The Equine Sympathetic and Parasympathetic Systems

Explaining the Autonomic Nervous

System in Horses

How the Sympathetic and Parasympathetic Systems Shape Behaviour, Physiology, and Welfare.

Horses, as prey animals, have an autonomic nervous system (ANS) that is exquisitely tuned for survival. The ANS has two major branches:

Sympathetic Nervous System (SNS) — mobilises the horse for action.

Parasympathetic Nervous System (PNS) — restores calm, digestion, and recovery.

These systems are always active, but one tends to dominate depending on context.

The Sympathetic Nervous System (SNS) “Fight, Flight, Freeze, or Fidget”

The SNS is the horse’s emergency response system. It activates when the horse perceives threat, pain, confusion, or high arousal.

The Key Hormones & Neurochemicals

• Adrenaline (epinephrine)- Rapid mobilisation: increased heart rate, respiration, muscle perfusion.

• Noradrenaline (norepinephrine)- Heightened vigilance, tension, readiness to react.

• Cortisol- Sustained stress response, energy mobilisation, learning under pressure.

• Glucose release- Fuels rapid movement

SNS Creating Predictable Behaviour in Horses

Horses show highly consistent SNS-driven patterns:1.

Flight behaviours– Spooking, bolting, spinning. Sudden acceleration. “Scooting” forward or sideways

2. Freeze behaviours– Immobility with high tension. Fixed stare, nostril flare. Elevated head and neck. Shallow breathing

3. Fight behaviours (Often misunderstood and labelled as “naughty”)- Striking, kicking, biting. Rearing. Charging or pushing through pressure

4. Fidget behaviours– A subtle but important welfare indicator: Pawing. Repeated head tossing. Tail swishing. Inability to stand still. “Busy” mouth behaviours.

These are displacement behaviours — the horse is trying to cope with rising SNS activation.

Physiological Signs

• Increased heart rate.

• Increased respiratory rate.

• Sweating.

• Dilated pupils.

• Tight muscles and reduced gut motility (risk factor for colic)

The Parasympathetic Nervous System (PNS)

“Rest, Digest, Learn, and Socialise”

The PNS is the system responsible for recovery and regulation. It dominates when the horse feels safe, comfortable, and pain-free.

Key Hormones & Neurochemicals

• Acetylcholine- Slows heart rate, supports digestion, and promotes relaxation.

• Serotonin- Mood regulation, social bonding, calmness

• Oxytocin- Social bonding, relaxation, affiliative behaviour

• Endorphins- Pleasure, pain relief, relaxation

Predictable Behaviours in Horses

1. Relaxed postures- Soft-looking eyes, blinking. Lowered head and neck. Relaxed lips and jaw. Sighing, blowing, chewing (true relaxation, not conflict behaviours)

2. Social behaviours- Mutual grooming. Standing in close contact. Synchronised grazing. Seeking companionship

3. Learning behaviours- The PNS is essential for: Retaining new information. Problem-solving. Responding to cues with softness. Developing new motor patterns

4. Digestive behaviours- Rhythmic chewing. Gut sounds. Grazing. Healthy manure output

 Physiological Signs- Slower heart rate. Deeper, slower breathing. Warm, relaxed muscles. Active gut motility. Normal salivation

How These Systems Interact in Horses

Horses constantly oscillate between SNS and PNS states. Welfare and training quality depend on: How quickly the horse can return to PNS after stress. Whether the horse becomes “stuck” in SNS activation. Whether the horse is experiencing chronic low-grade stress (cortisol load)

A horse that cannot down-regulate may show: Hypervigilance. Reactivity. Shutdown or learned helplessness. Aggression. Stereotypies (cribbing, weaving).

Why This Matters for Training & Welfare

Horses learn best when: PNS is dominant. They feel safe. Pressure is minimal and predictable. Pain is absent. They have agency and clarity.

Horses struggle when: SNS activation is high. They are confused, over-faced or overwhelmed. Pain is present. The environment is unpredictable. They lack social support.

This is why my ethical horse training focuses on: Reducing threat and increasing clarity. Allowing as much time as needed for emotional decompression. Building predictable patterns and supporting the horse’s ability to return to PNS.

All my training decisions are based on observable arousal levels. https://doi.org/10.1016/j.jveb.2018.10.002

If you understand what you are seeing, avoid labelling the horse with anthropomorphic terms and give the horse full autonomy, then marvellous changes can occur in helping horses learn how to let go of past trauma and open their eyes to trust. They then become trainable because they feel safe, understood and listened to.