Nervous System Dysregulation as a Major Factor in Disease: A Comparative Review in Humans and Horses

The nervous system is the central regulator of balance in both humans and horses. It governs critical processes such as cardiovascular function, digestion, stress responses, and recovery. Recent research highlights that nervous system dysregulation—an inability to shift smoothly between sympathetic (“fight-or-flight”) and parasympathetic (“rest-and-digest”) states—plays a key role in the development and progression of disease. In humans, dysregulation contributes to chronic pain, cardiovascular disease, stress-related disorders, and impaired resilience. In horses, it is evident in conditions such as equine grass sickness, pituitary pars intermedia dysfunction (PPID), performance issues, and stress-related behaviors. This paper reviews the evidence from both species, comparing mechanisms and manifestations, and argues that nervous system dysregulation should be recognized as a central factor in health and disease.

Introduction

The nervous system functions as the body’s command center, maintaining homeostasis and allowing organisms to adapt to internal and external challenges. In both humans and horses, the autonomic nervous system (ANS)—comprising sympathetic and parasympathetic branches—regulates critical automatic functions including heart rate, digestion, and stress responses (Thayer & Lane, 2009). A balanced nervous system dynamically shifts between these branches depending on circumstances.

Dysregulation occurs when this balance is lost, leaving the body stuck in prolonged activation, unable to rest effectively, or compromised by degeneration. This imbalance contributes to disease across species, not merely as a secondary effect but often as a driving force. By comparing human and equine research, this paper demonstrates how nervous system dysregulation underlies both chronic and acute illness, and why it deserves greater attention in medical and veterinary fields.

Nervous System Dysregulation: Conceptual Framework

A common metaphor for understanding the ANS is that of a car with two pedals: the sympathetic system acts as the “gas,” increasing alertness and preparing the body for action, while the parasympathetic system acts as the “brake,” restoring rest and repair (McEwen, 2008). In a healthy state, these systems work in dynamic balance. Dysregulation occurs when:

• The “gas” is overactive, leading to chronic stress, tension, and overexertion.

• The “brakes” fail, preventing recovery, healing, or digestion.

• The system becomes rigid, unable to adapt to new challenges.

This imbalance sets off cascades affecting cardiovascular, immune, endocrine, and musculoskeletal systems (Goldstein, 2019).

Evidence in Humans

Chronic Pain

Chronic pain is increasingly understood as a nervous system disorder rather than a purely physical one. Research shows that the brain can continue sending pain signals long after tissue healing, reflecting dysregulation in protective neural circuits (Edwards et al., 2018). Neuroimaging studies reveal changes in the central autonomic network, including the insula and anterior cingulate cortex, linking chronic pain to autonomic imbalance (Ichesco et al., 2012; Loggia et al., 2015).

Stress and Cardiovascular Disease

Prolonged stress dysregulates both the ANS and the hypothalamic-pituitary-adrenal (HPA) axis, producing inflammation, oxidative stress, and immune dysfunction (Juster et al., 2010). Autonomic imbalance—characterized by increased sympathetic drive and reduced parasympathetic activity—is strongly associated with higher cardiovascular risk, arrhythmias, and poor outcomes following cardiac events (Thayer et al., 2010).

Neurodevelopment and Trauma

Children and trauma survivors often display signs of nervous system dysregulation, including sleep disturbances, hyperarousal, and difficulty calming down (Bell et al., 2019). Polyvagal theory provides a framework for understanding these phenomena, suggesting that disrupted vagal tone limits adaptive responses and promotes vulnerability to both emotional and physical illness (Porges, 2011).

Evidence in Horses

Equine Grass Sickness (Equine Dysautonomia)

Equine grass sickness is the most direct example of autonomic nervous system failure in horses. It involves degeneration of autonomic neurons, leading to gastrointestinal hypomotility, weight loss, and colic (Merck Veterinary Manual, 2023). The pathology confirms that nervous system breakdown alone can precipitate systemic disease (Doxey et al., 1991). Long-term survivors show partial neural compensation, highlighting the nervous system’s plasticity (Newton & McGorum, 2019).

Stress and Behavior

Horses exhibit measurable autonomic changes under stress, with heart rate variability (HRV) studies showing reduced adaptability during transport, competition, or social separation (von Borstel et al., 2017). Low HRV reflects poor balance between sympathetic and parasympathetic systems, paralleling findings in human stress research.

Performance and Pain

Autonomic dysregulation can also arise from pain. Haussler (2017) documented a case in which spinal joint dysfunction caused abnormal autonomic patterns observable through thermography, which normalized following chiropractic intervention. This underscores the interplay between local pain and systemic nervous system regulation.

Pituitary Pars Intermedia Dysfunction (PPID)

In older horses, PPID demonstrates how nervous system degeneration drives systemic disease. Caused by neurodegeneration in the pituitary gland, PPID results in laminitis, muscle wasting, abnormal hair growth, and immune dysfunction (McFarlane, 2007). It exemplifies how nervous system imbalance can affect endocrine, metabolic, and musculoskeletal health.

Everyday Signs

Beyond clinical disease, subtle nervous system dysregulation may present as persistent tension, digestive upset, poor learning focus, or inadequate recovery after exercise. These early indicators mirror human experiences of chronic stress and warrant attention in horse management.

Comparative Insights

Cross-species analysis reveals shared principles:

1. Balance is central — health depends on smooth shifts between sympathetic and parasympathetic activity.

2. Dysregulation is an early warning — subtle signs often precede full disease.

3. Feedback loops matter — stress fuels dysregulation, which worsens illness, which in turn increases stress.

4. Plasticity exists — both humans and horses can recover or adapt with appropriate interventions.

These parallels suggest that studying nervous system dysregulation in one species can inform treatment and prevention strategies in the other.

Conclusion

Nervous system dysregulation is a powerful, cross-species driver of disease. In humans, it underlies chronic pain, cardiovascular illness, stress-related disorders, and neuroendocrine problems. In horses, it is central to equine grass sickness, PPID, pain-related performance issues, and stress-linked behaviors.

Recognizing the nervous system not only as a responder but as a root cause of disease opens new avenues for prevention, early detection, and therapy. Supporting nervous system balance—through stress management, pain relief, and targeted medical or rehabilitative interventions—represents a critical step in advancing health for both people and horses.

References

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