Module 5 - Polygraph Testing & Forensic Document Analysis

Stimulation of the Sympathetic Nervous System

An example of sympathetic nerve stimulation occurs when a person walking across a street suddenly realizes an approaching vehicle is honking loudly as it speeds towards him or her.  The sudden approach of this loud speeding car initiates a sympathetic nervous system response or a fight or flight response in this person.  Specifically, heart rate and blood pressure increase, breathing rate increases, the breakdown of glycogen into glucose increases suddenly, the pupils of the eyes dilate, perspiration increases, and digestive activities decrease.  All of these physiological activities help the body deal effectively with the stress to which it is exposed. (See the table below.)

People often call the polygraph instrument a lie detector; however, it is more accurate to call it simply what it really is& a stress monitor.

- Anonymous

Interestingly, the body becomes stressed when the person is lying.  The stress involved in being deceptive or lying can also trigger the sympathetic nervous system. Perhaps this changes is not to the same degree as a car speeding towards one, but it is a stress response nonetheless.  The stress responses that can be easily identified and monitored heart rate, blood pressure, breathing rate, pupil dilation, perspiration are analyzed and interpreted by the polygraph instrument and the trained polygraph expert.

When the stressful situation is over, the parasympathetic nervous system takes over from the sympathetic nervous system.  The parasympathetic nervous system allows the body to rest and digest by allowing the body to save energy by lowering the heart rate, blood pressure, and breathing rate, allowing the pupils and perspiration rate to return to normal, inhibiting the breakdown of glycogen, and allowing the digestive tract to properly digest food.

Did you know?

A very awkward thing can happen when an individual becomes very stressed. If the urinary bladder contains urine, the person may involuntarily urinate or "pee his pants'!  This occurs because the sympathetic nervous system diverts blood to the limbs and away from the sphincters that hold urine within the bladder. The sphincters relax, and the persons has difficulty holding urine present in the urinary bladder.