Now that evidence of a relationship between the autonomic nervous system and seborrheic dermatitis has been presented, let’s try to decipher the specific mechanism in which the brain and skin interact. Understanding this mechanism should provide the perspective needed to not only better understand the relationship, but also give the foundational framework for implementing beneficial changes.
The brain communicates with the skin via something that has been termed as the neuro-immunoendocrine system. The word neuro-immunoendocrine can sound quite daunting, but it’s meaning is simple: the interaction between the nervous system and the immune system (the skin being an immune system organ).
Consequently, this section will investigate how this circuitry operates and how the two systems interact.
Examination of the Stress Response for Clues
One of the most explored areas in this line of research is how our bodies respond to stress. And the most common way for researchers to study this connection has been through mice (poor mice).
In these animal models, the mice are placed under stressful conditions (loud sounds, bright lights, etc.) and researchers monitor how their organisms respond. The whole approach is undoubtedly cruel, but it has allowed humans to gain a much better understanding of how our own bodies operate under similar conditions.
In stressed mice, the production of several inflammatory substances is induced via a hierarchical pathway (one leading to the next) .
The key components of this pathway are (in hierarchical order):
- Corticotropin-releasing hormone
- Nerve growth factor
- Substance P
- Mast cells
As this process progresses, getting closer to the concluding components (Substance P and mast cell activation), the chance for dermatological damage increases; these later stages drive significant inflammation, which dissipates and impacts other nearby components of the immune system.
A difficult relationship to unravel
While it’s difficult to determine if the stress can cause dermatitis to appear in the first place, it has been demonstrated that it can drastically worsen existing dermatitis [2, 3].
A similar mechanism has been described in humans and researchers have theorized that abnormal activation of any individual part of the process may serve as the foundation for a variety of chronic inflammation driven skin conditions [4, 5, 6, 7, 8].
Why the Skin is So Prone to Neural Influences
Since the skin comes into direct contact with the outside environment, our skin is also a site of dense and intricate innervation (networks of nerves) . And these delicate sensory nerve endings need to be sensitive to both our external environment and internal environments; being able communicate external stimuli to the brain (in case of local issues – such as a wound) and being receptive to incoming signaling from our brain (in the case where issues at other sites require more nutrients/resources – fighting off an infection).
Returning to the animal model (mouse) of chronic stress, it is well established that many of the primary features of stable immune response become impaired; specifically an abnormal ratio of Th1 to Th2 and reduced Treg cell expression . This impairment is so significant that it’s been shown to be capable of severely inhibiting hair growth and driving excessive skin inflammation in the animals being studied [9, 10, 11].
Putting this in context, it means that without any external pathogens or irritants, the skin of these animals was severally degraded simply through the activity of the nervous system (as a response to psychological stressors) and it’s interaction with the immune system. Put more bluntly: stable skin function depends on stable nervous system function.
Focusing in on Mast Cells and Nerve Growth Factor
Undoubtedly, the communication between the central nervous system and the skin (including neurotransmitter, neuropeptides, neutrophils, and neurohormones) is significantly more complex than the general 5 step hierarchy outlined briefly above. Understanding it in its entirety could very well be highly valuable, but one may also gain a significant benefit from understanding two components that appear to be most directly involved with skin inflammation.
These components are:
- Mast cells
- Nerve growth factor
Mast Cells and Their Involvement in the Inflammatory Response
Mast cells appear to be at the center stage of our skins ability to communicate between the nervous system and the local immune system. Mast cells are responsible for activating a myriad of inflammation-driven immune responses (histamine, cytokines, tumor necrosis factor-a, vascular endothelial growth factor, and several others).
These cells are of particular interest to skin disease since they are:
- Positioned in close proximity to both nerves and blood vessels.
- Able to modulate both the innate immune response and the adaptive immune response.
- Act as a communication pathway between the immune response and the nervous system.
Their pivotal role is probably best demonstrated by the effectiveness of corticosteroids in reducing inflammation and eliminating skin symptoms for the large majority of skin conditions. Corticosteroids work by inhibiting mast cell activation . As the mast cells become inhibited, the inflammatory response is halted and the associated symptoms rapidly resolve, leading to symptom relief.
Mast cells are also required for the repair process
Corticosteroids are the most commonly prescribed topical agents for the treatment of seborrheic dermatitis. They work almost instantly and provide significant relief. However, long term use can impair the normal process of epidermal repair by driving away mast cells altogether .
While diminishing mast cell activation is one way to approach treatment, determining the source of overactive mast cells may be a more permanent solution. Since mast cells are near the end of the hierarchy chain discussed earlier, if we are able to understand what’s causing the over-activity in the first place, we may be able to stop the process from spiraling out of control in the first place.
Mast cells can be activated in a variety of ways:
- IgE and antigens
Now since we are talking about the nervous system, the most interesting of these to our discussion is the last one: neuropeptides – a specific type of peptides responsible for orchestrating the communication and development of neurons.
Nerve Growth Factor
Nerve growth factor (NGF) is one of these neuropeptides (that can activate mast cells) and its primary role is described in its name: regulation and maintenance of neurons . Its release by glial cells can rapidly be triggered by stress  and it has the peculiar ability to promote mast cell production and survival .
Interestingly, inhibition of nerve growth factor reverses the negative effects of stress on both hair growth and inflammation . And this appears to be related to the ability of nerve growth factor to regulate Substance P production (a pain molecule) , which then goes onto control mast cell activation .
Research of NGF has shown that chronic stress can increase the amount of nerve growth factor circulating through our bodies . This, in turn, may be resulting in overactivation of the nervous system fueling a state of overexpansion. As the system stumbles out of its regular state of homeostasis (balance), mast cell activation also spirals out of control. The end result of this process could very well be excessive – yet not properly developed – innervation in the epidermis.
While the above is just a theory, the plausibility of such mechanism has been described in the case of atopic dermatitis  and would in-part explain the beneficial impact of ultra-violet based therapies (a therapy that decreases epidermal nerve density ).
This section further evaluated the connection between the nervous system and the skin. It also introduced some potential areas which may be the key to understanding the mechanism which translates nervous system problems to skin issues down the line.
Key points include:
- The communication of the nervous system and the skin is scientifically referred to as the neuro immunoendocrine system; more broadly, it’s the relationship between the nervous system and the immune system
- Studies which examine the stress response of both animals and humans, and how this impacts skin function, provide the primary clues in this area of research
- Periods of high stress and activation of the sympathetic nervous system can induce inflammation via process composed of several individual steps
- The steps occur in a linear fashion (one after the other) and the longer the processed is allowed to continue – the greater the amount of inflammation
- Issues and instability at any stage of this process could potentially lead to abnormal levels of inflammation, eventually inducing dermatological damage
- The skin is very susceptible to being damaged by over-activation of this pathway due to the high amount and density of nerve-endings present
- Mast cells and nerve growth factor have garnered the most attention when it comes to nervous system related skin issues
- Mast cells are at the final stages of this process and carry much of the responsibility for activating inflammation; corticosteroids work by inhibiting mast cell activity
- Nerve growth factor is at the beginning stages of this process and it’s over-activity has been proposed as the key driver of skin inflammation, UV therapy is believed to work by inhibiting nerve growth