By Kevin K. Haussler, DVM, DC, PhD, DACVSMR
We often describe horses as having ‘increased sympathetic tone’ or ‘autonomic imbalance’. While these terms may be convenient shorthand, they oversimplify the true complexity of autonomic nervous system (ANS) regulation, and risk limiting our understanding of how horses actually respond to pain, stress, and therapy.
A deeper, more accurate understanding of the ANS is needed to provide insights into diagnosis, treatment, and rehabilitation strategies.
Interoception – monitoring the internal environment
The ANS plays a critical role in maintaining physiological homeostasis and coordinating involuntary bodily functions by regulating vital processes. It is responsible for both sensing the internal environment (i.e., interoception) and regulating involuntary body functions such as heart rate, blood flow, digestion, thermoregulation, and glandular secretion.
Interoception is the sensing, processing, and perception of the internal physiological state of the body. It includes sensations like:
- Heartbeat awareness
- Respiratory effort
- Hunger and thirst
- Gut motility and fullness
- Pain or discomfort from internal organs
- Body temperature
- Blood pressure and fluid balance
- Metabolic status and blood chemistry
- Sexual arousal
Interoception helps maintain homeostasis by allowing the brain to monitor and respond to internal conditions. Specialized visceral sensory receptors monitor internal states:
- Baroreceptors: blood pressure
- Chemoreceptors: oxygen, carbon dioxide (CO2), pH
- Visceral mechanoreceptors: stretch in the gut, bladder, lungs
- Thermoreceptors: body temperature
- Nociceptors: visceral pain
- Musculoskeletal mechanoreceptors: mechanical distortion, positional awareness
After detecting a change, the ANS responds by adjusting the heart rate, blood vessel tone, respiratory rate, salivation, gut motility, or sweating. Other changes include behavior modifications (e.g., seek water when dehydrated, rest when exhausted) and altered emotional and cognitive states (e.g., internal feelings of stress, calm, hunger, discomfort)
These autonomic changes help maintain homeostasis and support adaptation to stress, rest, movement, or environmental conditions.
The sympathetic and parasympathetic nervous systems
The autonomic nervous system (ANS) consists of two major components:
- Sympathetic nervous system (SNS)
- Parasympathetic nervous system (PSNS)
While the SNS and PSNS both play essential roles in regulating the body’s internal functions, they do so in different contexts and for different purposes.
The SNS is primarily involved in preparing the body for action (e.g., during stress, exercise, or perceived threats), whereas the PSNS is more concerned with the maintenance of normal, baseline functions during rest, relaxation, and recovery.
Rather than a simple on-off switch between the two systems, the SNS and PSNS work in a more dynamic, coordinated manner to support a range of physiological states.
Both systems can be active simultaneously, but their effects are not always in direct opposition.
Sympathetic nervous system (SNS) activation reallocates the body’s resources to systems that are essential for responding to danger (i.e., flight or fight). These systems include the musculoskeletal system (for movement), the cardiovascular system (to deliver oxygen and nutrients to muscles), and the nervous system (to enhance focus and alertness).
The digestive system is not immediately critical in a survival situation, so blood flow to the gut is reduced, and gut motility is slowed down. This allows the body to focus energy and resources on the systems that are needed to escape or fight.
Activation of the SNS in horses leads to predictable physiological changes, including:
- Heightened alertness and sensitivity
- Increased heart rate and cardiac output
- Vasoconstriction to skin, gut, kidneys
- Increased respiratory rate and bronchodilation
- Pupil dilation to increase visual acuity
- Redistribution of blood flow from the viscera to muscles
- Increased muscle and fascial tension for readiness to move rapidly
- Increased sweating for dissipation of body heat
- Reduced digestive activity
The PSNS facilitates the delivery of oxygen, nutrients, and immune cells to injured areas while helping to restore balance to the body’s physiological systems after stress or injury. PSNS activation promotes relaxation, energy conservation, and recovery by slowing the heart rate (bradycardia), lowering blood pressure, and stimulating digestive processes such as gastrointestinal motility and enzyme secretion. The vagus nerve, a major pathway of the PSNS, helps regulate many of these functions, making the PSNS essential for the restoration of bodily functions after periods of stress or physical exertion.
Activation of the PSNS promotes:
- Increased blood flow to gastrointestinal and abdominal organs
- Enhanced gastrointestinal motility and secretion of digestive enzymes
- Muscle relaxation and reduced muscle tone
- Vasodilation for improved circulation
- Anti-inflammatory and tissue healing effects
- Reduction in stress-related behaviors
- Reduced heart and respiratory rates
Common misconceptions
Since the SNS is typically associated with ‘flight or fight’ responses and the PSNS with ‘rest and digest’ functions, the fact that these systems have both sensory and motor components is often overlooked. The signals from the visceral sensory receptors and the central processing in the brain are not visible to the external observer. As practitioners, we can only observe the body’s response to internal signaling via changes in behavior (stress), increased heart and respiratory rates, reduced gut sounds, or sweating.
Other common misconceptions relate to an ‘all or none’ response, ‘sympathetic tone’, ‘autonomic imbalance’, where the thinking is that sympathetic tone can be reduced by simply increasing parasympathetic tone. The notion that horses are either resting (parasympathetic) or fleeing (sympathetic) is a gross oversimplification of the wide spectrum of behaviors and motor responses that can be observed.
‘Sympathetic tone’ implies a fixed, uniform level of sympathetic activity, ignoring the rapidly changing, highly adaptive, and tissue-specific modulation of sympathetic signals. The SNS is constantly active at some level and not simply ‘on’ or ‘off’. Otherwise, there would be severe disruptions in nociception, cardiovascular function, and blood flow to tissues.
Clinical signs associated with increased sympathetic activity can be observed in one organ or part of the body, and not be present in other areas. Sympathetic activity can either excite (e.g., increase heart rate) or inhibit (e.g., reduce gut motility). Increased heart rates can occur independent of changes in respiratory rate. Sweating can occur in relaxed horses. Increased muscle tone can be unrelated to pain or sympathetic nervous system activity. Heightened alertness and sensitivity to environmental stimuli can occur without sympathetic activation.
At any moment, different organs may be under different autonomic influences — highlighting the adaptive, flexible nature of true autonomic control.
Importantly, the sympathetic and parasympathetic nervous systems are not simply a see-saw where if one is increased, then the other is automatically decreased.
These systems operate independently, simultaneously, and sometimes cooperatively, depending on the needs of specific tissues and the situation at hand.
An increased heart rate (SNS), reduced respiratory rate (PSNS), increased gut motility (e.g., diarrhea – PSNS), and excessive sweating (e.g., high ambient temperature and humidity) can all occur simultaneously within the same patient.
The autonomic nervous system is dynamic, context-specific, and adaptable, not a static ‘balance’ between two opposing forces.
Autonomic dysregulation is characterized by a failure to adaptively modulate activity appropriately, rather than a simple surplus or deficiency of sympathetic or parasympathetic input. In horses, pain behaviors, colic signs, stress responses, behavioral withdrawal, anhidrosis, and even performance limitations often originate from altered or dysregulated interoceptive signaling mediated by the ANS.
Changes in blood flow
Horses have developed an exceptional ability to induce large changes in blood flow and red blood concentration required for exercise or escaping potential predators.
During sympathetic activation, blood is preferentially shunted towards skeletal muscles, heart, and brain, while significantly decreasing flow to digestive organs and kidneys. Blood shunted to the skeletal muscles allows rapid and sustained locomotion.
The relative percentage changes in blood flow to various tissues and organs in horses with an activated sympathetic (‘flight or fight’) state compared to a resting parasympathetic state include:
| Tissue or organ | Resting state | Sympathetic activation | |
| Skeletal muscle | Low (~15-20%) | Very high (~70-85%) | |
| Heart (coronary) | Moderate (~4-5%) | Increased (~6-8%) | |
| Heart rate | 28-44 beats per minute | 180-240 beats per minute | |
| Cardiac output | 25-40 liters per minute | 200-300 liters per minute | |
| Lungs (pulmonary) | Stable (~100%) | Stable (~100%) | |
| Respiratory rate | 8-16 breaths per minute | 120-180 breaths per minute | |
| Adrenal glands | Low (<1%) | Increased (~2-3%) | |
| Brain | Stable (~12-15%) | Stable (~12-15%) | |
| Liver and spleen | Moderate (~20-25%) | Reduced (~3-5%) | |
| Gastrointestinal tract | High (~20-25%) | Markedly reduced (~3-5%) | |
| Kidneys | Moderate-high (~20%) | Markedly reduced (~2-4%) | |
| Skin | Moderate (~5-10%) | Initially decreased (~3-5%), then increased due to sweating | |
The spleen acts as a blood reservoir, especially rich in red blood cells and plasma. Sympathetic nervous system activation causes contraction of smooth muscle fibers in the spleen that causes a release of stored blood (especially red blood cells) into the general circulation. Sympathetic-mediated splenic contraction during exercise can boost red blood cell numbers by 30-50%, thereby dramatically enhancing oxygen delivery to muscles during intense athletic effort.
Role of the autonomic nervous system in pain
Detection and transmission of pain (nociception)
Visceral nociceptors and mechanoreceptors are sensory receptors that detect harmful or potentially harmful stimuli and mechanical changes within the body. These receptors are found in various locations within the visceral organs (stomach, intestines, heart, and lungs), as well as in the capsules, fascia, and ligaments that surround and support these organs.
Visceral nociceptors respond to painful stimuli such as ischemia, inflammation, stretching, or chemical irritation of the mucosal linings. These pain signals travel via autonomic sensory afferents (primarily through sympathetic nerves, vagus nerve, and pelvic nerves) to central integration centers in the brain.
Autonomic response to pain
Sympathetic activation:- Cardiovascular: increased heart rate, increased blood pressure, vasoconstriction
- Respiratory: increased respiratory rate and depth
- Gastrointestinal: decreased gastrointestinal motility
- Skin and muscles: increased sweating, muscle tension, altered posture
- Can occur during severe visceral pain (colic), resulting in vagal-mediated bradycardia, hypotension, nausea, or even collapse
Autonomic nervous system and pain perception
Persistent or chronic pain states often result in sustained sympathetic activation, creating a cycle of heightened pain sensitivity. Autonomic dysregulation can cause exaggerated pain responses, altered pain thresholds, or development of neuropathic pain syndromes.
How manual therapy modulates the autonomic nervous system
Manual therapy acts as a powerful regulator of autonomic activity by stimulating mechanoreceptors, improving circulation, reducing nociceptive input, and promoting parasympathetic activation.
Key neurotransmitters and hormones involved in this shift include:
- Acetylcholine: promotes parasympathetic responses (slowed heart rate, tissue relaxation)
- Endorphins: reduces pain perception, induces euphoria, promotes relaxation
- Serotonin: enhances mood, regulates pain pathways, supports parasympathetic activation
- Dopamine: contributes to a sense of well-being, supports parasympathetic dominance
- Oxytocin: promotes social bonding, deep relaxation, reduces stress responses
- Nitric oxide: facilitates smooth muscle relaxation, vasodilation, enhances blood flow and gastrointestinal function
- Reduced norepinephrine release: decreased heart rate, blood pressure, and muscle tension
Clinical signs observed in horses attributed to parasympathetic activation during manual therapy include:
- Yawning
- Sighing or deep breathing
- Dropped penis in geldings or stallions
- Lowered head and neck posture
- Relaxed lip, nostril, and eye muscles
- Improved gut sounds
- Increased willingness to move and stretch
While increased yawning is often attributed to a successful manual therapy session, there are many other musculoskeletal, neurologic and behavioral changes that are likely more important parameters to assess.
In closing…
Understanding the autonomic nervous system requires moving beyond simplistic notions of ‘sympathetic tone’ or ‘autonomic imbalance’.
The ANS is a dynamic, adaptive network that continuously senses and regulates internal functions.
It operates in a tissue-specific, context-dependent manner, with sympathetic and parasympathetic branches often working independently or cooperatively.
Clinical signs of dysfunction reflect a failure of adaptability, not merely an excess of one branch over the other.
Treatment strategies should therefore be focused on helping to support the different tissues or organs in autonomic nervous system adaptability instead of just attempting to suppress sympathetic activity throughout the body. By focusing on restoring healthy autonomic responsiveness, manual therapies can profoundly influence equine health, performance, and emotional resilience.