In detail

The Autonomic Nervous System: Sympathetic and Parasympathetic System

The Autonomic Nervous System: Sympathetic and Parasympathetic System

The Autonomic Nervous System (SNA) is the part of the nervous system that controls and regulates internal organs like the heart, stomach and intestines, without the need for conscious effort on the part of the body. It is part of the peripheral nervous system (which includes the Somatic Nervous System or SNS and SNA) and for this reason it also controls some of the body's muscles.

It controls functions automatically such as heartbeat, digestion, breathing, sweat, blood pressure, etc.


  • 1 SNA Functions
  • 2 Sympathetic Nervous System
  • 3 Parasympathetic Nervous System

SNA Functions

The autonomic nervous system controls the following internal processes:

  • Blood pressure
  • Heart and respiratory rate
  • Body temperature
  • Digestion
  • Metabolism (which affects body weight)
  • The balance of water and electrolytes (such as sodium and calcium)
  • The production of body fluids (saliva, sweat and tears)
  • Urination
  • Defecation
  • Sexual response

Thus, most organs are controlled by the sympathetic and parasympathetic nervous system. Sometimes both have opposite effects on the same organ. For example, the sympathetic system increases blood pressure while the parasympathetic system decreases it. In general, both systems work together to ensure that the body responds adequately to different situations.

The main function of the SNA is maintain the balance of the internal environment, homeostasis, and control involuntary functions. For this reason it is able to modify the activity of smooth muscles, glands and heart muscle in response to information that comes from higher levels of the brain (especially emotions and stimuli of the environment).

The SNA is composed of two large systems:

  • The sympathetic nervous system
  • The parasympathetic nervous system

Sympathetic Nervous System

The sympathetic nervous system prepares the body for situations that require alertness or strength, such as situations that arouse fear, anger, emotion or shame (situations of "fight or flight"). In these types of situations, the sympathetic nervous system stimulates the heart muscles to increase the heart rate, dilates the bronchi of the lungs (increases oxygen retention) and causes the dilation of blood vessels that supply the heart and skeletal muscles ( increasing the blood supply).

The adrenal medulla is stimulated to release epinephrine (adrenalin) and norepinephrine (norepinephrine), which in turn increases the metabolic rate of cells and stimulates the liver to release glucose in the blood. Sweat glands are prepared to produce sweat. In addition, the sympathetic nervous system reduces the activity of other bodily functions that are less important in emergencies, such as digestion and urination.

Sympathetic activation tends to produce widespread (diffuse) effects, which are usually lasting.

Sympathetic fibers originate in neurons of thoracic and lumbar segments of the spinal cord, in the lateral horn. Therefore, the sympathetic division can also be called thoracicolumbar division. They leave the medulla and travel through thoracic and lumbar spinal nerves, until they reach an interconnected ganglion chain that is parallel and very close to the spinal cord, the sympathetic chain.

In sympathetic ganglia and preganglionic neurons synapt with postganglionic, and release acetylcholine. Postganglionic fibers are distributed very widely and release norepinephrine on effector organs.

Parasympathetic Nervous System

The parasympathetic nervous system is active during periods of digestion and rest. Stimulates the production of digestive enzymes and stimulates the processes of digestion, urination and defecation. It reduces blood pressure and heart and respiratory frequencies, and conserves energy through relaxation and rest.

The neuronal fibers leave the CNS (trunk and medulla) and travel through cranial nerves and sacral spinal nerves (especially the vagus nerve). They reach nodes that are located in the viscera or very close to them; unlike the sympathetic division that synapses between preganglionic and postganglionic neurons in nodes located very close to the medulla, usually the effector organs.

In parasympathetic ganglia, preganglionic neurons synapt with postganglionic and release acetylcholine.

The parasympathetic system stimulates activities that facilitate storage or energy savings. It produces changes aimed at conserving and restoring energy and ensuring long-term well-being (for example, digestion), while the activation of the sympathetic serves to face short-term emergencies.

Both the sympathetic and the parasympathetic system are involved in sexual activity, as are the parts of the nervous system that control voluntary actions and transmit the sensation of the skin (somatic nervous system).


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