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Pain fibres

Created: 1/6/2004
 

Cutaneous sensation is mediated by specific sensory receptors that are located in the skin. These can be broadly classified into low and high threshold primary afferents. Low threshold afferents are myelinated fibres with specialised nerve endings that convey innocuous sensations such as light touch, vibration, pressure (all Ab) and proprioception (Aa). High threshold afferents are thinly myelinated (Ad) or unmyelinated (C) fibres located in the dermis and epidermis, which convey pain and temperature.

Table 1: Comparative properties of primary afferent fibres

Fibre class
Threshold
Main transmitters
Main receptor activated
Laminar location
Target spinal cord neurones
Normal sensation
Pathological sensation
C
High
Peptides
NK1,2
I-II, V
NS
Slow pain
Hyperalgesia
Ad
EAA
NMDA
AMPA
mGlu
WDR
Fast pain
Allodynia
Ab
Low
EAA
AMPA
III-VI
LT
WDR
Touch
vibration
pressure
Mechanical allodynia

Key: EAA = Excitatory amino acids; NS = Nociceptive specific; LT = Low threshold; WDR = wide dynamic range; NK = neurokinin (peptide) receptor; NMDA, AMPA, mGlu are different types of glutamate receptors

Pain and temperature afferents do not have any specialised receptors; they use “free nerve endings”. They are polymodal, i.e. they respond to more than one kind of stimulus, e.g. chemical, thermal or mechanical stimuli. Free nerve endings are found in all parts of the body except the interior of the bones and the interior of the brain itself. In the cornea of the eye, only free nerve endings are found and abrasions of the cornea can be extremely painful. Most of these respond only to tissue damaging stimuli and are called nociceptors. Pain sensations can be broadly divided up into bright, sharp, stabbing types of pain, and dull, throbbing, aching types. Ad fibres mediate the former or ‘fast’ pain, C-fibres signal the latter or ‘slow pain’. Not all Ad and C fibres are nociceptors. Some respond to low threshold stimuli such as touching or brushing the skin. Many C fibres are thermoreceptors, and respond to warm or cold.

Although pain results from damage to these free nerve endings, in reality the pain is a result of substances released by damaged tissues: prostaglandins, histamine and peptides. These activate receptors located on the free nerve endings.


ArticleDate:20040601
SiteSection: Article
 
   
    
                                            
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