**1. Introduction**

Stress, which can be acute or chronic, is a term that has not been strictly defined, especially in the field of neuroscience. Scientists have not yet fully characterized the complete set of neuronal mechanisms that can explain stress. In this article, however, I use the term "stress" because it is the most appropriate one.

Stress is a physiological reaction of an organism to an uncomfortable or unfamiliar physical or psychological stimulus. Stress-inducing stimuli trigger reflex behavior, which results from alteration in the activity of the autonomic nervous system (ANS) and hormones. Reflex behavior includes a heightened state of alertness and increased heart rate. Acute stress is a short-term response that lasts for seconds, minutes, or days. In this article, "stress" refers to acute stress.

Scientists have not yet determined a method that can efficiently quantify stress. In fact, scientists have difficulty objectively defining whether an organism is expressing stress in response to stimuli. Through studies of crustacean hearts using electrophysiological methods, I have discovered that stressful and non-stressful states can be distinguished by heartbeat recordings: i.e., via electrocardiograms (EKGs). The methods used to measure stress in model animals can be applied to humans because model animals and humans are fundamentally similar in terms of autonomic nerve physiology. The autonomic cardio-regulatory function can simply described as the combined action of acceleration and inhibition. In this article, I present empirical results of stress observations in both model animals and humans. The presented data provide convincing evidence that stress can be quantified.

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