Psychophysiology of pain

• Pain, Nociception, and Pain Perception

  Pain:

  • Definition: An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.

  Nociception:

  • Definition: The neural processes of encoding and processing noxious stimuli. It involves the transduction, transmission, and modulation of signals that could potentially cause damage to tissues.

  Pain Perception:

  • Definition: The subjective interpretation of the nociceptive signals by the brain, influenced by sensory, emotional, and cognitive factors.
• Difference Between Nociception and Pain Perception

  • Nociception: Refers to the objective, physiological processes of detecting harmful stimuli.
  • Pain Perception: Refers to the subjective experience of pain, which is influenced by psychological, emotional, and cognitive factors.
• Direct vs. Indirect Spinothalamic Tract

  Direct Spinothalamic Tract (Anterior/Lateral Spinothalamic Tract):

  • Pathway: Transmits pain and temperature signals directly to the thalamus.
  • Function: Primarily responsible for the immediate perception of sharp, localized pain.

  Indirect Spinothalamic Tract:

  • Pathway: Transmits pain signals through the brainstem and various other brain regions before reaching the thalamus.
  • Function: Associated with the affective and motivational aspects of pain, such as discomfort and suffering.
• IASP Definitions of Pain

  International Association for the Study of Pain (IASP):

  • Definition: Pain is "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage."
  • Subjectivity: Emphasizes the subjective nature of pain, recognizing that it is influenced by various individual factors and cannot be objectively measured.
• Neuromatrix Theory of Pain

  Neuromatrix Theory:

  • Concept: Pain is produced by a network of neurons (the neuromatrix) in the brain, which integrates sensory, cognitive, and emotional information.
  • Implication: Pain is not merely a direct result of nociceptive signals but a complex experience shaped by the brain's interpretation.
• Pain as a Multifactorial Phenomenon

  Multifactorial Nature:

  • Factors Involved: Sensory input, emotional state, cognitive appraisal, past experiences, cultural background, and environmental context all contribute to the perception of pain.
  • Implication: Pain is a complex and individualized experience that cannot be fully understood by examining only physiological signals.
• Aδ and C Fibres

  Aδ Fibres:

  • Nature: Myelinated, fast-conducting fibres.
  • Function: Transmit sharp, localized pain and temperature information.

  C Fibres:

  • Nature: Unmyelinated, slow-conducting fibres.
  • Function: Transmit dull, aching, and diffuse pain, as well as some temperature information.

  Pathway:

  • Course: Both types of fibres transmit signals from the periphery to the dorsal horn of the spinal cord, then through the spinothalamic tract to the thalamus and other brain regions.
• Pain as a Personal, Subjective Experience

  Personal Experience:

  • Reason: Pain is influenced by individual factors such as genetics, psychology, past experiences, and cultural background.
  • Implication: Pain perception varies widely among individuals, making it a unique and subjective experience for each person.
• Allodynia and Hyperalgesia

  Allodynia:

  • Definition: Pain due to a stimulus that does not normally provoke pain (e.g., light touch).

  Hyperalgesia:

  • Definition: Increased sensitivity to pain or enhanced intensity of pain sensation in response to a normally painful stimulus.
• Types of Pain

  Visceral Pain:

  • Origin: Arises from internal organs.
  • Nature: Often diffuse, deep, and difficult to localize.

  Deep Somatic Pain:

  • Origin: Arises from muscles, bones, and joints.
  • Nature: Usually dull and aching, localized.

  Superficial Somatic Pain:

  • Origin: Arises from the skin or subcutaneous tissues.
  • Nature: Sharp, well-defined, and localized.
• Chronic vs. Acute Pain

  Chronic Pain:

  • Duration: Persists for more than 3-6 months.
  • Nature: Often less related to tissue damage and more influenced by ongoing physiological and psychological factors.

  Acute Pain:

  • Duration: Short-term, usually lasting less than 3 months.
  • Nature: Directly related to tissue damage or injury, often resolving as the injury heals.
• Pain Gate Theory

  Pain Gate Theory:

  • Concept: The spinal cord contains a neurological "gate" that controls the transmission of pain signals to the brain.
  • Mechanism: Non-painful input (e.g., touch) can close the gate to painful input, thus reducing the perception of pain.
• Spinothalamic Tract

  Structure:

  • Neurons: Consists of three neurons.
  • First Neuron: Cell body in the dorsal root ganglion, synapses in the dorsal horn.
  • Second Neuron: Cell body in the dorsal horn, crosses to the opposite side and ascends to the thalamus.
  • Third Neuron: Cell body in the thalamus, projects to the sensory cortex.

  Function:

  • Information: Carries pain, temperature, and crude touch information.
• Descending Modulatory Pain Pathways

  Function:

  • Mechanism: These pathways modulate pain signals at the spinal cord level, often inhibiting the transmission of pain.
  • Neurotransmitters: Involves neurotransmitters like serotonin and norepinephrine.
• Activation of Free Nerve Endings as Nociceptors

  Activation:

  • Mechanism: Free nerve endings act as nociceptors when activated by noxious stimuli or inflammatory mediators.
  • Inflammatory Mediators: Substances like bradykinin, prostaglandins, and substance P sensitize and activate nociceptors.
• Peripheral Sensitisation and Substance P

  Peripheral Sensitisation:

  • Concept: Increased sensitivity of nociceptors due to inflammatory mediators, leading to heightened pain response.

  Substance P:

  • Role: Released from nociceptors, promoting inflammation and further sensitizing the nociceptors, creating a positive feedback loop.
• Neuropathic Pain

  Concept:

  • Definition: Pain caused by damage to the nervous system, either central or peripheral.

  Example (Herniated Disk):

  • Mechanism: A herniated disk can compress spinal nerves, leading to nerve damage and the perception of pain even in the absence of ongoing tissue damage.
• Psychological Factors Affecting Pain Perception

  Factors:

  • Perceived Threat: Higher perceived threat can amplify pain perception.
  • Attention: Focused attention on pain can increase its intensity.
  • Expectation: Expecting pain can heighten its experience.
  • Experience: Past pain experiences can shape current pain perception.
• Importance of Pain Perception

  Necessity:

  • Function: Pain serves as a protective mechanism, alerting individuals to potential harm and prompting behaviors to avoid injury and promote healing.
• Referred Pain

  Concept:

  • Definition: Pain perceived in a location different from the site of the actual tissue damage.
  • Mechanism: Due to the convergence of nerve fibers from different body regions onto the same neurons in the spinal cord.
• Aδ, Aβ, and C Fibres

  Aδ Fibres:

  • Characteristics: Myelinated, fast-conducting.
  • Function: Transmit sharp, localized pain.

  Aβ Fibres:

  • Characteristics: Myelinated, fast-conducting.
  • Function: Transmit touch and pressure information.

  C Fibres:

  • Characteristics: Unmyelinated, slow-conducting.
  • Function: Transmit dull, aching pain and temperature sensations.