Pain- general

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Property

C-fibre

A(delta)-fibre

A(beta)

fibre

Neuronal

size

Small-

0.4±1.2 mm

Medium-

2±6 mm

Large->10

mm

Conduction

velocity

Slow-

unmyelinated - 2 m/s

Fast-

myelinated- 20 m/s[C1]

Fast-

myelinated- 30±100 m/sec

Adaption

rate

Threshold

for activation

High

High

High

Termination

in spinal cord laminae

Laminae

I & II, and then projects to V (polysynaptic)

Laminae

I & V (monosynaptic)

Laminae

I & II & V >> not very well studied

Sensation

Poorly localised[C2] - Burning pain

Well

localised[C3] - Sharp pain

Sharp

pain

modality

thermal,

mechanical, and chemical stimuli- polymodal

dangerously

intense mechanical or to mechanothermal stimuli

Receptive

field

Relatively

large- presumably because the detection

of pain is more important than its precise localization

Relatively

large- clusters of sensitive spots

[C1]That a proportion of

A-fibre nociceptive neurones have fibres that conduct in the Aa/b

range has been confirmed with

intracellular recordings in DRG neurones in vivo in

several species.

[C2]effective in causing

reflex changes such as cardiovascular or respiratory changes

[C3]important property of an exteroceptive

pathway, enabling precise and rapid withdrawal responses from a specific

localised noxious stimulus

Property

C-fibre

(nociceptive)

C-fibre (mechanoreceptor)

Stimulus

High

intensity mechanical stimuli

low

intensity mechanical stimuli moving very slowly

across

the skin

Neurotransmitter

release

SP,

CGRP

Low

levels of CGRP

A-Fibre (nociceptive)

A-fibre (non-nociceptive)

Action

potentials

longer

somatic AP and AHP durations than A-fibre LTM neurones

The AP

overshoot is greater than that for LTM units

Conduction

velocity

Slower

conduction velocity

Ad-fibre

nociceptive

Aa/b-nociceptive[C1]

Thresholds

High thresholds

Lower thresholds

[C1]Why is it important to

distinguish between Ad- and Aa/bfibre

nociceptive neurones?

One reason is the need to distinguish between the contributions of Aa/b-fibre

LTM

units and Aa/b-fibre

nociceptive units to the development of hypersensitivity (hyperalgesia and

allodynia) after

peripheral nerve injury.

Stimulus type

DRG neurons

Molecule

Mechanical

Chemical

Thermal (degrees celcius)

Size (S/M/L)

Afferent type

TRPV1

Low pH,

capsaicin

>42

S

C

TRPV2

>52

SML

A

TRPV4/3

Noxious- mech/

stretch

>32

SM

NF-rich

A

TRPM8[C1]

Methanol

<22

S

C-cooling

TRPA1[C2] / ANK TM1

Nox mech

<17

S

C-cold (visceral)

ASIC / ASIC1

Low pH

C

DRASIC / ASIC3

Mech?

Low pH

Heat

P2X3

Mech?

ATP

S

Noxious

[C1]Responsive

to changes in temp

>> not noxious cold receptor

[C2]Some

controversy- is it noxious cold or not?? See evidence

Adequate

Stimulus

Receptor

Adaptn

Afferent

Innn

CUTANEOUS

RECEPTORS

Mechanoreceptors

mechanical

Meissner

Pacinian

Hair Cells

Ruffini

Merkel

RA

RA

RA

RA

SA

Aβ

Aβ

Aβ

Aβ

Aβ

Thermoreceptors (LT)

heat & Cool

Free endings

variable

Aδ & C

Nociceptors:

HTMs

Thermal

Polymodal

mechanical

heat or Cold

chem/heat/mech

Free endings

Free endings

Free endings

SA/RA

slow

slow

Aδ most

Aδ & C

C, few Aδ

DEEP

RECEPTORS

Muscle: spindles

nociceptors

mechanical

polymodal/mech

Spindle

Free endings

SA/RA

SA/RA

Ia, Ib, II

Aδ & C

Viscera: pressure

nociceptors

mechanical

polymodal/mech

Free endings

SA/RA

Aβ

Aδ & C

CLASS

DESCRIPTION

AFF

INPUT

LAMINA

1

Low

Threshold

Mechanoreceptive

Aβ

III,

IV

V,VI

2

Wide Dynamic

Range

Polymodal

Multireceptive

Convergent

Lam V Type[C1]

Aβ,

Aδ & C

Aβ

& Aδ

I

IV,

V, X

VII,

VIII

3[C2]

High

Threshold

Nociceptive

Specific (NS)

Nocireceptive

Aδ

Aδ

& C

C

I

V,X,

VII, VIII

[C1]Different

names

[C2]Class

2 and 3 are involved in nociception

C vs A-fibres

nociceptive vs non-nocicpetive

TRP receptors

cutenous and deep receptors summary

classification of Dorsal horn neurons