The itchy scalp--scratching for an explanation

Abstract

Scalp pruritus is a common complaint that is considered a diagnostically and therapeutically challenging situation. Scalp skin has a unique neural structure that contains densely innervated hair follicles and dermal vasculature. In spite of the recent advances in our understanding of itch pathophysiology, scalp itching has not been studied as yet. In this review, we summarize the current knowledge on the neurobiology of scalp and hair follicles as well as itch mediators and provide a putative mechanism for scalp itch with special emphasis on neuroanatomy and pathophysiology.

Figure 1. Sensory innervations of the scalp

V1: Ophthalmic division of trigeminal nerve; V2: Maxillary division of the trigeminal nerve; V3: Mandibular division of the trigeminal nerve; C2: Second cervical nerve; C3: Third cervical nerve.

Figure 2. Innervation around a human scalp hair and melanocytes of the dermal papilla and epidermis

Projection of 55 one-micron optical sections of human scalp hair follicle captured using laser scanning confocal microscopy. 4-mm punch biopsy from human scalp, was cryo-sectioned into 180-micron, vertical section. Sample was immunostained with antibodies to a pan-neuronal marker PGP9.5 (pseudo-colored aqua), and Mel5 (pseudo-colored yellow). 100XMAG

Figure 3. Itch mediators in the scalp

Scalp pruritus involves a complex interaction between different cells, mediators and receptors, most of them are shown above. PAR-2, proteinase-activated receptor 2; H1R, Histamine 1 receptor; H3R, Histamine 3 receptor; H4R, Histamine 4 receptor; NGF, nerve growth factor; ET-1; TrkA, high-affinity NGF receptor; endothelin-1; ETAR, endothelin receptor A; eCB, endogenous cannibinoids; NT-3, neurotrophin 3; NT-4, neurotrophin 4; LTB4, leukotriene B4; TXA2, thromboxane A2; GRP, gastrin-related peptide; SP, substance P; CGRP, calcitonin gene related peptide; Mrgpr, mas-related G protein-coupled receptor; IL-2, interleukin-2; IL-4, interleukin-4; IL-8, interleukin-8; IL-31, interleukin-31; BDNF: brain-derived neurotrophic factor; EDN, eosinophil-derived neurotoxin; ECP, eosinophil cationic protein; PGD2, prostaglandin D2; ORS, outer root sheath; IRS, inner root sheath; CB1, cannabinoid receptor1; CB2, cannabinoid receptor2; NK1R, neurokinin-1 receptor; TRPV, transient receptor potential vanilloid; MOR, Mu-opioid receptor.