The Human Nervous System

When determining deception, we pay close attention to a person’s nonverbal behavior—emotions, gestures, body posture, and, of course, changes in the body’s functioning when presented with various stimuli. In previous articles, we have often discussed the signs of lying that can be observed thanks to the autonomic nervous system, and we have even described the specific functions of this system. In this article, we will not focus heavily on lie detection cues; instead, we will take a closer look at the human nervous system itself to provide a more complete understanding of why the autonomic nervous system is so essential in the process of detecting deception.

The functioning of the autonomic nervous system is extremely important in lie detection, both in non-instrumental methods and in instrumental ones (such as using a polygraph). During a polygraph examination, we can observe even the slightest changes in the body’s functioning—changes that a person may not even be able to feel. Of course, in non-instrumental lie detection, it is impossible to catch every minor change, since we rely only on what we can see and hear, but nevertheless, we can still observe many changes in the body’s functioning. These observations are very useful for drawing further conclusions.

Basic Structure of the Nervous System

To understand why the autonomic nervous system is so important and what it is responsible for, let’s take a closer look at the human nervous system as a whole. However, we won’t go into every detail and nuance, as our goal is not to explore every aspect of how the nervous system works. What matters is understanding the basic principles.

The human nervous system is divided into the central and peripheral nervous systems. The central nervous system includes the brain and spinal cord. The peripheral nervous system consists of the somatic and autonomic nervous systems. Our main interest is the autonomic nervous system, which affects all organs in three ways: functional (speeding up or slowing down), trophic (metabolism), and vasomotor (hormonal regulation and homeostasis). The autonomic nervous system is made up of two divisions: the sympathetic and parasympathetic systems.

The Autonomic Nervous System in Detail

The main difference between the two divisions of the autonomic system is that the sympathetic nervous system tends to activate as a whole, causing reactions throughout the body. It is typically triggered in situations of fear, attack, or flight, as well as during intense physical activity. The symptoms are well known: dilated pupils, dry mouth, rapid heartbeat, sweating, and strong emotions. In contrast, the parasympathetic nervous system is more selective in its activity.

Functions of the Sympathetic and Parasympathetic Systems

• Sympathetic Nervous System: This system is dominant during the daytime, roughly from 9 a.m. to 6 p.m. It is responsible for arousing the body and operates during wakefulness. It controls catabolism—the burning of nutrients in cells—with the help of adrenaline. Under stress, all processes governed by the sympathetic nervous system speed up: heart rate and strength increase, breathing becomes faster and deeper (bronchi dilate), and sweat glands are activated to cool the body.
• Parasympathetic Nervous System: This system is dominant after 6/8 p.m. and until about 9 a.m. It is responsible for calming the body and operates during sleep and rest. It controls anabolism—the accumulation of nutrients. Under stress, it acts in a targeted way as a safeguard: heart rate and strength decrease, and as adrenaline levels drop, the salivary glands begin to secrete enzymes, eliminating dry mouth.

Understanding these basic principles of the nervous system, especially the autonomic nervous system, helps explain why it is so crucial in the process of lie detection and in interpreting human behavior in general.