The Preception of An Itch

There are no special itch sensors in the skin. It is normally agreed that itch and pain are elicited by unspecialised free nerve endings in the skin. However, recent research indicates that the actual transmission of the itch sensation is through a separate subset of dedicated nerve fibres.
The itch-transmitting nerve fibres enter the spinal cord where it links up with a relay nerve cell, which then carries the impulse to the thalamus and then to the brain. At this linking junction in the spinal cord, the nerve impulse may be dampened or amplified by inhibitory or excitory circuits related to local nerve cells or to nerve cells from higher up in the central nervous system.
Apart from peripheral stimulation of nerve endings causing itch, there is also the important concept of “central itch”, i.e. itching which is perceived to occur in the skin but which actually originates in the brain due to abnormal processing of sensory information.
Itch can be caused by peripheral stimulation in the skin by:
Physical factors like light touch, stroking and vibration.
Chemical factors like mild acids, mild alkalis and venoms (for example, from caterpillars and moths).
Special substances released by cells. The most important of these is histamine. Others include substance P, interleukins and prostaglandins. These mediators are important in the causation of itch in the majority of skin disorders.
Apart from peripheral stimulation, factors that affect the control mechanism for regulation and interpretation of itch impulses in the brain may cause itch. For instance, heat aggravates itch and the heat stimulus appears to act centrally. Parasitophobia (delusions of parasitic infections of the skin) is a well-known cause of psychogenic itch, which can result in widespread excoriations in the skin.
An itch can be trivial and scratching certainly can be pleasurable. However, in certain disease states, itching can be so severe that scratching brings no relief. In fact, severe itch can be a great distress, causing poor sleep, depression, agitation, difficultly in concentrating and reduced sexual desire and function. Some severely affected patients may even contemplate suicide.