Sympathetic Nervous System

Sympathetic nervous Graphics courtesy of PhysiopediaOpens in new window

The sympathetic nervous (SNS) is a division of the autonomic nervous system (ANS). Neurons in the ANS monitor the organs and internal activities such as heart rate, digestion, breathing, energy mobilization, and glandular activity.

The ANS regulates these internal body functions to maintain the body’s homeostasis (balance). The SNS and the other primary division of the ANS, the parasympathetic nervous system (PNS), immediately respond to environmental circumstances and work together to achieve the body’s homeostasis.

The SNS and PNS have different yet complementary functions: the SNS is involved in the functioning of the active body, whereas the PNS is involved in the functioning of the body at rest. Both systems operate automatically without the involvement of human consciousness.

The SNS originates in the thoracic and lumbar (middle and lower) regions of the spinal cord. Most sympathetic neurons are part of the peripheral nervous system. The peripheral nervous system mainly controls the functioning of internal organs and muscles in the periphery of the body. Through chains of sympathetic ganglia (nerve complexes), sympathetic neurons of the spinal cord connect to peripheral sympathetic neurons. This connection leads to the physiological reactions throughout one’s body.

sympathetic and parasympathetic nervous system Diagram courtesy of Backyard BrainsOpens in new window

The primary function of the SNS is to prepare the body for action and stress. Known as the fight-or-flight system, the SNS is active in a state of arousal or emergency.

Epinephrine and norepinphrine, hormones secreted by the adrenal glands (which are controlled by the SNS), help produce general arousal and emotional reactions. Other actions that occur when the SNS is aroused are increased heart rate, increased blood pressure, release of glucose by the liver (so that glucose may be used as an energy source), slowing down of digestion, dilation of the pupils, and other effects. Most of the physiological changes found in the state of arousal arise from SNS activity.

Experiencing a negative emotion (e.g., anger, sadness, fear) typically involves activation of the SNS; the stronger the emotion, the more intense the fight-or-flight response can be. In contrast, a negative emotion that appears to be associated with a different physiological response—that is, activation of the PNS—is disgust (Levenson, Ekman, & Friesen, 1990). The physiology of positive emotionsOpens in new window may also differ from the general physiology of negative emotionsOpens in new window. For instance, happinessOpens in new window is associated with both sympathetic and parasympathetic nervous system response (Levenson et al., 1990).

See also:
  1. Levenson, R.W., Ekman, P., & Friesen, W.V. (1990). Voluntary facial action generates emotion-specific autonomic nervous system activity. Psychophysiology, 27, 363 – 384.