Nociception Flashcards

1
Q

Adelta fibre diameter:

A

1-5um.

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2
Q

Adelta fibre CV:

A

3-30m/s.

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3
Q

C fibre diameter:

A

0.2-1.5um.

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4
Q

C fibre CV:

A

0.5-2m/s.

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5
Q

Abeta fibre diameter:

A

6-12um.

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6
Q

Abeta fibre CV:

A

33-75 m/s.

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7
Q

Membrane of nociceptors contain receptors that …

A

Covert energy of noxious stimuli into depolarising electrical potential.

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8
Q

What ion channel mediates pain producing action of capsaicin, and is activated by temperatures over 45C?

A

TRPV1.

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9
Q

What is the menthol-responsive and cold-sensing channel?

A

TRPM8.

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10
Q

What does congenital pain insensitivity show about the role of other ion channels (v-g sodium channels) in nociception?

A

Caused by loss of function mutation in SCN9A gene encoding NaV1.7 v-g Na+ channel.

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11
Q

Mutations in SCN9A gene can also cause what? Implicated in what condition?

A

Hyperexcitability of nociceptors. Inherited erythromelalgia, ongoing burning pain of extremities.

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12
Q

Role of v-g Na+ channels also highlighted by what pharmacological evidence?

A

Local anaesthetics, lidocaine, blocks v-g Na+ in primary afferent fibre.

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13
Q

What mechanism highlights the role of inflammation in pain sensitivity?

A

TRPv1-mediated membrane currents are enhanced by a reduction in pH, characteristic of the chemical milieu of inflammation.

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14
Q

Describe the spinothalamic tract:

A

Sensory neurons synapse in dorsal root ganglion, second order neurons decussate and ascend in anterolateral white matter to the VPL. Then somatosensory cortex.

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15
Q

Describe the spinoreticular tract:

A

Fibres decussate after DRG and ascend the contralateral cord to reach the brainstem reticular formation, before projecting to the thalamus and hypothalamus.

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16
Q

Describe the spinomesencephallic tract:

A

DRG, decussate, travel upwards in anterolateral column to midbrain, terminating in PAG and superior colliculus.

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17
Q

Role of the spinothalamic tract:

A

Localisation.

18
Q

Role of the spinoreticular tract:

A

Arousal and emotional responses to pain.

19
Q

Role of the spinomesencephalic tract:

A

Modulates pain by connections with the PAG.

20
Q

What evidence is there that the VPL organises input from the spinothalamic tract according to body region, preserving spatial fidelity of signal to somatosensory areas?

A

Neurons in VPL have small RFs matching those of presynaptic spinal neurons .

21
Q

What evidence is there that the thalamus plays a role in pain?

A

Electrical stimulation of thalamus leads to intense pain. And lesions can cause neuropathic pain condition Dejerine-Roussy syndrome. Causes spontaneous burning pain and abnormal sensations (dysesthesias).

22
Q

Gate control theory suggests that pain arises from the …

A

Balance of activity in nociceptive and non-nociceptive afferent fibres.

23
Q

What is the mechanism behind gate control theory?

A

Activation of non-n neurons engages inhibitory interneurons in the dorsal horn, closing a gate for the transmission of afferent signals. Activation of n neurons opens the gate.

24
Q

Evidence for gate control theory?

A

Activation of low threshold afferent fibres can attenuate pain, TENS machine. Electrodes at peripheral locations can activate large diameter fibres innervating areas that overlap and surround area of pain.

25
Q

What empirical observation may gate control theory explain?

A

Reflexive behaviour of shaking the hand after hammer blow or burn. May activate fibres that suppress pain transmission.

26
Q

What shows that the PAG modulates pain?

A

Stim. produces analgesia. Also blocks spinally mediated withdrawal reflexes normally evoked by noxious stimuli.

27
Q

How do PAG neurons modulate pain?

A

Make connections with neurons in rostroventral medulla, including serotonergic neurons in nucleus raphe magnus. Axons project through dorsal region of lateral funiculus to spinal cord, forming inhibitory connections.

28
Q

What could a noadrenergic modulatory system originating in locus coeruleus explain?

A

Stress-induced analgesia.

29
Q

What does morphine do?

A

Binds to brainstem opiate receptors, activates brainstem descending serotonergic pathway to spinal cord.

30
Q

What shows that morphine acts by activating monaminergic systems?

A

Blocked if naloxone injected into PAG or NRM. Also bilateral transection of lateral funiculus blocks. This central analgesic actions involve activation of descending pathways.

31
Q

What does C fibres having more mu receptors than Adelta fibres explain?

A

Why morphine is more effective at treating persistent> acute pain.

32
Q

What is hyperalgesia vs allodynia?

A

Increased sensitivity to pain, perception of a tactile stimulus as painful.

33
Q

Who showed that after prolonged stimulation, ‘silent’ fibres become responsive?

A

Raja, Campbell and Meyer (1984). Burnt areas on palm , mechanical threshold significantly decreased, and in an area larger than burn.

34
Q

Why causes peripheral hyperalgesia ?

A

Activation of nociceptors releases substance P. Acts on mast cells in area which release histamine. Histamine directly excites C fibres.

Damaged tissue also releases prostaglandins which potentiate effects of histamine, lowering activation threshold of nerves for activation by it.

35
Q

What highlights role of histamine in peripheral hyperalgesia?

A

NSAIDs are COX1 and COX2 inhibitors. They reduce prostaglandin synthesis.

36
Q

When can central facilitation happen?

A

More signals to CNS, long term changes in activity at level of SC an brain. “Wind up”.

37
Q

Mechanism behind central facilitation:

A

C fibres fire repeatedly, co-release of Glu and P. P enhances Glu-induced NMDAR activation by increasing response to glutamate. LTP.

38
Q

What highlights role of NMDARs in central hyperalgesia?

A

Chronic pain can be treated with ketamine (Orhurhu et al., 2019).

39
Q

Referred pain is what:

A

When pain spreads from visceral organs to distant site.

40
Q

Referred pain, heart: , liver: .

A

Left arm, shoulder blades.

41
Q

What causes referred pain:

A

Nociceptive axons from viscera enter spinal cord by same route as cutaneous axons. Convergence of information on same nocineurons.

42
Q

Verbal-induced hyperalgesia is associated with what?

A

Increased adrencorticotropic hormone and cortisol plasma concentrations.