thesis
Post-Traumatic Neuropathic Pain: The Role Of The Skin
- Publication date
- 18 September 2013
- Publisher
- In the past decades, a body of evidence has shown that the uppermost layer of the
skin, the epidermis, not only serves as a protective layer but also plays a pivotal role
in the transmission of painful and non-painful stimuli to the central nervous system. The epidermis consists of five layers (Fig. 1): stratum basale, stratum spinosum,
stratum granulosum, stratum lucidum (only in palmar and plantar skin) and stratum
corneum, from the inner-most to the outer-most layer, respectively. The epidermis
contains keratinocytes, which undergo gradual differentiation as they progress from
the basal layer to the stratum corneum, with a turnover time of about 27 days. Keratinocytes
relay sensory information as they are activated by mechanical, thermal
and noxious stimuli and they transmit their information indirectly to the spinal cord
by activating nearby located peripheral nerve fibers . For example, upon activation,
keratinocytes secrete adenosine triphosphate (ATP), which binds to and activates
P2X purinoceptor 3 (P2X3), receptors expressed on adjacent epidermal nerve fibers. In addition to keratinocytes, the epidermis contains Merkel cells, melanocytes and
Langerhans cells, all of which serve different sensory purposes (Fig. 1). Merkel cells
are believed to play a prominent role in the transduction of light touch . Melanocytes
are UV sensitive cells that produce melanin, a pigment that has photo protectant
properties. Langerhans cells are antigen-presenting cells, but are also sensitive to
thermal stimulation, such as the increasing skin temperature occurring during local
inflammation.