Sympathetic nervous system

Medical Specialties: Internal medicine, Neurology

Clinical Definition

The sympathetic nervous system is a division of the autonomic nervous system. It consists of nerves, which originate in the thoracic region of the spinal cord and extends to the lumbar area. In response to a stimulus the body senses is dangerous, the neurons of the sympathetic nervous system release norepinephrine and then adrenaline is secreted from the adrenal medulla. The chemicals cause the reaction associated with the fight or flight response, such as increased heart rate, dilated pupils, sweating and increased blood pressure.

In Our Own Words

A part of our nervous system called the autonomic nervous system is devoted to automatic reactions or involuntary things like breathing, sweating, and sleeping. Within the autonomic nervous system, there are two opposing sub-systems that work together: sympathetic and parasympathetic. The sympathetic nervous system has many roles, one of which is kicking us into gear when our brain perceives something dangerous or stressful. A series of chemical reactions occur, which prepare the body for action. Involuntary reactions occur including increased blood pressure and heart rate, dilated pupils and increased energy. This reaction is commonly referred to as the fight or flight response. The response is the body’s way of preparing for an effective and immediate response.

Relevant Conditions
  • Fight or flight
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  • Tortora, Gerald, Derrickson, Bryan. Principals of Anatomy and Physiology. Wiley 2011. Accessed October 2013.
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