioral response

Responses to Pain

• The body's response is complex, with both physiological and psychosocial aspects
• Initially, the sympathetic nervous system responds, resulting in the fight-or-flight response (increased pulse and blood pressure)
• People may hold their breath, have shallow breathing, or exhibit reflexive movements as they withdraw from the painful stimulus
• Over minutes or hours, pulse and blood pressure generally return to baseline even while pain persists.
• Unrelieved pain can have harmful effects on well-being, sleep, appetite and quality of life.

Clinical Application: Pain Control Methods

• Opioids & Alpha2 agonists: Used for pain management
• Local anesthetics: Used to numb or block pain signals
• Anti-inflammatory drugs: Used to reduce inflammation contributing to pain
• Epidural analgesia: Injection of medication into the epidural space to block pain transmission
• Patient Control Analgesia (PCA): Allows patient to self-administer pain medication through a pump.

Referred Pain

• Visceral pain is often felt in an area distant from the source of the pain. The nerves serving the organ and the skin area may converge on the same neurons in the spinal cord. Thus, pain from the organ is perceived in a location distant from the organ itself
• The pain experienced is the result of the overlap of both pain fibers from the stimulated organ and the body's sensory nerves

Gate Control Theory

• Melzack and Wall's 1965 theory.
• Sensory input from peripheral fibers modulates before being transmitted to the brain.
• Gates at afferent synapses in the spinal cord and brain control pain signal transmission
• When gates are open, pain signals ascend via the spinothalamic tract
• Pain can be moderated or reduced when the gates are closed

Gate Control Theory & Clinical Application

• Peripherally, large-diameter A-delta nerve fibers (touch, temperature) have an inhibitory effect.
• Higher brain centers (affect and motivation) modify and influence gate opening/closing.
• Clinically, nurses use this model for therapies like topical, massage, or addressing mood and emotion