Definitions and Types of Pain

Definitions of Pain

• Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage.
• The International Association for the Study of Pain (IASP) definition (2020) encompasses individuals who are unable to articulate pain verbally.

Pain Receptors

• Pain receptors are free nerve endings that are slowly adapting.

Types of Pain Receptors

• Mechanical pain receptors: triggered by mechanical injuries.
• Thermal pain receptors: activated by extreme temperatures.
• Chemical pain receptors: stimulated by harmful chemicals.
• Polymodal: respond to all types of stimulation.

Pain Threshold & Response

• The pain threshold is similar across individuals, while the reaction to pain varies.
• Pain fibers project to various areas:
  • Reticular formation: responsible for cortical arousal reactions..
  • Limbic system: governs emotional reactions, anxiety, and restlessness.
  • Anterior horn cells of the spinal cord: mediate motor responses, such as the flexion withdrawal reflex.
  • Hypothalamus: governs autonomic reactions.

Pain Responses

• Acute pain: characterized by sympathetic stimulation resulting in increased heart rate, respiratory rate, and blood pressure, along with profuse sweating.
• Saver pain: characterized by parasympathetic activation, leading to hypotension, bradycardia, and gastrointestinal disturbances.
• Hyperalgesia: an exaggerated pain sensation from the skin, where non-painful stimuli become painful in the healthy skin surrounding the injured area.

Hyperalgesia vs. Sensitization

• Hyperalgesia: a subjective heightened pain response.
• Sensitization: an enhanced responsiveness of nerve fibers to stimuli.

Mechanism of Pain Stimulation

• Damaged tissues release chemical mediators, including histamine, bradykinin, potassium ions, and prostaglandins.

Tissue Damage

• Tissue damage creates an "inflammatory soup" involving the release of various substances, including:
  • Substance P: increases capillary permeability and contributes to inflammation.
  • Histamine: activates nociceptor endings.

Receptors on Nociceptive Terminals

• Direct activation: Nociceptive stimuli, such as heat, can directly activate receptors through transduction of the stimulus energy, like with the transient receptor potential (TRP) channel TRPV1.
• Indirect activation: Activation can occur indirectly through TRP channels on keratinocytes, such as TRPV3.
• Intermediate molecule activation: Nociceptors, like mechanoreceptors, can be activated by the release of molecules like ATP, ASIC (acid-sensitive ion channel), P2X (ionotropic purinoceptor), and P2Y (G-protein-coupled purinergic receptor).

Chemical Mediators in Tissue Injury and Inflammation

• Tissue injury and inflammation lead to the release of chemical mediators from various cells, including:

  • Mast cells
  • Macrophages
  • Platelets
  • Immune cells
  • Endothelial cells
  • Schwann cells
  • Keratinocytes
  • Fibroblasts
  • Peripheral nociceptor terminals

• These mediators include:

  • Protons (H+)
  • Purines (adenosine, adenosine triphosphate)
  • Nerve growth factor (NGF)
  • Cytokines, such as tumor necrosis factor (TNF-α) and interleukins (IL-1β, IL-6)
  • Leukemia inhibitory factor (LIF)
  • Prostaglandin E2 (PGE2)
  • Bradykinin
  • Histamine
  • Serotonin (5-HT)
  • Platelet activating factor (PAF)
  • Endothelin

Peptidergic Neurons

• Peptidergic neurons contain peptides like substance P (SP), calcitonin gene-related peptide (CGRP), and somatostatin (SST).
• Sensory neurons with cell bodies in the dorsal root ganglia (DRGs) are responsible for transmitting somatic and visceral pain signals.
• Slowly conducting Ad and C fibers, including nociceptors, have small cell bodies.

Somatosensory Pathways

• Dorsal Column System (A): relays fine touch, pressure, and proprioception information.
  • Receptor: Sensory receptors located in skin, muscles, and joints.
  • First-order neuron: neuron with cell body in dorsal root ganglion, sending axons to the spinal cord.
  • Second-order neuron: neuron located in brainstem (nucleus gracilis for lower body, nucleus cuneatus for upper body), sending axons to the thalamus.
  • Third-order neuron: neuron in thalamus sending axons to somatosensory cortex.
• Anterolateral System (B): transmits pain, temperature, and light touch information.
  • Receptor: Sensory receptors located in skin, muscles, and joints
  • First-order neuron: neuron with cell body in dorsal root ganglion, sending axons to the spinal cord.
  • Second-order neuron: neuron located in spinal cord, sending axons to thalamus, which relays to the somatosensory cortex.

Ascending Sensory Tracts

• Ventral Spinothalamic Tract: carries information related to crude touch, crude pressure, and tickling/itching sensation.
• Lateral Spinothalamic Tract: transmits pain and temperature sensations.
• Dorsal Column: relays information about fine touch, tactile localization, tactile discrimination, stereognosis, texture, fine pressure, vibration sense, and proprioception.

Physiological Classification of Somatic Sensation

• Mechanoreceptive Sensation: includes tactile sensations (crude touch, tickling/itching, fine touch, pressure, vibration) and position senses (static, kinetic).
• Thermal Sensation: includes sensations of warm and cold.
• Pain Sensation: includes cutaneous (skin), deep (muscles, tendons, joints, ligaments), visceral (internal organs), and neuropathic pain.

Neospinothalamic vs Paleospinothalamic Pathways

Types of Pain Sensation

• According to site of pain sensation:

  • Cutaneous: Pain originating from the skin.
  • Deep: Pain arising from muscles, tendons, joints, and ligaments.
  • Visceral: Pain coming from internal organs.
  • Neuropathic: Pain caused by damage to nerves.

• According to quality of pain sensation:

  • Fast (sharp pain, pricking pain, acute pain, electric pain): A rapid and well-defined pain sensation.
  • Slow (slow burning pain, aching pain, throbbing pain, nauseous pain, chronic pain): A prolonged, dull, and often diffuse pain sensation.

• According to type of pain sensation:

  • Somatic pain (musculoskeletal): Originating from the skin, muscles, and soft tissues.
  • Visceral: Originating from internal organs.

1- Fast and Slow Cutaneous Pain

2- Deep Pain

• Deep pain originates from injuries to muscles, tendons, joints, ligaments, and bones, transmitted via C fibers.
• Causes:
  • Ischemic pain: Caused by reduced blood supply to muscles due to vessel wall narrowing, compression, or intravascular thrombosis.
    • Examples:
      • Cardiac muscle ischemia: Anginal pain.
      • Skeletal muscle ischemia: Intermittent claudication (pain in the legs during exercise).

3- Visceral Pain

• Visceral pain originates from internal organs located in the abdomen and pelvis.
• Causes:
  • Overdistention of a hollow viscus: Stretching or expansion of a hollow organ.
  • Spasm of a hollow viscus: Contractions or spasms of a hollow organ, leading to pain due to blood vessel blockage or mechanical stimulation of pain receptors.
  • Chemical irritation: Caused by substances like hydrochloric acid (HCl) in a peptic ulcer.
  • Ischemia: Reduced blood flow to an organ, leading to accumulation of pain-inducing metabolites.
  • Inflammation: Inflammation of visceral tissues, such as appendicitis.

Characters of Visceral Pain

• Diffuse and poorly localized: Due to limited cortical representation and low nerve density in visceral organs.
• Rhythmic cramps (colic): Occur because of rhythmic smooth muscle contractions. Each peristaltic wave triggers a cramp.
• Referred to surface structures: Pain may be felt in areas of the body that correlate with the affected organ.
• Associated with autonomic changes: Hypotension, bradycardia, nausea, vomiting, sweating.
• Guarding phenomenon: Involuntary muscle spasms to protect the affected area.

4- Referred Pain

• Definition: Pain perceived in a surface area that shares the same dermatome as the diseased internal organ.
• Examples:
  • Cardiac pain: Felt retrosternally, in the epigastrium, root of the neck, and inner part of the left arm.
  • Appendicitis pain: Felt around the umbilicus.
  • Gallbladder pain: Felt in the epigastrium and at the tip of the right scapula.
  • Renal pain: Felt in the back, inguinal region, and testicles.

This quiz explores the definitions of pain and the various types of pain receptors. It includes information on pain thresholds, individual pain responses, and the neurological pathways involved in pain perception. Test your understanding of these crucial concepts related to pain and its impact on the body.