Subclinical sensitivity to any drug may be related to pruritus. Activation may be the result of internal or external thermal, mechanical, chemical, or electrical stimulation. The itch impulse is transmitted along the same neural pathway as pain impulses, i.e., traveling from peripheral nerves to the dorsal horn of the spinal cord, across the cord via the anterior commissure, and ascending along the spinothalamic tract to the laminar nuclei of the contralateral thalamus. Thalamocortical tracts of tertiary neurons are believed to relay the impulse through the integrating reticular activating system of the thalamus to several areas of the cerebral cortex. Factors that are believed to enhance the sensation of itch include dryness of the epidermis and dermis, anoxia of tissues, dilation of the capillaries, irritating stimuli, and psychological responses.
Scratching is modulated at the corticothalamic center and is a spinal reflex.
After scratching, itching may be relieved for 15 to 25 minutes. The mechanism through which the itch is relieved by scratching is unknown. It is hypothesized that scratching generates sensory impulses, which break circuits in the relay areas of the spinal cord. A pinprick near or in the same dermatome as an itchy point will abolish the itch sensation. Biliary, hepatic, renal, and malignant diseases are thought to produce pruritus through circulating toxic substances.