The autonomic nervous system (ANS) is a part of the peripheral nervous system that controls numerous body systems and does so mostly outside of conscious awareness. The ANS is divided into two branches: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS)[1]. The operation of the SNS and PNS branches of the ANS work in a complementary manner to control vital functions and maintain homeostasis.
Sensory signals from throughout the body enter the homeostatic centers of the brain in the hypothalamus, pons and medulla. These centers monitor the signals and act to regulate functions such as body temperature, blood pressure and water balance. Homeostatic responses that require behavioral responses such as drinking and eating involve brain regions involved in motivation and movement. In addition to input via physical sensors, input can also come by way of cerebral and limbic systems, meaning that cognition and affect can influence autonomic responses. The ANS works together with the CNS to monitor vital bodily parameters and works via communication with the endocrine and immune systems to maintain homeostasis.
We can thank the operation of the ANS every time we stand up from a sitting or lying position. Blood pressure would fall precipitously and consciousness would likely be lost were it not for the quick compensatory action of the SNS.
SNS activity is typically associated with bodily responses that prepare the body to respond to threats to homeostasis, and PNS activity is associated with restorative functions. A dramatic example of the SNS in action is during a fight-flight response in which the individual is faced with a severe threat and a body-wide response is mounted to prepare the body to either fight or flee. Among other measures, heart rate quickens, pupils dilate, glucose is released into the bloodstream, digestion slows, and sensitivity to pain is decreased.
Most organs of the body are innervated by both branches of the ANS although a notable exception are the sweat glands. This fact is exploited in the use of skin conductance methods as a measure of sympathetic specific activity.
All pathways (with one exception) of both the sympathetic and parasympathetic systems consist of 2 neurons in series, one that originates in the spinal cord (preganglionic) and another that ends at target tissues, with the preganglionic neuron synapsing at ganglia.
The sympathetic and parasympathetic branches differ based on where they emanate from the spinal cord as well as on the locations of the autonomic ganglia at which they synapse. Most sympathetic pathways originate in a cluster within the thoracic and lumbar portions of the spinal cord and the ganglia at which preganglionic neurons synapse are located mainly in regions that run down both sides of the spinal column. Long nerve fibers (axons) run from these ganglia to the various target organs. Spinal preganglonic neurons originate primarily from two locations: in the brain stem and along the lower (sacral) portion of the spinal cord. Unlike sympathetic pathways in which their ganglia are close to the spinal cord, with long postganglionic axons extending out to target tissues, preganglionic fibers of the parasympathetic system extend out to ganglia that are close to the target tissue. The main parasympathetic tract is the vagus nerve, which contains about 75% of all parasympathetic fibers.
The two neurotransmitters primarily involved in ANS pathways are acetylcholine and norepinerphrine. In almost all cases, preganglionic neurons secrete acetylcholine onto nicotinic cholinergic receptors at the ganglia. Things diverge at the postganglionic neurons, however, where postganglionic sympathetic neurons secrete norepinephrine onto adrenergic receptors and postganglionic parasympathetic neurons secrete acetylcholine onto muscarinic cholinergic receptors.
Advantages of ANS measures
Typically, when we want to know how people think or feel, we simply ask them. But these self-reports are susceptible to a wide range of biases not the least of which is the fact that humans are very often not even aware of how they are affected by various stimuli or situations.
ANS activity can be influenced by a wide range of physical factors such as posture, sleep, and temperature, but for psychologists the most intriguing fact is that a wide range of mental factors such as emotion, attention, and motivations can also influence ANS activity. What this means is that ANS responses can provide a window into the vast array of mental processes that are usually out the reach of consciousness.
ANS measures have several important advantages: 1) they are not susceptible to self-report biases, 2) they can be obtained without interrupting individuals or interfering with normal activities, 3) they can indicate situational effects on the body of which individuals are unaware, 4) they can reveal emotional states and preferences before individuals are able to report these, and 5) patterns of ANS responses can be used to ascertain risk of different mental and physical health conditions.
There are a number of methods by which ANS activity can be measured. The one that will be discussed here is through electrodermal activity (EDA)[2], the measurement of electrical activity at the surface of the skin.
The fingers, palms of the hands and soles of the feet are replete with eccrine sweat glands and these glands are innervated by sympathetic branch fibers but not parasympathetic branch fibers. In fact the skin is one of the only organs not innervated by both branches of the ANS. This fact means that EDA measures can be assumed to reflect SNS activity specifically. Arousal fo the SNS provokes sweat gland activity, making the hands more sweaty, and thereby reducing electrical resistance and increasing conductance. Given that EDA provides a good measure of SNS responses and heart rate variability provides a good measure of PNS activity, it is recommended that these two measures be obtained together in order to produce a complete picture of ANS function.
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Two methods of assessing EDA are skin conductance and skin potential. Skin conductance involves passing a small electrical current through the skin and measuring how easily the current flows from one electrode to another. Skin potential involves no externally applied current and involves only measuring the magnitude voltage between two electrodes. In addition, EDA can be assessed in response to particular events where the time frame is on the order of seconds, or across longer periods of time, over several minutes. These 2 types and 2 timeframes of EDA can be combined into 4 measures: skin conductance response (SCR), skin conductance level (SCL), skin potential response (SPR), and skin potential level (SPL).
Measuring EDA
The typical EDA setup is the placement of sensors on the fingers, palms or soles of the feet, where eccrine sweat glands are most dense. For skin conductance, it is recommended that electrodes be placed on 2nd and 3rd fingers or 4th and 5th fingers. For skin potential, one electrode is place on a finger or palm of the hand and another electrode is placed at an inactive site such as the forearm. Rapid developments in sensors and electronics is beginning to result in ambulatory devices capable of measuring skin conductance outside the lab.
Notes
[1] There is actually a third branch of the ANS, called the enteric nervous system, which controls the gastrointestinal system but we will constrain our discussion to the SNS and PNS systems as it is the activity of these branches that can be detected though ambulatory monitoring.
[2] You may know EDA by its outdated term, galvanic skin response (GSR).
License Attributions
Fig 1: Sympathetic and Parasympathetic pathways. From Unglaub-Silverthorn (2007), Fig 11-7, Fig 2, p. 382.
Fig 2: Eccrine sweat glands. http://s.hswstatic.com/gif/eccrine-sweat-glands-2.gif.
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