Nociception, Pain, and Analgesia Flashcards
How can pain be classified?
Acute and chronic.
What is acute pain?
Pain that lasts less than 3-6 months. Can be caused by tissue injury, trauma, and childbirth.
What is chronic pain?
Pain that lasts longer than 3-6 months, outlasting tissue injury.
What is hyperalgesia?
Enhanced pain response to noxious stimuli, due to sensitization of peripheral nociceptors.
What is allodynia?
Pain caused by innocuous stimuli.
What are the pain classifications based on cause?
Nociceptive pain, inflammatory pain, neuropathic pain.
What is nociceptive pain?
Direct activation of pain fibres by noxious stimuli.
What is inflammatory pain?
Pain caused by inflammatory mediators.
What is neuropathic pain?
Pain to the somatosensory nervous system.
What are nociceptors?
Nociceptors work like pain signal generators that convert damaging stimuli into nociceptive signals.
Describe TRPV1 nociceptors.
Heat activated ion channels, also activated by capsaicin, H+ ions (pH <5.5). Permeable to Na+ and Ca2+.
Describe TRPM8 nociceptors.
Cold (<26°C) activated ion channel also activated by menthol.
Describe TRPA1 nociceptors.
Detects noxious cold (colder than above) and mechanical stimuli, oxidative stress, lipopolysaccharide (LPS), irritating chemicals (e.g. AITC from wasabi, mustard oil and cinnamon, allicin from garlic), Δ-9-tetrahydrocannabinol (marijuana), Methylglyoxal (DN), bradykinin etc.
Describe TRPV4 nociceptors.
Senses hypo-osmolarity and arachidonic acid metabolites.
Describe ASICs (acid sensing ion channels).
Activated by acid (pH <6.4).
Describe P2X3 nociceptors.
ATP-gated ion channels; detects ATP released during tissue damage.
Name some mechanically activated ion channels implicated in nociception.
ENac, PIEZO.
What neurotransmitters are involved in nociception?
Glutamate, tachykinins.
What neurotransmitters are involved in analgesia?
Opioid peptides, 5-HT, noradrenaline, GABA, adenosine.
Describe glutamate’s role in nociception.
Released from primary afferents, Fast transmission (AMPA subtype of receptors), Slower, NMDA (component is important in wind up).
What are inflammatory mediators?
These are mediators released from damaged cells and trigger the nociceptive pathway. There is an element of positive feedback (Substance P, Mast Cells, Histamine) which increases the pain response.
Describe bradykinin’s role in nociception.
Generated in damaged tissue, couples to the B1 and B2 receptors, sensitizes TRPV1, TRPA1, and TRPV4 via activating PKC.
Describe the role of prostaglandins E and F in nocicaption.
Increase the excitability of afferents by acting on Na+ and K+ ion channels, and sensitized TRPV1 and TRPV4 via activating PKA.
Describe the role of NGF (nerve growth factor) in nociception.
Sensitizes TRPV1, Na+, and K+ channels, increasing the expression of these ion channels. Released from damaged and inflamed tissues, and cells.
What metabolites, involved in nociception, are released from damage cells?
Acid, ATP, 5-HT, histamine.
What neuropeptides are involved in nociception?
Substance P and calcitonin gene-related peptide (CGRP) released from afferent nerve fibres. Bidirectional signalling.
What are the two types of afferent nerve fibres used in nociception?
A-delta and C fibres.
Describe the characteristics of A-delta afferent nerve fibres used in nociception.
Myelinated, high conduction velocity, cause a sharp, localised, and fast pain.