120. Physiology of Pain, Itch and Temperature – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue injury. It is a protective sensation that initiates withdrawal reflexes and appropriate behavioral responses. Pain perception is influenced by sensory, emotional, cognitive, and psychological factors.
• Types of pain:
  • Fast pain: Sharp, well localized, transmitted by Aδ fibers.
  • Slow pain: Dull, burning, aching, poorly localized, transmitted by C fibers.
  • Other classifications include superficial/deep, somatic/visceral, peripheral/central, and acute/chronic pain. Neuropathic and inflammatory pain are important pathological types.
• Nociceptors are free nerve endings that detect harmful mechanical, thermal, and chemical stimuli. Aδ mechanical nociceptors transmit fast pain, whereas polymodal C-fiber nociceptors respond to mechanical, thermal, and chemical stimuli and transmit slow pain. TRPV1 is activated by temperatures above 43°C, tissue acidity, and capsaicin.
• Pain fibers enter the spinal cord through the dorsal roots. Aδ fibers terminate mainly in laminae I and V, while C fibers terminate in lamina II (substantia gelatinosa). Second-order neurons cross through the anterior white commissure and ascend in the lateral spinothalamic tract.
• Two major ascending pain pathways are:
  • Neospinothalamic pathway: Carries fast pain through Aδ fibers to the ventral posterolateral (VPL) nucleus of the thalamus and then to the primary somatosensory cortex for accurate localization.
  • Paleospinothalamic pathway: Carries slow pain through C fibers to the reticular formation, hypothalamus, limbic system, and thalamus, producing emotional, autonomic, and behavioral responses.
• Deep pain is dull, poorly localized, transmitted mainly by C fibers, and commonly results from ischemia, inflammation, or muscle spasm. Visceral pain is diffuse, poorly localized, commonly caused by distension, ischemia, inflammation, or smooth muscle spasm, and is frequently associated with autonomic responses and referred pain.
• Referred pain is pain perceived at a site distant from its origin. The convergence theory is the most widely accepted explanation. Other theories include dermatomal, facilitation, and experience theories. Classic examples include pain of myocardial infarction radiating to the left arm and cholecystitis radiating to the right shoulder.
• Tissue injury releases prostaglandins, bradykinin, histamine, serotonin, leukotrienes, potassium ions, and substance P, which sensitize nociceptors and produce peripheral sensitization. Pain perception is modified by inflammation, attention, emotions, placebo effect, local anesthetics, and centrally acting analgesics.
• Endogenous pain control includes:
  • Descending analgesic pathway: Periaqueductal gray → nucleus raphe magnus → dorsal horn, where serotonin activates interneurons releasing enkephalins, which inhibit substance P release.
  • Endogenous opioid system: Enkephalins, β-endorphin, and dynorphins act on μ, δ, and κ receptors to suppress pain transmission. Naloxone blocks opioid receptors.
• Gate control theory states that stimulation of large Aβ fibers activates inhibitory interneurons in the dorsal horn, reducing pain transmission. This explains pain relief by rubbing the skin, transcutaneous electrical nerve stimulation, and acupuncture.
• Important pain syndromes include hyperalgesia, hypoalgesia, allodynia, causalgia (complex regional pain syndrome type II), inflammatory pain, neuropathic pain, trigeminal neuralgia (tic douloureux), and thalamic syndrome.
• Itch is a distinct sensation producing an urge to scratch. It is mediated by specialized C fibers, mainly stimulated by histamine, and ascends through the lateral spinothalamic tract.
• Temperature receptors are free nerve endings. Cold receptors respond best between 10–40°C, warm receptors between 30–50°C. Cold is transmitted mainly by Aδ and C fibers, warmth mainly by C fibers, through the lateral spinothalamic tract to the thalamus and sensory cortex.