Chapter 12b - Pain Flashcards

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1
Q

What are nociceptors?

A

Nociceptors are free, branching, unmyelinated nerve endings that signal that body tissue is being damaged or is at risk of being damaged.

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2
Q

How do nociceptor pathways relate to mechanoreceptor pathways?

A

The information from nociceptors takes a different path and produces different sensations. Selective activation of nociceptors can lead to conscious experience of pain.

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3
Q

How do nociception and pain relate to each other?

A

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.

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4
Q

What kind of stimuli can activate nociceptors?

A

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.

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5
Q

What kind of mechanisms can activate nociceptors when cells are damaged?

A
  • 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
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6
Q

What kind of mechanisms can activate nociceptors when tissue is burned?

A

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.

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7
Q

What causes the excruciating dull ache associated with very hard exercise?

A

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.

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8
Q

What causes the pain associated with bee stings?

A

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]

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9
Q

What are the 4 types of nociceptors?

A

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

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10
Q

Where are nociceptors located?

A

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.

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11
Q

Where does the transduction of painful stimuli occur?

A

In the free nerve endings of unmyelinated C fibers and lightly myelinated Aδ fibers.

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12
Q

What is hyperalgesia and where does it occur?

A

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.

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13
Q

What is inflammation?

A

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.

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14
Q

What are prostaglandins?

A

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.

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15
Q

What is substance P?

A

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.

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16
Q

What is a CNS cause for hyperalgesia?

A

The cross-talk between the touch and pain pathways in the spinal cord.

17
Q

How do itch and pain relate to each other?

A
  • Both are mediated by thin sensory axons, although they seem to be different axons.
  • Both can be triggered by various types of stimuli, including chemicals and touch.
  • Some pain regulating drugs and compounds can also trigger itch.
  • Some signaling molecules transduce both sensations.
  • They interact: pain can suppress itch.
18
Q

What are first pain and second pain?

A

Aδ and C fibers bring information to CNS at different rates because of conduction velocity differences. Skin nociceptors, therefore, produce two different perceptions of pain: sharp first pain, and duller long lasting second pain.

First pain is caused by the activation of Aδ fibers, second pain is caused by activation of C fibers.

19
Q

What is the zone of Lissauer?

A

A region of the spinal cord where the Aδ and C fibers travel through, before synapsing on cells in the outer part of the dorsal horn in the substantia gelatinosa region.

20
Q

What are the primary afferents of the pain system?

A

Aδ and C fibers.

21
Q

What is referred pain?

A

A phenomenon where a visceral nociceptor activation is perceived as a cutaneous sensation.

Classic example: Angina, where heart fails to receive sufficient oxygen, is localized to the upper chest wall and the left arm.

22
Q

What are some of the main differences between the touch and pain pathways?

A

1) Nerve endings: Touch pathway is characterized by specialized structures in the skin, the pain pathway has only free nerve endings.
2) Diameter of axons: Touch pathway is swift, using fat, myelinated Aβ fibers; pain pathway is slow, using thin, lightly myelinated Aδ fibers and unmyelinated C fibers.
3) Connections in the spinal cord: Branches of the Aβ axons terminate in the deep dorsal horn; Aδ and C fibers branch, run within the zone of Lissauer, terminate within the substantia gelatinosa.

23
Q

What is the spinothalamic pain pathway?

A

The pathway in the body from the spinal cord to the brain, transferring information about pain and temperature.

24
Q

Describe the 6 parts of the spinothalamic pathway for PAIN.

A

1) Small dorsal root axon Aδ, C
2) Dorsal column
3) Lateral spinothalamic tract
4) Medulla
5) Thalamus (intralaminar and VP nuclei)
6) Primary somatosensory cortex S1

25
Q

What are the 7 parts of the dorsal column-medial lemniscal pathway for TOUCH?

A

1) Dorsal root axon Aα, Aβ, Aδ
2) Dorsal column
3) Dorsal colum nuclei
4) Medulla
5) Medial lemniscus
6) Thalamus
7) Cerebral cortex

26
Q

What are the 3 parts of the trigeminal pain pathway?

A

1) Spinal trigeminal nucleus of the brain stem
2) Trigeminal lemniscus
3) Thalamus

27
Q

What is afferent regulation of pain?

A

Pain evoked by activity can be reduced by simultaneous activity in low-threshold mechanoreceptors (Aβ fibers).

28
Q

What is descending regulation of pains?

A

Strong emotion, stress, or stoic determination can powerfully suppress feelings of pain.

29
Q

What is the periventricular and periaqueductal gray matter (PAG)?

A

A zone of neurons that is implicated in pain suppression. Electrical stimulation of the PAG can cause profound analgesia (pain suppression).

30
Q

What are opioids?

A

Opium, its active narcotic ingredients, and their analogues (e.g., morphine, codeine, heroin), and other drugs with similar actions.

31
Q

How do opioids affect pain?

A

They produce profound analgesia when taken systemically. They bind tightly and specifically to several types of opioid receptors in the brain.

32
Q

What are endorphins?

A

Endogenous morphine-like substances. They are relatively small proteins, or peptides.

33
Q

How do endorphins affect pain?

A

Extensive systems of endorphin-containing neurons in the spinal cord and brai nstem prevent the passage of nociceptive signals through the dorsal horn and into higher levels of the brain where the perception of pain is generated.