Class notes
NEU 7: Somatosensory system - chronic pain
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Lecture notes from Imperial College London, Medical Biosciences BSc, 2nd year, Neuroscience (NEU) module.
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Somatosensory system - Chronic pain- afferent (sensory) division of the nervous system provides vital info about outside world
What are somatosensory modalities?
- sensory neurons: pseudounipolar (peripheral branch + central branch) + in the dorsal root ganglia
pressure, vibration...
- ≠ types of somatosensory modalities (convey info about the body) travel via ≠ pathways to brain
dorsal column pathway (medial lemniscus system)
=> fine touch, pressure, vibration, proprioception
unconscious tract
spinothalamic pathway (anterolateral system)
=> nociception, temperature
- special senses also have their modalities (vision, hearing...) // visceral sensation via interoceptors
Somatosensory receptors
encapsulated within
Mechanoreceptors (touch, vibration, pressure....) non-neural cells
- nerve endings enclosed within capsules (layers of connective tissue)
- ≠ encapsulated receptors:
Merkel cell Meissner corpuscle Ruffini ending Pacinian corpuscle
⑦@°-
light touch (Braille), fine discriminative continuous pressure deep pressure, i
⑨
superficial pressure touch (grip force) touch, stretch vibration, tickling
in epidermis closest to skin elongated capsules deep in dermis
,- mechanoreceptors specialised for proprioception (info on position body): proprioceptors
=> muscle spindles
=> Golgi organs (in tendons)
=> joint receptors
Thermoreceptors (temperature)
- free sensory nerve endings in skin (see previous image)
- specific temperatures activate transient receptor potential (TRP) channels
=> cold-activated: TRPM8, TRPA1 thermal nociceptor & chemoceptor
=> heat-activated: TRPV1, TRPV2, TRPV3, TRPV4
n
noxious heat
=> thermal nociceptors
cold pain innocuous - cool painfully hot (50+)
moderate cold painfully hot (40+)
(menthol) (capsaicin chili)
Nociceptors (nociception)
- free sensory nerve endings in skin, joints & other tissues
- respond to damaging/ potentially damaging (noxious) stimuli
=> thermal (extreme temperature)
=> mechanical (intense pressure)
=> polymodal (thermal, mechanical, chemical: low pH, ATP, immune mediators...)
- specific receptors sensitive to histamine => itch sensation (urge to scratch)
Sensory transduction & stimulus threshold
, - sensory transduction (name of process):
=> receptors have specific cation channels on the nerve endings: open in response to stimuli
=> to activate mechanoreceptors:
- stretch of the membrane
- force applied to connected extracellular proteins
- force applied to intracellular cytoskeletal components
=> TRP thermoreceptors: sodium & calcium channels activated by changes in temp
- the stimulus intensity needs to reach the threshold to activate the voltage gated Na channels
Stimulus intensity
- firing rate of action potentials correlates to the stimulus intensity:
1) stimulus of sufficient duration & amplitude
2) generator potential (= depolarisation)
3) above threshold: induces action potentials
4) transmission & release neurotransmitters
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