Lecture 2
PS 1003
Somatosensory perception:
Touch, pain and analgesia
Somatosensory perception
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Sensory receptors
Sensory receptors
Peripheral
Cranial
Spinal cord
Brainstem
Thalamus
Somatosensory cortex
Other cortical areas
Cingulate cortex
Peripheral sensory receptors
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Free nerve endings
(pain, temperature)
Meissner’s corpuscle (touch)
Pacinian corpuscle (vibration)
Ruffini’s ending (stretch)
Hair follicle receptor
Spinal cord
Merkel’s disc (touch)
Converts mechanical
energy to electrical
energy
Labelled
lines
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Nociception
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Nociception –the perception of a noxious stimulus
Pain – the subjective ‘feeling’ due to a noxious stimulus
Analgesia – the modulation of nociception or pain
Transduction of a noxious stimulus into electrical activity in
appropriate nerve endings
Transmission of the electrical signal through nerves, to the
spinal cord and the brain
Perception of the noxious stimulus in brain areas, and the
conscious ‘feeling’ of pain
Modulation of signal at various levels (analgesia)
Levels of nociception
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Transduction
• Noxious stimuli activate peripheral nociceptors
• Mostly polymodal, responding to mechanical, thermal
and chemical stimuli
• Some unimodal
• Responses mediated through histamine, prostoglandins,
bradykinin etc.
Peripheral transmission
• Peripheral nociceptive fibres enter spinal cord through the dorsal
root and terminate in the dorsal horn
• Synapse in the dorsal horn with both ascending axons and spinal
interneurones
• Neurotransmitters at the spinal cord: glutamate and substance P
Central transmission
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• Ascending axons cross midline and ascend
through anteroelateral column of the spinal cord
• Terminate in the ventral posterior nucleus of the
thalamus
• Collateral terminations in the brain stem
• Projections from brainstem to thalamus
Figures from :
Carlson: Physiology of Behaviour
• Thalamus projects to somatosensory cortex
• Also to cingulate cortex
• Reciprocal connections between
somatosensory and cingulate cortices.
‘Pain’ perception
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• Perception of pain is probably in the cingulate cortex
• Cingulate cortex is activated in people during illusory pain
• Pain is a subjective experience based on the information
received from nociceptive fibres
• The brain can be fooled into thinking something is painful,
even though no tissue damage has occurred
• Mechanism of pain perception is very poorly understood
Opium and opioid drugs
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Opium is one of the earliest known drugs used by man
Comprises the dried sap of the opium poppy
• Sedative and analgesic
• Well known to ancient Greeks
Two main active components of opium
• Morphine (~10%)
• Codeine (~0.5%)
• Morphine is poorly absorbed orally.
• Hence, to produce central effects, it is injected.
• Other opioid drugs.
• Heroin, methadone.
Morphine
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Morphine acts through binding sites in the brain and spinal cord
• What are the endogenous receptors which morphine acts on?
At first believed to act as an antagonist at one or more stages of the pain
pathway
• But effects of morphine are blocked by naloxone
• Suggests that morphine is an agonist not an antagonist
Morphine is not endogenous
• Endogenous opioids were isolated in 1970s
• Enkephalins, endorphins, dynorphins
Opioid Analgesia
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Morphine injected into lateral ventricles relieves severe pain
• effective at doses 10-fold lower that for
systemic injection
• therefore acting in the brain
Intracerebral injection of morphine induces analgesia
• periaqueductal grey matter (PAG)
• periventricular grey matter (PVG)
• rostroventral medulla (RVM)
These effects are blocked by naloxone
Naloxone injected into PAG, PVG or RVM partially
reverses analgesic action of systemically
administered morphine
Stimulation Produced Analgesia
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Electrical stimulation of localised brain areas
suppress pain perception
• periaqueductal grey matter (PAG)
• periventricular grey matter (PVG)
• rostroventral medulla (RVM)
• Analgesia is an active process
How does stimulation produced analgesia relate to opioid analgesia?
Similarities between opiate analgesia and
stimulation produced analgesia
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• Effective loci are the same in each case
(i.e. PAG, PVG, RVM)
• Both are blocked by naloxone
• Combining sub-analgesic levels of both produces analgesia
• Cross-tolerance develops between the two.
• Both effects cause blockade of spinal reflexes
• therefore mediated at the level of the spinal cord
• Both effects activate the same descending spinal pathway
• dorsolateral funiculus
Therefore stimulus produced analgesia is mediated through
opioid mechanisms
Levels of opiate analgesia
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Supraspinal
• opioid receptor activation in brain stem
• mediated via spinal cord mechanisms
• mu-receptor mediated (i.e. enkephalins)
Spinal
• opoid receptors activation in spinal cord
• delta- & kappa- receptor mediated (enkephalins & dynorphins)
Hormonal
• stress-induced analgesia is reversed by naloxone
• also reversed by removal of adrenal glands
• mechanism is unclear
Non-opiate analgesia
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Brain stem
• Noradrenaline and 5HT modulate analgesia
• Especially in PAG and PVG
• Mechanism not clearly understood
Spinal cord
• Noradrenaline injected into spinal cord blocks responses to
noxious stimuli
• 5HT injected into spinal cord is analgesic
• blocks spinal cord nociceptive neurones
• blocks spino-thalamic neurones
Control of analgesia
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(+) (+) (+)
Central grey matter
(+)
(+)
Rostroventral
medulla
Raphe
nuclei
(+)
5HT
Nociceptive neurone
Enk (-)
Morphine/SPA
Na
(-) (-)
(+)
Ascending
nociceptive
pathway
Alternative methods of analgesia
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Transcutaneous electrical nerve stimulation (TENS):
• Alters nociceptive signal to brain, or brain’s perception of pain
• Mechanism unclear but may activate endogenous opiate systems
Acupuncture
• Greater than 80% increase in pain threshold
• Increased enkephalin levels in the brain
• Increased biosynthesis of enkephalins
• Effects were enhanced by enkephalinase inhibitors
• May be mediated via enkephalin release in PAG and PVG
Placebo : may activate endogenous pain-control systems
Hypnosis : alters brains perception of pain
Stress : both opiate and non-opiate mechanisms
Cognitive : may activate endogenous pain-control systems
Summary
PS 1003
• Sensory perception occurs at peripheral modality-specific receptors
• Information transmitted to the brain through cranial or spinal nerves
• Makes connections in brainstem and thalamus
• Information enters cortex at unimodal primary sensory cortices
• Hierarchical processing occurs in unimodal association cortices
• Highly processed unimodal signals enter multimodal association
cortex, where sensory integration occurs