itch is a protective cutaneous somatosensory modality that drives reflexive scratching to remove harmful parasites and irritants from the skin.
the itching response with subsequent scratching is activated by two distinct neural pathways:
mechanical itch pathway
begins with low-threshold mechanoreceptors (LTMRs) in hairy skin that respond to light mechanical stimulation
these signals are transmitted to certain excitatory dorsal horn neurons expressing two proteins called Y1 and Lbx1 (Y1Cre neurons) 1)
the dorsal horn response is modulated by inhibitory interneurons (INs) that express neuropeptide Y::Cre (NPY::Cre INs) and this prevents an itch response to certain types of light touch such as hair, etc and thus allows humans to enjoy light touch rather than be annoyed with itch
chemical itch pathway
generally triggered by histamine and serotonin and is transmitted via pruriceptive A delta and C fibers to the dorsal horn of the spinal cord, and then via the cells carrying the gastrin-releasing peptide receptor (GRPR), to the thalamus and parabrachial nucleus by projection neurons, including those marked by neurokinin-1 receptor (NK1R) expression