"The skin is an efficient barrier which protects our bodies from the external environment but it is also an important site for the perception of various stimuli. Sensory neurones of the peripheral nervous system send many primary afferent fibres to the skin. They pass through the dermis and penetrate the basement membrane to innervate epidermal cells or remain as free endings. Nerve fibres are clearly involved in somatosensation. However, they are not always so numerous, for example in distal parts of the limbs, and some kinds of sensors can be at a distance of hundreds of micrometers from each other. The skin can detect patterns at a very fine and smaller scale, which suggests that nerve terminals are helped by epidermal sensors. All epidermal cells (keratinocytes, melanocytes, Langerhans cells and Merkel cells) express sensor proteins and neuropeptides regulating the neuro-immuno-cutaneous system. Hence, they must play a part in the epidermal sensory system. This review will consider the epidermal components of this forefront sensory system and the stimulations they perceive. The epidermis can be considered a true sensory tissue where sensor proteins and neurone-like properties enable epidermal cells to participate in the skin surface perception through interactions with nerve fibres."
Consider that skin is an organ, a continuous half inch or so thick membrane layer around the entire body, held on by connective tissue filaments, many millions of them, some thick and tubular, through which pass neural fascicles on their way to skin, and others (most) spider web thin. Skin is heavy. The BodyWorlds exhibit states that skin is equal in weight to the skeleton. If you are an obese person, you could well be carrying around skin that weighs as much or more than the rest of you does.
What does it do, other than be a burden? Well, a lot of things, but from a thermodynamic perspective, the skin is there to regulate temperature/promote heat loss. It's useful to remember that life is more a verb than a noun. The human body (any mammal body, but we'll look at our own body) is essentially a 100-trillion cell highly-controlled furnace operation reducing the oxygen gradient of the planet's atmosphere in order to produce carbon dioxide (in more ways than just metabolically.. but they are beyond the scope of this discussion). Plenty of heat is produced in this transformation, and must be eliminated from the multi-cell organism we are. Skin has 10 times the amount of blood flow than it needs to maintain its own existence (Gray's). So, its biggest job is to be a radiator/heat regulator for the entire organism.
It is also useful, I think, to remember (always) that ectoderm builds the body (if "life promotion" is going to be found in anything, it will be in ectoderm- I'm almost sure about that). Ectoderm builds the body in such a way that it kicks off layers, layers that seem to range from least "communicative" to most "communicative." It's first "cells" are germ cells, which lie completely dormant until stimulated. The next layer is mesoderm, which in addition to being the next least communicative, has to grow most of the body, 98% of it - all the bones and muscles and so on we are more familiar with. Next, it builds brain, spinal cord, and PNS- very very communicative. Lastly, what's left of the ectoderm encircles the entire body in a layer of lively communicative non-neural cells, which, even though they are non-"neural," strictly speaking, are still highly communicative, maybe the most communicative of all.
The neuro-immune-cutaneous system (NICS) refers to the communication system skin cells use in their efforts to keep the body as safe as possible through appropriate communication of environmental conditions. Here are some of the points made in the paper:
1. Epidermal cells connect the skin to the mind through a complex communication network, tightly related to the neuroendocrine and the immune systems.
(I would have used the word "perception" instead of "mind," but that's just me being picky, probably.. Had they used the word "perception," it would have been easier to include all the other mammals whose fur rises in fury or fright, reptiles and even invertebrates like cuttlefish and octopuses who can change their color to match their surroundings. I suspect our proclivity to blush or blanch is part of this system rather than primarily something to do with "mind," whatever that is.. all critters can be safely assumed to have perceptual capabilities, but I expect few will be found to be able to deduce meaning from perceptions with something called "mind.")
2. Langerhans cells and mast cells bridge the gap between neuroendocrine and immune systems in the skin. They participate in endocrine function through metabolism of vitamin D, production of neurohormones. They affect the permeability of blood vessels, are implicated in wound healing, pruritus (itching) and other dermatological disorders like psoriasis.
3. Epidermal cells act on the nervous system at local and central levels:
- 30 to 40% of dermatological patients also have psychological problems (understandable if you were being tortured by your own skin...)
- modulate the sensory information of touch or "pain." After ultraviolet (UV) exposure, they lead to a decrease in the pain threshold and immunomodulatory effects through pro-opiomelanocortin (POMC)-peptide release.
4. Brain can act on skin:
- can affect cutaneous functions in an efferent manner to stimulate target tissues; for example during neurogenic inflammation ("pain"-ful).
The authors go on to explain NICS further:
1. NICS consists of a common language shared by sensory neurones, keratinocytes, melanocytes, Langerhans cells and Merkel cells, with the neuromediators as letters. These powerful molecules are widely involved in skin physiology and the response to a stimulus.
2. Skin cells are able to recognize the relevant biological signals transmitted through neuromediators with high specificity because they synthesize the receptors themselves. Such neuroendocrine capabilities are critical for the activity of the NICS.
3. In the NICS, it is currently understood that:
- substance P (SP) plays a key role in pain sensitization and leads to mast cell degranulation
- POMC and derivatives are immunomodulators
- neurotrophines, like the nerve growth factor (NGF), are mitogenic proteins which also stimulate nerve fibre sprouting, regulate neuropeptides synthesis and probably take part in psoriasis
- catecholamine acts as an inflammatory factor
4. Acetylcholine, calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY) seem to act differentially, depending on the skin environment.
5. Therefore, the NICS acts locally, at the level of the neurogenic inflammation, but it is also considered to affect the whole organism via the endocrine and neurocrine pathways
6. "Until now, the concept of NICS mainly described the effects of the nervous system on skin cells through the presence of synapses, neurotransmitters and specific receptors in the skin. We now know that the epidermis also appears at the forefront of the sensory system, as revealed by new data on the sensory abilities of epidermal cells."
My bold. The authors carefully break down the picture by explaining which sensory proteins are made by which cell and commonly transducted by which size nerve fiber. They discuss the TRP (transient receptor protein) family, in particular TRP vanilloid 1 (TRPV1). It turns out that these receptors function exactly the same no matter which surface of the body they happen to be on - out in the epidermis of the body or that of the tongue - they are the equivalent of what we could perhaps call the nervous system's "weather" channels, sensing temperatures and tastes/ "tasting" temperatures both comfortable and noxious, and their fluctuations.
More about TRPV1:
1. is the most characterized receptor and probably the most expressed within the epidermis
2. TRPV1 is highly expressed in neurones involved in pain transmission and neurogenic inflammation (C and Aδ-fibres)
3. also shows a strong immunoreactivity in keratinocytes from the upper and the basal layers of the epidermis
4. In humans, the temperature responsiveness ranges from – 10 to 60 °C
5. plays a major role in the detection of temperatures over 42 °C and acidic conditions below a pH of 6.6
6. has the ability to bind capsaicin, the molecule which confers spiciness to chili peppers, with high affinity.
7. TRPV1 activation evokes sensations ranging from warmth to burning pain, as well as piquant taste
8. Consequences of its activation vary according to the context.
- once activated by capsaicin, the TRPV1 channel first leads to calcium influx and neuropeptide release.
- the lasting calcium influx, with too high intracellular calcium concentrations, leaves the neurone desensitized, thus it loses its ability to induce the release of neuropeptides such as SP, which is co-localized
- This is responsible for a transient insensitivity, which is exploited by dermatologists to induce analgesia or anti-inflammatory effects.
TRPV2 channel is heat-gated, strongly expressed in Aδ-fibres; it is activated for temperatures above 53 °C, warns of a burn.
TRPV3 channel is camphor sensitive, found in sensory neurones and keratinocytes of the inner boundary of the epidermis, is activated by heat from 31 °C to 39 °C.
TRPV4 channel is present in keratinocytes and Merkel cells, exhibits an apparent threshold of about 27 °C, and reacts to hypo-osmolarity.
TRPM8 (melastin cation channel) is menthol-sensitive and transduces cold; it "gates at temperatures below 30 °C. TRPM8 is expressed almost exclusively in a subpopulation of C-fibres representing 10% of the sensory neurones."
"TRPA1, a member of the TRP ankyrin-repeats family has been reported to be activated below 18 °C, so it may also participate in the cold responsive behaviour."
The authors state that there is no firm model yet. They discuss three possible models:
1. high speed channels convert stimuli into an electrical signal (this is what is thought to occur in hair cells of the organ of Corti (hearing) because of their remarkable transduction speed)
2. ion channels are tethered to the cytoskeleton or extracellular matrix
3. a mechanosensory protein initiates a second messenger cascade leading to the opening of the ion channels, thus producing depolarization
Re: the third model, in research done with invertebrates two mechanosensitive proteins have been discovered, MEC-4 and MEC-10. These belong to the Degenerin/Epithelial sodium channel family, or Deg/ENaC. "This family is characterized by common N and C terminals, two membrane-spanning sequences and a large extracellular loop with 14 conserved cysteins. The receptors are organized into homo- or heteromultimers of 4 to 9 subunits, forming nine voltage-insensitive Na+ permeable channels in mammals. Thus the mechanosensitive Deg/ENaC is composed of α, β, γ and δ ENaC, the acid-sensing ion channel (ASIC), the brain Na+ channel 1 (BNC1 or ASIC2), the dorsal root acid-sensing ion channel (DRASIC or ASIC3), the brain-liver-intestine amiloride-sensitive Na+ channel (BLINaC) and the ASIC4, which is not proton-gated despite its name."
Although there is still some conflicting data and gaps, and the work done on invertebrates has yet to be perfectly aligned with mammal models, these proteins are found produced by pacinian and Meissner corpuscles, lanceolate endings of hair follicles and the neurites contacting to Merkel cells, mechanosensory neurones of the dorsal root and trigeminal ganglia and hair cells of the inner ear.
Going back to the TRP family, TRPV4 rescues mechanosensory deficit in C. elegans (worm), TRPC1 are stretch-sensitive ion channels gated by membrane deformation, mutated TRPA1 attenuates mechanical responsiveness, NOMPC (analogue to TRPN1 in Xenopus) is implicated in the somatosensation of Drosophila and is newly found in the vertebrate zebrafish, where it behaves as a mechanically-gated ion channel in sensory hair cells. TRPV4 can do two things: it is expressed in the Merkel cell-neurite complexes, anatomical structures composed of the association of mainly Aβ-fibres and Merkel cells, which play a key role in the slowly adapting type I mechanoreception; furthermore, "TRPV4 is highly expressed in non-sensory tissues too. There, TRPV4 is believed to control the systemic fluid balance by its osmolarity-sensitive capability."
Sensor proteins also include purinergic receptors, thought to participate in many cutaneous phenomena. "They are involved in cell growth, differentiation, neuronal regeneration, wound healing, inflammation, etc." There are two types of these receptors grouped according to the ligand they bind:
1. P1 receptors bind adenosine and are divided into 4 subtypes
2. P2 receptors, which bind ATP, ADP, and UTP, are divided into ionotropic P2X receptors and metabotropic G protein-coupled P2Y receptors.
"Keratinocytes express both the P2Y receptors, implicated in the mobilisation of intracellular calcium stores in response to noxious stimulation, and the P2X ion channel. The latter is involved in the initiation of afferent signals on sensory neurones and plays a key role in sensing tissue-damaging and inflammatory stimuli. Immunohistochemical investigation into Merkel cells has revealed expression of P2Y2 receptors, which could argue for a putative role of this channel in mechanoreception."
The paper goes on to discuss sensory nerve endings. It is worth remembering that single nerve cells span the entire distance between skin and spinal cord. They can be various sizes and have varied degrees of myelination and neuropeptide expression, and convey varied information. Functional properties are not strictly related to morphological aspects. However:
1. "it is currently accepted that cutaneous large myelinated Aβ-fibres of low-threshold are suited to be mechanoreceptors which feel pressure, stretch or hair movement."
2. Unmyelinated C-fibres and lightly myelinated Aδ-fibres are often thermoreceptors which respond to heat and cold with different thresholds of activation.
3. Nociceptors, containing opioid receptors, are mainly high-threshold C-fibres and Aδ-fibres which transduce painful sensations.
4. A pruritus-specific pathway was recently defined - the pathway processing the itch is functionally and anatomically separate from the pain pathway.
"The itch pathway implies its own subgroup of peripheral, mainly mechano-insensitive, C-fibres in the skin. In the central nervous system, histaminergic spinal neurones transduce the itch sensation initiated by dedicated pruriceptors, to the thalamus. The pruriceptors are activated by histamine which consistently provokes pruritus, and rarely pain. However, other inflammatory molecules such as prostaglandin E2, serotonin, acetylcholine, bradykinin or even capsaicin may induce a moderate itching sensation. Thus a complex interaction exists between the pain and the itch pathway. - Scratching that induces pain is well-known to inhibit the pruritus and conversely, the inhibition of pain-processing by µ-opioïd can generate pruritus. Therefore, the distinction between cutaneous fibres is not easy and disrupting criteria are frequently evoked, like nociceptive signalling, normally particular to Aδ and C-fibres, with the conductance speed of Aβ-neurones.
- Further investigations have revealed that Aβ-fibres can phenotypically switch into fibres expressing SP whereas normally, SP is only contained in a subpopulation of small C and Aδ-fibres involved in pain perception. This occurs following nerve injury but also after inflammation. Thus the peripheral endings of primary sensory neurones participate in neurotransmission. But they also participate in the immune response by the release of proinflammatory peptides, from unmyelinated C-fibres or myelinated Aδ-fibres, leading to the set of changes referred to as neurogenic inflammation."
5. Two kinds of nociceptor have been identified based on their ability to bind isolectin B4 (IB4). "Those which bind IB4 are usually small diameter non-peptidergic neurones involved in acute pain." But:
- "only half of them seem to answer to noxious stimuli, with the remainder containing less mechanosensory C-fibres
- Within the epidermis, nerve viability and sensitivity can be modulated by neurotrophic factors secreted by epidermal cells."
- IB4-negative neurones containing SP and CGRP are NGF-responsive, small diameter nociceptors
- IB4-positive neurones, which lack such neuropeptides, respond to glial-derived neurotrophic factor (GDNF)
- NGF (nerve growth factor) produced in large quantities by keratinocytes increases nociceptive-neurone survival while brain-derived neurotrophic factor (BDNF) decreases the activation threshold of mechanosensory Aβ-fibres
- neurotrophin-3 (NT3) enhances the innervation by slow adapting mechanosensory neurones.
6. Cutaneous neurites play a major role in the sensory behaviour, but there is much evidence suggesting a modulation of their sensitivity by epidermal cells:
- stimulated cutaneous sensory neurones induce action potentials, but also the release of neurotransmitters, which modulate inflammation, cell growth or pruritus.
- Such neuronal modulations of cutaneous properties regularly bring heterotrimeric G proteins into play at the beginning of the metabolic cascade, and endopeptidases at the end, for termination of the response degrading the messengers
Fascinating stuff. The paper goes on to discuss each type of skin cell separately in detail, which I won't bring here as I am more interested in the overall picture of touch and what occurs neurologically as a result, which of course involves neurochemistry.
Here are tidbits from the conclusion:
- Ion channels have been discovered on epidermal cells: TRP, purinergic and Deg/ENa channels are putative transducers of touch, thermal sensation and nociception, as shown in invertebrate models and knockout mice. Thus they must start the signalling of the stimulus at the molecular level, based on their thermo-dynamical properties.
- Merkel cells are excitable cells containing the molecular components of synaptic connections so they should transduce the stimuli synaptically.
- "The mechanisms of communication between keratinocytes, Langerhans cells or melanocytes and sensory neurones are more mysterious. They are non-excitable cells with no molecular basis of synaptic connections. Paracrine function is supposed, but the mediator used to transmit rapid stimuli as fast as they occur must exhibit the characteristics of a neurotransmitter. It must be specific enough to carry a unique signal and quickly degraded to transmit a short stimulation. We have started to gain insight into this phenomenon so that some non-peptidic candidates are now being considered, like calcium, which can activate neighbouring cells, once released by keratinocytes."
This is reminiscent of glial communication.
Finally: "Acceptance of the epidermis as a sensory and endocrine tissue as part of the NICS has increased, as some authors define skin as spread brain. However, the relationship between skin and brain, although fascinating, remains poorly understood."
I don't think it is beyond understanding if one considers ectodermal behavior - it is the layer that would seem to be in charge of who knows what and when and how much, like a general manager. It seems to like taking "membran-eity" and playing with it, seeing how small it can fold up a single epithelial membrane (brain) and how much surface area it can cover (skin). Ectoderm is like the protective sensing membrane boundary of single cell creatures that ended up inheriting the job of figuring out how to cover monstrous multi-cell creatures. I think it did a brilliant job of it.
Online version of the whole paper (caution: format errors are displayed in table 2).
Boulais N, Misery L (2008). The epidermis: a sensory tissue Eur J Dermatol , 18, 119-127 DOI: 10.1684/ejd.2008.0348