Gate Theory & Pain Pathway – Lecture
Notes
Course / Subject: Physiology / Neuroanatomy
Topic: Pain Mechanisms
1. Introduction
● Pain is not just a simple signal from tissue → brain.
● It is modulated at multiple levels: peripheral nerves, spinal cord, brain.
● Understanding pain pathways helps explain clinical observations (e.g., why
rubbing an area can reduce pain).
2. Pain Pathway (Nociceptive Pathway)
A. Nociceptors
● Free nerve endings in skin, joints, periosteum, viscera.
● Two main fiber types:
○ A-delta fibers: thin, myelinated, fast, sharp pain, well-localized.
○ C fibers: unmyelinated, slow, dull/burning pain, poorly localized.
● Respond to mechanical, thermal, chemical stimuli (e.g., bradykinin,
prostaglandins).
B. First-order neurons
● Cell bodies in dorsal root ganglion (DRG).
● Enter spinal cord → synapse in dorsal horn (lamina I, II, V).
● Neurotransmitters: glutamate (fast), substance P (slow), CGRP.
C. Second-order neurons
● Cross to opposite side (anterior white commissure).
● Ascend via spinothalamic tract:
○ Lateral STT: pain & temperature.
○ Anterior STT: crude touch.
● Project to thalamus (VPL).
D. Third-order neurons
● From thalamus → cortex:
○ S1: localization.
○ Insula & anterior cingulate: emotional component.
○ Prefrontal cortex: interpretation & attention.
3. Gate Control Theory (Melzack & Wall, 1965)
● Pain signals are modulated in the dorsal horn before ascending.
● “Gate” can open or close depending on inputs.
A. Key Components
1. Small fibers (A-delta & C) → transmit pain, open gate.
2. Large fibers (A-beta) → touch, pressure, vibration, close gate.
3. Substantia Gelatinosa (Lamina II) → inhibitory interneurons.
4. Transmission cells (T-cells) → send signals up spinothalamic tract.
B. Mechanism
● Pain fibers inhibit inhibitory interneurons → gate opens → pain transmitted.
● Touch fibers activate inhibitory interneurons → gate closes → less pain.
● Descending pathways (PAG, RVM, locus coeruleus) modulate gate from brain →
release endorphins, serotonin, norepinephrine.
4. Clinical Applications
● Rubbing injured area → activates A-beta fibers → reduces pain.
● TENS units → stimulate A-beta fibers → decrease chronic pain.
● Opioids → increase inhibition in dorsal horn → gate partially closed.
● Stress / anxiety → gate can open → pain perception increases.
● Placebo effect → descending modulation via endogenous opioids.
5. Summary / Key Points
● Pain is processed, modulated, not just transmitted.
● Small fibers → open gate; large fibers → close gate.
● Dorsal horn interneurons are central to modulation.
● Descending brain pathways influence pain perception.
● Clinical interventions (rubbing, TENS, drugs) act on this gating mechanism.
