Sensory pathways of pain and acute versus chronic pain (Leo, 2007)

Leo, R. J. (2007). Sensory pathways of pain and acute versus chronic pain. In Clinical manual of pain management in psychiatry (pp. 11-33). Washington DC: American Psychiatric Publishing, Inc

It has been suggested that pain is a multidimensional concept, incorporating the biological, psychological and social dimensions.

Pain-relaying pathways and mechanisms

Conventionally, the pain relay pathway is considered to involve three sets of neurons:

First order neurons: noxious information is transmitted from the peripheral to the spinal cord particularly ending in dorsal horns.

Second order neurons: Information in the spinal cord is sent to thalamus. At this level, fibers go all the way up to the brain stem and ending in the countralateral thalamus while a small number of them go into the ipsilateral thalamus. Within the spinothalamic system, the pathways diverge into two pain pathways: the paleospinothalamic (affective-motivational) pathway and the neospinothalamic (sensory discrimination) pathway.

Third order neurons: these neurons reside in the thalamus and beyond.

• The involvement of somatosensory cortex (parietal lobe) makes possible the discriminative aspects of pain, localization of the pain and motor-coordination response to pain.
  
• Information from the paleospinothalamic pathway lands in reticular formation, media thalamus, hypothalamus and prefrontal cortex—resulting in painful sensory information in the the affect, attention, cognition and memory domains.
  
• Stress reactions involve hypothalamic-pituitary axis and autonomic nervous system.
  
• The affective quality and pain experiences involve information landing in both cortices.
  
• The experiences of pain are shaped by the affective influences (e.g., anger, alarm, surprise) due to the involvement of limbic system; mood state, for instance, can affect the cognitive strategies one takes to deal with pain.
  
• The cognitive processing (e.g., identify, evaluation, decision making) of pain stimuli involves the frontal lobe.
  

Pain-Modulating Process within the Nervous system

• Neurochemicals in pain processing
  



• Endogenous Opiates: β-endorphin, enkephalins and dynorphins
  
• Pain-reducing pathways:
  
  
• Four regions of the CNS functions to reduce the sensation of pain or to make the individual be aware of the pain:
  
  • the cortex and the limbic structure
    
  • the midbrain
    
  • the rostral ventromedial medulla
    
  • the spinal dorsal horn (and yes, I thought I might have dorsal horn reorganization)
    
• If my understanding is correct, neurochemicals are required for these regions to modulate pain perception such as the following
  
  
• Opiate receptors and descending inhibitions of pain pathways: 4 classes of opiate receptors have been recognized to date.
  
• The rise of chronic pain
  
• Essentially, one thing leads to the other… towards the end, the more dysfunctional your pathways become, the harder it is to break the hardwired habit of pain.
  
  
  

Hansen's disease: people with no pain….

Acute vs. Chronic pain

Categories of chronic pain: Chronic pain could be categorized as nociceptive, neuropathic or psychogenic.

The impact of pain on the quality of life