Aspects of Pain

General Pain Pathways

These pathways are the primary components that connect, receive, and process all the pain information generated by neurons. The pain pathway comprises three orders of neurons that transmit pain signals to the brain. Sensory or afferent neurons carrying pain are the Aδ-fibers, which are myelinated, and the unmyelinated C-fibers.

Mediators of Pain

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  Beta-endorphin: A metabolite of beta-lipotropin produced in the anterior lobe of the pituitary gland in response to pain. It carries mu-opioid receptors agonist properties and acts as a synaptic transmitter agent and neural hormone.

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  Substance P: A chain of 11 amino acid residues with inflammatory effects in immune and epithelial cells. Substance P induces vasodilation, increases vascular permeability, and facilitates leukocyte release to the site of injury.

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  Bradykinin: An inflammatory mediator that produces edema and vasodilatation in the site of injury.

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  Cytokines: Involved in the initiation of pain and chronic pain by directly activating nociceptive sensory neurons and in nerve injury–induced central sensitization.

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  Prostaglandins: Produced in nearly all body tissue cells and function as enhancers of other chemical mediators, such as serotonin substance P.

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  Nerve growth factor: Expressed after the infla­mmatory lesions are formed causing mast cell degranulation and the release of serotonin and histamine. The nerve growth factor receptors have become potential therapeutic targets in treating acute and chronic pain states.

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  γ-Aminobutyric acid (GABA): The most widely distributed inhibitory transmitter in the central nervous system. It can bind to the ionotropic GABA-A receptors or metabotropic GABA-B receptors.

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  Cannabinoids: Bind to CB1 receptors in the brain and spinal cord and peripheral CB2 receptors inhibiting intracellular cyclic adenosine monophosphate formation regulating pain responses.

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  Norepinephrine: Synthesized from phenylalanine in the nerve terminals and directly inhibits pain through alpha-2-adrenergic receptors.

Types of Pain

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  Nociceptive pain: Seen after acute tissue damage and not a direct nerve injury. It presents as dull aching pain at the site of the injury. Examples include pain from arthritis, inflammation, or bone fractures.

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  Neuropathic pain: Consists of direct nerve tissue injury and presents as a sharp, lancinating, burning, or electric sensation in the affected area. Some examples of neuropathic pain are carpal tunnel pain, phantom limb pain, or postviral neuralgias. Common causes of peripheral neuropathy are diabetes, vitamin B1 or B12 deficiency, and hypothyroidism.

The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, and Pain

The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, pain disorder diagnosis was eliminated and largely replaced by the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, of somatic symptom disorder. People suffering from this disorder have a significant focus on physical symptoms that result in concern and problems functioning.