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Lecture 2

Somatosensory perception:Touch, pain and analgesia

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Somatosensory perception

Sensory receptors

Sensory receptors

Spinal cord

Brainstem

Thalamus

Somatosensory cortex

Other cortical areas

Cingulate cortex

Peripheral

Cranial

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Peripheral sensory receptors

Free nerve endings (pain, temperature)

Merkel’s disc (touch)

Meissner’s corpuscle (touch)

Pacinian corpuscle (vibration)

Ruffini’s ending (stretch)

Hair follicle receptor

Converts mechanical energy to electrical energy S

pina

l cor

d

Labelledlines

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Nociception

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)

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Levels of nociception

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

Transduction• Noxious stimuli activate peripheral nociceptors

• Mostly polymodal, responding to mechanical, thermal and chemical stimuli

• Some unimodal• Responses mediated through histamine, prostoglandins,

bradykinin etc.

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Central transmission

• 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.

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‘Pain’ perception

• 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

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Opium and opioid drugs

Opium is one of the earliest known drugs used by manComprises 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.

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Morphine

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

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Opioid Analgesia

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

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Stimulation Produced Analgesia

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?

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Similarities between opiate analgesia and stimulation produced analgesia

• 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

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Levels of opiate analgesia

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

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Non-opiate analgesia

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

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Control of analgesia

Central grey matter

Raphenuclei

Rostroventralmedulla

Nociceptive neurone

Ascending nociceptive

pathway

Na(-)(-)

5HT

(-)Enk

(+)

(+) (+) (+)

(+) (+)

Morphine/SPA

(+)

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Alternative methods of analgesia

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

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Summary

• 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