Sweaty Paws, Racing Heart: The Neurology and Physiology of Fear

False Alarm or Truly Threatening, It’s All the Same to Your Dog

Dogs exhibit behavior, any behavior, because it achieves results that they believe to be most beneficial to them.

All animals react to a threat, whether it is perceived or real, in one of four ways: freeze, flight, fidget/fool around or fight.  Fear serves an important evolutionary purpose: the ability to survive.  Your dog’s split second reaction time can mean the different between life and death. The trouble with fear is that based on genetics, early socialization or past experience, an individual may exhibit a fear based response disproportionate to the perceived threat.

The word “reaction” is critical because your dog’s fear response is involuntary, which means it is not pre-meditated or even conscious!  Your dog cannot control her sweaty paws, dilated pupils, racing heart, involuntary urination or explosive diarrhea any more than you can when faced with a scary situation. Eating the tastiest treat being offered or performing a “sit” on cue is literally impossible as her body swiftly funnels resources to the organs necessary to defend itself: lungs, heart and muscles.

The Physiology of Fear

Fear, as well as other emotions and emotional memories, are regulated by the limbic system, which in turn governs the autonomic nervous system (ANS). 

The limbic system is housed in several structures sitting on top of the brain stem: the thalamus, hypothalamus, amygdala, and hippocampus. These structures work together to relay sensory information to appropriate areas of the brain, trigger emotions (including anxiety and fear), form long term memories and regulate the ANS by triggering the release of hormones in response to a perceived threat.

The autonomic nervous system is comprised of the sympathetic nervous system (“fight or flight”), the parasympathetic nervous system (“rest and digest”) and (depending on the scientist questioned), the enteric system, which regulates the functions of the gastrointestinal tract. 

The sympathetic nervous system governs the response to a threat by controlling the endocrine system. When the thalamus receives sensory information, it relays that information to the other parts of the limbic system. The hippocampus determines the magnitude of the threat and regulates the response, and even before the sense is fully processed, the amygdala immediately shuts down conscious thought in order to divert all essential processes to survival. Research suggests that this is unconscious or reflexive, bypassing input from the conscious brain. The amygdala instigates the fight or flight response by stimulating the brain stem to release adrenaline and norepinephrine. Adrenaline and norepinephrine instantaneously surge through the body, boosting energy and focus.  Heart rate, blood pressure and respiration increase, as well as blood flow to the muscles, in anticipation of the conflict.  

The amygdala also plays a substantial role in anxiety, the worry that something bad could potentially happen. It is additionally responsible for constructing memories built around fearful experiences, as well as around positive experiences.  

Impulses travel at lightening speed through the limbic system to the hypothalamus, which then fully activates the “fight or flight” response via the pituitary gland and the endocrine system.  The hypothalamus regulates the ANS, and together with the rest of the limbic system, is responsible for the physical manifestations of fear: accelerated heart rate, decreased digestion, dilation of the blood vessels in the muscles, widening of the bronchial passages, increased perspiration, rigid muscles, dilated pupils and raised hackles (piloerection). These physiological responses contribute to a dog (or a human!) being ready run or fight. Your dog cannot consciously control these bodily changes, nor can she disregard the overwhelming need to get away from (or vanquish) the source of her fear. 

Cortisol, also known as the “stress hormone”, is released by the adrenal gland and suppresses non-essential functions like digestion, growth, immune responses and reproductive capabilities. Cortisol peaks twenty minutes after the initial response to a threat. If your dog is fighting for survival, thinking about her next meal or romantic encounter are the last things on her mind, with good reason! Cortisol also activates chemicals that repair tissues and increases a type of sugar called glucose in the bloodstream as energy to boost brain function.

Your dog’s body is primed for action in just a few heartbeats, but what happens when the threat (real or imagined) has passed?

In order to regain balance, or homeostasis, hormone levels are self regulated by a negative feedback loop.  When hormone levels reach peak concentrations, they inhibit their own release, allowing the body to gradually return to baseline. As part of your dog’s functioning stress response, the negative feedback loop regulates the levels of cortisol and reduces the initial surge 50% about twenty minutes after the initial event. Studies suggest that extended periods of stress can inhibit the negative feedback loop and deregulate cortisol production. Cortisol levels have been measured up to four times greater than baseline for several days after the precipitating stressful event.   Prolonged periods of elevated cortisol levels can lead to chronic pain, disease, weight gain, poor sleep patterns, reduced cognitive functions and lower immune function.

Once the threat of danger is over, the parasympathetic nervous system takes over. In contrast to the “fight or flight” sympathetic nervous system, the parasympathetic nervous system regulates “rest and digest”: energy conservation, lower heart rate and respiration, inhibited production of adrenaline, increased digestion, constriction of pupils and contraction of the bladder and intestinal sphincter muscles to move waste out of the body.

The Physical Manifestations of Chronic Stress

If left unresolved, repeated or prolonged exposure to stressors can lead to chronic stress. Physiologically, hormone levels become or remain unbalanced, which affects virtually every system in the body. Outward signs of this imbalance may include:

• Increased activity/arousal levels, pacing, whining, inability to settle, separation distress, disrupted sleep patterns
• Decreased activity including lethargy and/ or excessive sleep, reduced interactions (with people/animals) 
• Inattentiveness, distractibility, confusion, shut down
• Increase or decrease in appetite/thirst
• Nausea, vomiting, gastrointestinal upset
• Eating inedible objects (pica)
• Excessive or obsessive grooming 
• Inappropriate (or excessive or limited) elimination
• Poor coat
• Reactivity
• Hyper-vigilance 
• Slow healing of wounds
• Chronic pain
• Chronic disease
• Premature aging
• Stereotypic behaviors including repetitive circling, vocalization, staring, spinning, grooming, biting oneself, jumping 

How Can You Help?

The physiological and emotional effects wrought by continuous fear are not short-term. Chronic stress can lead to atypical behaviors, pain and chronic illness.  In order to break the cycle of chronic fear, you need to remove the stressors (also known as triggers) from your dog’s life, manage her environment, and modify her behavior.  Depending on the severity of her fears, behavioral medication may be prescribed as an aid in regulating chemical levels in the brain.  The right medication combined with behavior modification can literally save a severely fearful dog’s life.

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