Specificity Theory of Pain

• The presence of dedicated pathways for each somatosensory modality

• Each modality has a specific receptor and associated sensory fiber that is sensitive to one specific stimulus

• Noxious stimuli would activate a nociceptor, which would project to higher pain centers through a pain fiber

• Its popularity decreased

Descartes’ description of the pain system

• René Descartes: one of the 1st philosophers to describe a detailed somatosensory pathway in humans

• Pain = perception that exists in the brain

• Nerves: hollow tubules that convey both sensory and motor information

Anatomical discoveries inform physiology

• Dedicated pain pathway developed by Charles Bell

• Brain is not a common sensorium but is a heterogenous structure

• Nerves: bundles of heterogenous neurons that have specialized functions and their bundling was only for ease of distribution

• Different sensory neurons for different types of stimuli

• Specificity Theory of Pain: there is a dedicated fiber that leads to a dedicated pain pathway to the sensory modality’s region of the brain

von Frey’s and Goldscheider’s skin spots

• von Frey hairs

  • Determine the pressure required to elicit a sensation at each of the skin spots

  • Which spots responded to innocuous pressure and which ones responded to noxious pressure

  • Different spots for innocuous pressure and noxious pressure

• The main function of the receptor is to lower the excitability threshold of the reflex arc for one kind of stimulus and heighten it for all others

Intensity Theory of Pain

• Pain: emotion that occurs when a stimulus is stronger than usual

• Repeated subthreshold stimulation or suprathreshold hyperintensive stimulation could cause pain

• Increased sensory input would converge and summate in the gray matter of the spinal cord

• Theory lost support

Pattern Theory of Pain

• Any somaesthetic sensation occurs by a specific and particular pattern of neural firing and the spatial and temporal profile of firing of the peripheral nerves encoded and the stimulus type and intensity

• Cutaneous sensory nerve fibers are the same

Gate Control Theory of Pain

• There are nociceptors and touch fibers and these fibers synapse in 2 different regions within the dorsal horn of the spinal cord: substantia gelatinosa and transmission cells

• Large-fiber activity inhibits the gate whereas small-fiber activity facilitates the gate

• When nociceptive information reaches a threshold that exceeds the inhibition elicited, it opens the gate and activates pathways that lead to the experience of pain and its related behaviours

Shortcomings of the Competing Pain Theories

• None account for the complexity of the pain system

• Focus on cutaneous pain and do not address issues pertaining to deep-tissue, visceral or muscular pain

• Focus on acute pain and do not address mechanisms of persistent pain or the chronification of pain

Contemporary Views and the Multidimensional Aspects of Pain

• Pain can affect and interact with motor systems

• Concept of pain = multidimensional experience

• Dimensions:

  • Sensory-discriminative

  • Affective-motivational

  • Cognitive-evaluative

• Those dimensions interact with one another

• The more intense a noxious stimulus is, the more unpleasant it will be

  • However, hypnosis has been shown to modulate pain unpleasantness without affecting intensity

• Cognitive modulation: cognitive state can modulate the percept of the affective-motivational component of pain

• Chronic pain conditions, brain structure and function undergo plasticity and network dynamics are altered