The autonomic nervous system is an integral component of the peripheral nervous system that controls things that we don’t need to consciously think about.
This includes things such as:
- Heart rate and force (vascular system)
- Sweat and sebaceous gland secretion
- Hormone secretion
- Gastrointestinal movement
- Metabolic function
Basically, it’s all the stuff that our bodies do without us giving it any thought and without its work, we wouldn’t be alive.
As we have learned in previous sections of this book, seborrheic dermatitis symptoms are closely related to sebaceous gland output and immune function; both of which fall (at-least partially) under the control of the autonomic nervous system. Accordingly, stable function of the autonomic nervous system is potentially at the center of the relationship between the nervous system activity and healthy skin function.
This section introduces the autonomic nervous system and some terminology that will be useful in later parts of this book.
Two Primary Arms of the Autonomic Nervous System
The autonomic system is divided into two primary components, the sympathetic and parasympathetic nervous systems.
These two systems typically function in opposite directions of each other depending on the circumstances. However, they never operate independently and normally both systems are active to some degree.
The sympathetic system takes control in situations where increased alertness or physical exertion is required. This includes physical exercise, mentally challenging circumstances, triggered fear, sensed threats, and everything else that requires rapid action.
Another, more common, way to think of the actions of the sympathetic arm is as the “fight or flight” response. Basically, this describes a response where increased metabolic activity is required.
Activation of the sympathetic nervous system has the following key characteristics:
- Heart rate and blood pressure increase
- Hormone excretion increases
- Blood flow is directed to muscles and brain
- Blood flow to the skin and gastrointestinal tract is restricted
- Digestion slows
- Pupils dilate (more light reaches retina)
Other more subtle, but equally telling changes can be seen in brain wave patterns and electrodermal activity (lie detectors operate by monitoring these parameters).
For our ancestors, the sympathetic system would usually become active during an encounter with a wild animal, exploration of unknown territory, or simple disputes with other humans. And during the relaxation part of the day, the sympathetic nervous system would usually be taking a break.
For the modern human, sympathetic nervous system activation is more likely to come from disputes with others, work stress, negative emotions, or perhaps even the ongoing interruptions we receive from our always-connected electronic devices. In today’s world, it’s easy to presume the sympathetic nervous system activation is less pronounced but spans over longer periods of time.
The parasympathetic nervous system takes over when our body is at rest and no immediate external thread is detectable. Essentially, this state dominates when we relax, eat, and unwind; also referred to as “rest and digest” phase.
This arm of the autonomic nervous system operates in the opposite direction of the sympathetic nervous system, with key characteristics of its function being:
- Decreased heart rate and pressure
- Blood flow directed to the gastrointestinal tract
- Blood flow to muscles and brain is restricted
- Digestion intensifies
- Pupils constrict
When we’re relaxing at home, going for a walk in the park, or enjoying a pleasant dinner with friends/family, the parasympathetic nervous system is usually showing dominance. And due to this more passive nature of its activities, discussion of its function is usually quite limited, but it would be unwise to underestimate its importance.
Heart Rate Variability as a Way of Measurement
Control of blood circulation throughout our bodies is one of the autonomic nervous systems most vital tasks. So much so, that many of its other functions critically depend on modulation of blood delivery to the organ/site being influenced.
For example, in order to increase sweat production (a task for the sympathetic system) a stable supply of oxygen and nutrients is required, the delivery of which is achieved through the circulatory system. And in order to optimize delivery, dilation of blood vessels and increased heart rate are both beneficial.
A simplified way of looking at the relationship between heart rate and autonomic activity is:
- Faster heart rate – sympathetic nervous system is engaged.
- Slower heart rate – parasympathetic nervous system is engaged
In light of this intricate relationship between heart rate and autonomic activity, one of the most common methods to measure autonomic function is through the analysis of our heart rate and its variability.
This is where the Heart Rate Variability (HRV for short) measure comes in. The HRV is a measurement that looks at the variation in time between each heartbeat (the heart is not always beating at a constant speed); the more variation, the better (usually). One way to think of it is that more variation simply tells us that our bodies are able to adapt and regulate autonomic activity with high precision and coordination.
A high HRV has been associated with not only better cardiovascular function, but individuals with a high HRV tend to be healthier and live longer, with a reduced risk of developing disease.
While there does not appear to be any studies linking a low HRV with seborrheic dermatitis, we will look at some evidence from conditions with a frequently reported low HRV later in this chapter. For now, the key takeaway is that HRV is one of the primary ways researchers examine the state of autonomic nervous system function and a term that will be used in later parts of this chapter.
Another interesting component of the autonomic nervous system that is worth reviewing is its integral connection to the digestive system. On the one hand, the balance between sympathetic and parasympathetic activation directly regulates the rate and efficiency of the digestive process (one of the big reasons why we shouldn’t exercise right after eating). On the other hand, the gut microbiome can also influence the autonomic nervous system .
Short Chain Fatty Acids as Key Mediators
Short-chain fatty acids produced through the fermentation of fiber by the beneficial bacteria in the colon have been identified to be one of the key messengers in this connection.
Inline with this two-way relationship, individuals with digestive disorders (such as inflammatory bowel disease, Crohn’s disease, and ulcerative colitis) have a higher incidence of autonomic nervous system issues where damage to these important nerve networks is evident (autonomic neuropathy) [7, 8, 9].
This section introduced the autonomic nervous system and its pivotal role in the regulation of a variety of bodily functions. And since the skin and its environment is under the direct influence of the autonomic nervous system, further examination of this topic could potentially uncover additional clues as to the source of seborrheic dermatitis
Key points of this section include:
- The autonomic nervous system is a major component of the peripheral nervous system and controls bodily functions that we do not need to think about
- Things controlled by the autonomic nervous system include hormone section, sweat and sebum section, heart rate and force, and metabolic function
- The system is further divided into two components, the sympathetic and parasympathetic, each of which dominate in certain conditions
- When increased alertness is required, such as periods of high stress or physical activity, the sympathetic system dominates
- Sympathetic dominance is characterized by reduced heart rate, blood pressure, hormone secretion, blood flow to muscles and brain; and reduced blood flow to the skin and digestive system
- When we are relaxed and increased alertness is not necessary, such as having dinner or walking through the park, the parasympathetic system dominates
- Parasympathetic dominance is characterized by decreased heart rate, blood pressure, hormone secretion, blood flow to muscles and brain; and increased blood flow to the skin and digestive system
- Heart rate variability measures the natural variation in our heart rate over a set period of time and has been proposed as a reliable method for assessing autonomic function
- Emerging evidence from the last decade has suggested that the gut microbiome also plays a big role in influencing our autonomic nervous system via two-way relationship