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Nocioception

Pain and Nocioception

A.Pain is the perception of aversive or unpleasant (painful)sensations. It is a conscious experience, not a sensory

experience.

B.Nocioception is the sensory process necessary forthe perception of pain. This sensory process is dependentupon the activation of specific sensory receptors,

nocioceptors.

C. Nocioceptors are activated by a wide variety of stimuli,mechanical, chemical, and thermal.

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A. Nocioceptors are relatively undifferentiated as compared to other types ofsomatosensory neurons (Merkel cells, Pacinian corpuscles etc).

B. They fall into two broad categories:A. Specialized thermal or mechanical nocioceptors: small diameter, thin myelin

sheaths, conduct at 5-30 m/s A fibers (A-delta) (sharp, highly localized)

B. Polymodal nocioceptors: activated by a wide spectrum of stimuli, smalldiameter, unmyelinated, slow conducting (C) (dull, poorly localized, persists)

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1. Capsaicin2.Allyl isothyiocyanate3.Heat >43C

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A. Nocioceptors project toseveral layers of the dorsalhorn.

A.Inhibitory interneuronsB.Excitatory interneuronsC.Projection neurons

B. Layer I projection neuronsA.Nocioceptive specificB.Wide dynamic range

C. Layer II: interneuronsD. Layer IV-VI

A.Skin and visceraB.Wide dynamic rangeC.Referred pain

E. Wind-up (central pain)

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Figure 10.6 Discriminative pain pathways (anterolateral pathway)

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Figure 10.6 Discriminative pain pathways

Figure 10.6 Dorsal column pathway for visceral pain

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Brown-Squard Syndrome

Referred Pain or where is the pain really coming from?

A. Nocioceptors from viscera converge onto neurons that receive inputfrom cutaneous nocioceptors

B. Heart attacks

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Hyperalgesia

A.Tissue damage sensitizesnocioceptors, calledhyperalgesia.

B.Two types:A.Primary occurs in damaged

tissue

B.Secondary occurs in surroundingtissue

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Hyperalgesia: mechanisms

A. Substances released as aresult of injury from avariety of sources act eitherdirectly or indirectly tostimulate nocioceptors.

B. Decrease threshold foractivation or directlyactivate nocioceptors.

C. Wind-upA.Persistent firing of C

fibers increasesresponsiveness ofdorsal horn neurons

B.NMDA receptors,glutamate, LTP, centralsensitization

Gate control hypothesis

A.Activation ofmechanoreceptors in theskin eases pain.

B. Convergence of non-nocioceptors andnocioceptors ontocommon projectionneurons.

A.Inhibitory interneuronschronically active

B.Suppresses activity inprojection neuron

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Descending Pathways

1. Periaquaductal Gray1. Descending control of

pain

2. Stimulation inducesanalgesia

3. Projections to raphenucleus (and nucleus

paragigantocellularis)

2. The nuclei project to thedorsal horn where they

synapse on interneurons

and projection neurons in

the anterolateral system

Descending control of analgesia

1. Descending projections1. Norepinephrine2. Serotonin

2. Local inhibitory interneuronsin the dorsal horn

1. Enkephalins2. Pre- and post-synaptic

effects

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