Lecture 25 - Pleasure and Pain I Flashcards
Aspects of the pain experience
1)
2)
3)
1) Pain is always subjective
2) It is an experience
3) Relationship between pain and tissue damage is variable
Way that touch and joint sense are projected to the brain
1)
2)
1) Receptors in skin project to dorsal column nuclei.
2) Dorsal column nuclei project to the thalamus via the Medial lemniscus
Way that nociceptive signals project to the brain
1)
2)
1) Receptors in the skin project to neurons in the dorsal horn
2) Receptors in the dorsal horn project to thalamus, via the spinothalamic tract
Example of nociceptors that project to the dorsal column
GIT nociceptors
Cutaneous mechanoreceptors 1) 2) 3) 4) 5)
1) Hair follicles
2) Meissner corpuscle
3) Pacinian corpuscle
4) Merkel cell-neurite complex
5) Ruffini corpuscle
Skin stimulus detected by Meissner corpuscle
Dynamic deformation. Objects slipping over skin.
Skin stimulus detected by Pacinian corpuscle
Vibration
Skin stimulus detected by Merkel cell-neurite complex
Indentation depth. For fine tactile discrimination.
Skin stimulus detected by Ruffini corpuscle
Stretch.
Two types of C-fibre mechanoreceptors
1) LTM, responsible for pleasant contact.
2) Polymodal nociceptor (with Adelta fibres)
Example of a transducer in polymodal nociceptors
TRPV1
TRPV1
A cation channel (Ca2+, Na+) in Adelta and C-fibres (nociceptors)
Four functional types of nociceptors
1) Thermal (Adelta fibre)
2) Mechanical (Adelta fibre)
3) Polymodal (C-fibre)
4) Silent (C-fibre)
Things that polymodal C-type fibres can detect 1) 2) 3) 4)
1) Noxious heat
2) Noxious cold
3) Peptidergic receptors (noxious peptides)
4) Nonpeptidergic
Ion channel types in nociceptive neurons 1) 2) 3) 4) 5)
1) Transient Receptor Potential channels (EG: TRPV1)
2) Voltage-gated Na+ channels
3) Hyperpolarisation-activated cation channels
4) K+ channels
5) Voltage-gated Ca2+ channels
Functional difference between Adelta and C fibres
Adelta are larger, myelinated, faster conducting.
C-fibres are smaller, unmyelinated, slower conducting
Silent nociceptors
C-fibres.
Not active in normal tissues. Activated when a tissue is damaged.
Nociceptors in hairy skin
Adelta, C
Nociceptors in glabrous skin
Only C-fibres
Pain that Adelta are responsible for
Sharp pain
Pain that C-fibres are responsible for
Slow, burning pain
Differential projection of Adelta and C-fibres into the spinothalamic tract
1)
2)
1) C-fibres project to superficial layers (layers I and II)
2) Adelta project to deeper layers (IV, V)
Proportion of neurons in the dorsal horn that project to the brain
5-10%
Evidence that nociceptor activation doesn’t have to result in emotional response to pain
Reflex pathways of the dorsal horn
What can lead to referred pain?
Two nociceptive inputs converging on the one spinal circuit
Primary hyperalgesia
Tissue is damaged, leading to release of factors (EG: prostaglandin, bradykinin from mast cells).
This leads to nociceptors being easier to depolarise.
Allodynia
Sensation of pain from light touch
Difference between primary and secondary hyperalgesia
Primary occurs in the tissues that were damaged.
Secondary occurs in undamaged tissues surrounding damaged tissues
Area responsible for sensory aspect of pain
Projections to primary somatosensory cortex
What correlates with phantom limb pain?
Functional remodelling in brain of somatosensory cortex (detected with fMRI)
Non-somatosensory parts of brain that are activated in nociception
Prefrontal cortex, anterior cortex, singulate cortex.
Example of pain not correlating with tissue damage
Hot/cold grill results in sensation of burning heat
Affective-motivational pain pathway
1) Anterolateral system
2) Middle thalamic nuclei
3) Insular cortex and anterior singulate gyrus
Sensory-discriminative pain pathway
1)
2)
3)
1) Anterolateral system
2) Ventral posterior nucleus
3) Somatosensory cortex (S1, S2)
Outcome of the complex network of centres stimulated by nociception in the brain.
Experience of pain is modulated by emotional factors
Positive emotion reduces unpleasantness of pain
Lack of attention to pain reduces intensity of pain
