Survival in Balance: Autonomic Nervous System Control in Wilderness Medicine

In the wilderness, balance is the foundation of survival. A misstep on uneven terrain, poor pacing on a long trek, or exposure to extreme weather can quickly tip the scales from stability to crisis.

Nature itself operates in balance: day and night regulate temperature, along with alternating periods of activity and rest; ecosystems rely on opposing forces working together (predator-prey dynamics, life and death, photosynthesis and respiration). Muscle and bone development is largely driven by the body’s efforts to overcome gravitational forces, and the very act of walking involves shifting balance from one leg to the other.

Nothing captures the concept of complementary opposites working together to establish balance and harmony than the yin and yang of ancient Chinese philosophy. It applies to all aspects of life — night and day, hot and cold, good and evil, masculinity and femininity, happiness and sadness, health and illness.

In the human body, that balance is maintained through homeostasis, largely regulated by the autonomic nervous system — the part of the nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, breathing, and digestion. The autonomic nervous system is able to perform its magic thanks to two complementary components — the sympathetic and parasympathetic nervous systems.

Understanding Homeostasis: Homeostasis is the process our bodies use to keep its internal environment stable, even when conditions around us change. For example, our bodies regulate things like temperature, blood sugar, and water levels in order for everything to remain balanced. This stability helps our cells function properly and keeps us alive.

For wilderness medicine providers such as outdoor educators, field guides, and emergency medical technicians (EMTs), understanding and recognizing disruptions in the balance between these two components — often when a patient is far from traditional healthcare — is critical.

This post discusses the differences between the sympathetic and parasympathetic nervous systems and provides guidance on how to determine when an overactivation or under activation of one or the other may be impairing the body’s ability to maintain homeostasis. Hence, the potential to threaten a patient’s survival.

Getting to Know the Sympathetic Nervous System

The sympathetic nervous system activates the “fight, flight, or freeze” response. In a wilderness setting, this response is often activated by real threats, such as injury, an encounter with a potentially dangerous animal, exposure to harsh environmental conditions, or navigational stress.

When activated, the sympathetic nervous system triggers the following physiological changes:

• Heart rate increases to deliver oxygen to muscles
• Respiratory rate increases to improve oxygen intake
• Blood pressure elevates to maintain perfusion (blood flow to tissues)
• Blood is diverted away from the digestive tract to the brain and other vital organs
• Pupils dilate to improve vision
• Stored fat and glucose are mobilized for energy
• Sweat increases to regulate body temperature

These physiological changes are essential for enabling human beings to respond effectively to threats and crises. Problems arise, however, when the sympathetic nervous system is overactive or underactive. 

For example, a traumatic event can increase a person’s heart rate and blood pressure to the point of causing a heart attack or stroke. Or it can trigger something less serious, such as loss of bowel or bladder control, as blood is diverted away from the digestive system. Chronic activation of the sympathetic nervous system can cause long-term health issues, including cardiovascular damage, metabolic disorders, digestive problems, anxiety, depression, and others.

On the flip side, when the sympathetic nervous system is underactive, a person may experience severe fatigue, hypotension (low blood pressure), slowed digestion (constipation), anhidrosis (not sweating enough), or other disorders.

Getting to Know the Parasympathetic System

The parasympathetic system is responsible for the “rest and digest” or “rest, recover, and repair” response. When the parasympathetic system is engaged, it triggers the following physiological changes:

• Slows heart rate and breathing
• Redirects blood flow to the gut to promote digestion and nutrient absorption
• Conserves energy
• Supports cellular repair

In wilderness settings like the backcountry, the parasympathetic nervous system allows the body to digest, adapt to stress, recover during periods of sleep, and maintain proper function over the long haul.

Recognizing Imbalances: Wilderness Red Flags

In wilderness medicine, imbalances between the sympathetic and parasympathetic nervous systems can be exacerbated by environmental extremes, fear and anxiety, and delayed care. Outdoor educators, field guides, and first responders alike must be able to recognize when one system dominates dangerously or is overly stimulated or inhibited.

The stimulations and inhibitions can come from myriad causes that we attempt to discover when sizing up the scene. During your scene size-up, ask yourself, “What’s disrupting homeostasis?” Look for the following possible causes of imbalance:

• Temperature extremes (hypothermia or heat illness)
• Altitude exposure
• Dehydration or lack of nutrition
• Substances (overdose or underdose)
• Trauma (falls, burns, blunt force)
• Allergies
• Creepy crawlies (snakes, spiders, bees, hornets)
• Water exposure (drowning, immersion)

You should also conduct a primary assessment of the patient, looking for any obvious imbalances. Read “The Role of Assessment in Wilderness Medicine” here on the NCOAE Blog for a deeper dive on assessment.

Here are some clear indications of disruptions in homeostasis:

Circulation

• Severe bleeding
• Rapid pulse (sympathetic overdrive)
• Weak or absent pulse
• Pale or blue skin (especially evident in the extremities)

Respiration

• Fast breathing (stress or shock)
• Slow or absent breathing (critical failure)

Skin

• Pale, cold, clammy (shock)
• Hot, dry (heat illness)

Mental Status

• Agitation or anxiety (sympathetic surge)
• Confusion, lethargy, or unresponsiveness (decline)

These signs and symptoms are magnified in a wilderness setting, where time, weather, terrain, and lack of resources all work against recovery.

Recommended Reading: For even more details about conducting an initial assessment and sizing up a scene, check out our previous blog post, “Just the Facts: Recognizing the Importance of Reporting Accurate Information in a Wilderness Emergency.”

The human body has an amazing ability to self-regulate, respond to challenging situations, adapt to changing environmental conditions, and repair itself. In addition, humans can cycle through periods of activity and rest/recovery much more rapidly than other creatures. These “superpowers” are largely the result of the sympathetic and parasympathetic nervous systems working alone and together to maintain homeostasis.

However, every now and then, these systems can suffer imbalances, some severe enough to threaten survival. It’s important to recognize the signs of any such imbalances early on and know how to respond in order to restore balance. Here at The National Center for Outdoor Adventure and Education (NCOAE), we offer several wilderness medicine courses that provide the knowledge, skills, and hands-on experience you need to meet this challenge.

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About the Author: Todd Mullenix is the Director of Wilderness Medicine Education at The National Center for Outdoor & Adventure Education in Wilmington, North Carolina.

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