Chapter 12b - Pain Flashcards
What are nociceptors?
Nociceptors are free, branching, unmyelinated nerve endings that signal that body tissue is being damaged or is at risk of being damaged.
How do nociceptor pathways relate to mechanoreceptor pathways?
The information from nociceptors takes a different path and produces different sensations. Selective activation of nociceptors can lead to conscious experience of pain.
How do nociception and pain relate to each other?
Nociception: Sensory process that provides signals that trigger pain.
Pain: The feeling or perception of irritating, sore, stinging, aching, throbbing, miserable, or unbearable sensations arising from a part of the body.
What kind of stimuli can activate nociceptors?
E.g., strong mechanical stimulation, extremes in temperature, oxygen deprivation, and exposure to certain chemicals.
The membranes of nociceptors contain ion channels that are activated by these types of stimuli.
What kind of mechanisms can activate nociceptors when cells are damaged?
- Damaged cells can release substances, such as proteases, ATP and K+, that cause ion channels on nociceptor membranes to open
- Proteases can break down an abundant extracellular peptide, kininogen, to form bradykinin, that binds to specific receptor molecules that actrivate ionic conductances in some nociceptors
- ATP causes nociceptors to depolarize by binding directly to ATP-gated ion channels
Elevation of extracellular [K+] directly depolarizes neuronal membranes
What kind of mechanisms can activate nociceptors when tissue is burned?
Heat-sensitive ion channels in nociceptors open at a temperatore of 43ºC, which causes pain.
The sensation of warmth (37ºC - 43ºC) and scalding (43ºC+) are mediated by separate neural mechanisms.
What causes the excruciating dull ache associated with very hard exercise?
1) When tissue oxygen levels do not meet oxygen demand, anaerobic metabolism is used to generate ATP.
2) As consequence of anaerobic metabolism, lactic acid is released.
3) Buildup of lactic acid leads to an excess of H+ in the extracellular fluid.
4) The H+ ions activate H+-gated ion channels on nociceptors.
What causes the pain associated with bee stings?
1) The skin and connective tissue contain mast cells, a component of the immune system.
2) Mast cells can be activated by exposure to foreign substances (e.g., bee venom)
3) The mast cells release histamine
4) Histamine binds to specific cell surface receptors on nociceptors and causes membrane depolarization
5) Histamine also causes blood capillaries to become leaky, which leads to swelling and redness at the site of injury
[6) Antihistamines can reduce the pain and the swelling]
What are the 4 types of nociceptors?
1) Polymodal nociceptors: Majority of nociceptors. These respond to mechanical, thermal, and chemical stimuli.
2) Mechanical nociceptors: Show selective responses to strong pressure
3) Thermal nociceptors: Show selective responses to burning heat or extreme cold
4) Chemical nociceptors: Show selective responses to histamine and other chemicals
Where are nociceptors located?
They are present in most body tissues, including skin, bone, muscle, most internal organs, blood vessels, and heart.
Notably absent in the brain itself, except for the meninges.
Where does the transduction of painful stimuli occur?
In the free nerve endings of unmyelinated C fibers and lightly myelinated Aδ fibers.
What is hyperalgesia and where does it occur?
A reduced threshold for pain, an increased intensity of painful stimuli, or even spontaneous pain.
Primary hyperalgesia occurs within the area of damaged tissue, secondary hyperalgesia in the tissues surrounding a damaged area.
What is inflammation?
When skin is damaged, a variety of substances (inflammatory soup) are released. This includes certain neurotransmitters, peptides, lipids, proteases, neurotrophins, cytokines, and chemokines, and other substances.
Together, they can trigger inflammation, which is a natural response of the body’s tissues as they attempt to eliminate injury and stimulate the healing process.
What are prostaglandins?
Chemicals generated by the enzymatic breakdown of lipid membrane. They greatly increase the sensitivity of nociceptors to other stimuli, even though they do not elicit overt pain.
What is substance P?
A peptide synthesized by the nociceptors themselves. Activation of one branch of a nociceptor axon can lead to the secretion of substance P by the other branches of that axon in the neighboring skin.
Substance P causes vasodilation (swelling of blood capillaries) and the release of histamine from mast cells.
One cause of secondary hyperalgesia.