Pain

What is the IASP definition of pain?

Unpleasant sensory and emotional experience associated with actual or potential tissue damage

What is nociception?

• Transmission of painful stimuli (distinct from perception)

• Pain occurs in the brain

What characterises acute somatic pain?

• Well localised, protective

• From skin or deep tissue.

What characterises acute visceral pain?

• Poorly localised

• From distension or inflammation

What is chronic pain?

• Long-term, debilitating; altered perception

• E.g. rheumatoid arthritis, phantom limb

What type of stimuli activate A-delta nociceptors?

• Mechanical

• Produce sharp, well-localised pain

• Small myelinated fibres

What stimuli activate C-fiber nociceptors?

• Mechanical, thermal, chemical (polymodal)

• Produce slow, burning, diffuse pain

• Unmyelinated

What lowers nociceptor thresholds?

• Inflammatory mediators

• COX inhibitors counteract

Where do A-delta fibers terminate?

Lamina I and V of dorsal horn

Where do C fibers terminate?

Lamina I and II (substantia gelatinosa)

What neurotransmitters help transmit pain in the dorsal horn?

Glutamate and Substance P

What is the flexion withdrawal reflex?

A-delta activation → excitatory/inhibitory interneurons → withdrawal of limb

What tract carries pain and temperature?

Spinothalamic tract (anterolateral system)

What is the sensory-discriminative component of pain?

• Location, intensity, quality

• Projected to SI and SII cortex

What is the affective-motivational component?

• Emotional/cognitive reactions

• Goes to prefrontal cortex, amygdala, hippocampus

How does acute stress affect pain?

Decreases pain via descending inhibition

How does chronic stress affect pain?

Increases pain via descending facilitation

What endogenous inhibitors exist in spinal cord?

Endogenous opioids, cannabinoids, noradrenaline, serotonin, GABA

What opens the pain gate?

Small unmyelinated C fibers (facilitate spinothalamic activity)

What closes the pain gate?

• Large A-beta fibers

• Activated by touch (rubbing, TENS)

Why does referred pain occur?

Convergence of visceral and somatic afferents onto same dorsal horn neurons