Slow pain

Fast pain may be described as sharp or prickling pain (see Table 8.2). This pain is perceived first (within 0.1 sec) as it is carried by the more rapidly

Table 8.1 Endogenous Chemicals Activating or Sensitizing Nociceptors

Enzyme involved in

Effect on first-order

Pharmacological

Chemical

Source

synthesis

sensory neuron

intervention

Potassium

Damaged cells

Activation

Serotonin

Platelets

Tryptophan hydroxylase

Activation

Bradykinin

Plasma kininogen

Kallikrein

Activation

Histamine

Mast cells

Activation

Hj receptor antagonists (e.g., diphenhydramine chloride, Benadryl®)

Prostaglandins

Arachidonic acid/ damaged cells

Cyclooxygenase

Sensitization

Non-steroidal anti-inflammatory drugs (e.g. aspirin, ibuprofen)

Leukotrienes

Arachidonic acid/ damaged cells

Lipoxygenase

Sensitization

Substance P

First order sensory neuron

Sensitization

Opioid receptor agonists (e.g., morphine)

Table 8.2 Characteristics of Fast and Slow Pain

Fast pain

Slow pain

Occurs first

Occurs second, persists longer Dull, aching, throbbing sensation; more unpleasant C fibers

Sharp, prickling sensation

A-delta fibers

Thermal or mechanical nociceptors Polymodal nociceptors

Easily localized

Poorly localized conducting A-delta fibers. Because fast pain is elicited by stimulation of specific thermal or mechanical nociceptors, it is easily localized. This type of pain is not felt in most of the deeper tissues of the body. Slow pain may be described as dull, aching, or throbbing pain. This pain is perceived second (only after 1 sec or more) because it is carried by C fibers. Slow pain persists longer and is typically more unpleasant; in fact, it tends to become greater over time. Slow pain is typically associated with tissue destruction. Noxious chemicals released from damaged cells or activated in the interstitial fluid can spread in the tissue, causing a relatively diffuse stimulation of polymodal receptors. As a result, slow pain is poorly localized; it may occur in the skin as well as almost any deep tissue or organ.

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