Study sheet: Pain Control Systems

📋 Course Outline

1. Central Pain Control Principles
2. Gate Theory of Pain Inhibition
3. Opioid Peptides and Analgesia
4. Supraspinal Descending Analgesia
5. Peripheral Gating and Treatment

📖 1. Central Pain Control Principles

🔑 Key Concepts & Definitions

• Pain control system : inhibits pain transmission and perception in central sensory pathways through ascending and descending impulses

📝 Essential Points

• Pain responses vary between individuals, injuries may be ignored during stressful conditions such as accidents or battles, and irritating the skin over a diseased viscus can relieve pain.

💡 Memory Hook

Ascending pathways transmit pain, whereas descending pathways inhibit it.

📖 2. Gate Theory of Pain Inhibition

🔑 Key Concepts & Definitions

• Gate theory : states that dorsal horn cells, especially substantia gelatinosa region cells, act as a gate controlling pain transmission

📝 Essential Points

• Substance P released from C pain fibers opens the dorsal-horn gate, whereas preventing substance P release produces presynaptic inhibition and decreases or blocks pain sensation.

• The three mechanisms producing presynaptic inhibition are:

  • opioid peptides
  • the supraspinal central descending analgesic system
  • spinal peripheral impulses

💡 Memory Hook

A dorsal-horn gate opens for substance P and closes when presynaptic release is blocked.

📖 3. Opioid Peptides and Analgesia

🔑 Key Concepts & Definitions

• Opioid peptides : morphine-like endogenous ligands that act on receptors for morphine in the central nervous system

★ Must-know

• Morphine is a powerful analgesic found in opium and acts by combining with receptors in the central nervous system.

• The three important opioid peptide types are:

  • enkephalins
  • β-endorphins
  • dynorphin

• β-endorphins are released from the hypothalamus and limbic system as neurotransmitters and from the pituitary as neurohormones, promoting well-being and producing stress analgesia when secreted with pituitary ACTH during stress.

Further detail

• Enkephalins are released in different areas of the central nervous system, including the periaqueductal gray area and spinal substantia gelatinosa region, where they produce analgesic effects.

• Dynorphin is released from neurons in the brain stem and is related to addiction.

💡 Memory Hook

EβD: Enkephalins, β-endorphins, Dynorphin.

📖 4. Supraspinal Descending Analgesia

★ Must-know

• The three components are:

  • periaqueductal gray area
  • raphe magnus nucleus
  • pain-inhibitory interneurons

• β-endorphin stimulates the periaqueductal gray area, which releases enkephalins to the raphe magnus nucleus; descending axons then release serotonin onto dorsal-horn inhibitory interneurons, which release enkephalins and inhibit substance P release from pain fibers at the substantia gelatinosa region.

Further detail

• The periaqueductal gray area lies in the midbrain and upper pons around the aqueduct of Sylvius, the raphe magnus nucleus lies in the upper medulla, and pain-inhibitory interneurons lie in the dorsal horn of the spinal cord.

• Collaterals from the lateral spinothalamic pain pathway stimulate the periaqueductal gray area and link ascending and descending pain pathways.

💡 Memory Hook

Periaqueductal gray → raphe magnus nucleus → dorsal-horn inhibitory interneurons.

📖 5. Peripheral Gating and Treatment

★ Must-know

• 🔄 Peripheral gate inhibition proceeds through: stimulation of large Aβ fibers by tactile receptors, activation of inhibitory interneurons through collaterals, release of GABA or enkephalins, presynaptic inhibition of pain C fibers at the substantia gelatinosa region, closure of the gate

• Medical treatment of pain includes analgesics such as NSAIDs or opiates and treatment of the cause, such as antacids, antispasmodics, or vasodilators for ischemic anginal pain.

• For uncontrollable severe pain, anterolateral cordotomy cuts the spinothalamic tract, while prefrontal lobectomy relieves the emotional reaction to intractable cancer pain.

Further detail

• Examples of peripheral gate inhibition are:

  • rubbing or massage of the skin
  • counter-irritation such as a mustard plaster
  • acupuncture

• Electric stimulation can target the analgesia system or large sensory Aβ fibers.

💡 Memory Hook

Aβ tactile stimulation → inhibitory interneurons → presynaptic inhibition of C fibers → analgesia.

📊 Synthesis Tables

Pain Inhibition Mechanisms

Mechanism	Initial stimulus	Main mediator	Effect
Opioid peptides	Opioid peptide activity	Enkephalins, β-endorphins, or dynorphin	Analgesia
Supraspinal descending system	β-endorphin or pain-pathway collaterals	Enkephalins and serotonin	Presynaptic inhibition in the dorsal horn
Spinal peripheral gate	Nonpainful Aβ tactile stimulation	GABA or enkephalins	C-fiber inhibition and gate closure