Difference Between Sympathetic and Parasympathetic Nervous System

What is the difference between sympathetic and parasympathetic nervous system? The sympathetic nervous system is responsible for alterations in the body in situations of stress or emergency. Thus, it leaves the individual in a state of alert, prepared for fight and escape reactions.

The parasympathetic nervous system has the function of making the organism return to the state of calm in which the individual was before the stressful situation.

  Sympathetic nervous system Parasympathetic nervous system
Definition The sympathetic nervous system is the first of the two divisions of the autonomic nervous system. The parasympathetic nervous system is the second of the two divisions of the autonomic nervous system.
Function Responsible for preparing the organism to respond to stress and emergency situations. Responsible for making the body return to a stable and calm emotional state, in addition to controlling some systems and non-conscious actions, such as breathing.
Main activities Increase heart rate and blood pressure, release adrenaline, contracting and relaxing muscles, dilating the bronchi, dilating the pupils, increasing sweating. Decrease the heart rate, decrease blood pressure, decrease adrenaline, decrease the amount of blood sugar, control the size of the pupils.

Both are parts of the autonomic nervous system. This is responsible for the spontaneous actions of the body, such as respiration, heartbeat, digestion, control of body temperature, among several other functions, administered by the sympathetic and parasympathetic systems.

What is the sympathetic nervous system?

Also known as sympathetic Ortossystem or thoracolumbar system, the sympathetic nervous system prepares the body to cope with stressful or emergency situations. In this sense, in times when the brain perceives a danger, the sympathetic nervous system comes into action.

During stress or emergency, the system will trigger a number of internal actions, such as increasing the heartbeat, releasing adrenaline, increasing blood pressure, contracting some muscles and relaxing others. All this for the individual to respond to the situation in order to get rid of the discomfort or danger.

There are still other physical changes at times when the sympathetic nervous system is triggered. These include:

  • Dilatation of the bronchi passages for greater oxygen retention;
  • Pupil dilation, to improve the sense of vision;
  • Contraction of blood vessels;
  • Increased esophageal contraction;
  • Sweating.

Other body responses are the sensation of heat and cold, as well as pain.


The sympathetic nervous system is triggered in stressful or emergency situations.So, if a person is crossing a street or avenue and realizes that a car is advancing towards it, the system is triggered so that the individual can handle the possible accident.

In this case, if there is time, the body will undergo all the changes described above and, in impulse to save one’s life, act to not be struck by the vehicle.

Except for the danger, it’s time for the parasympathetic nervous system to come into action.

What’s a parasympathetic nervous system?

In the opposite of the sympathetic nervous system, the parasympathetic is responsible for making the organism calm after a stressful or emergency situation. Thus, it will decrease the heartbeat, blood pressure, adrenaline and the sugar present in the blood.

In addition to calming the body and returning it to a stable emotional state, the parasympathetic nervous system also has the function of conserving the energy of the organism. Along with the sympathetic system, it also controls the pupils.

However, the parasympathetic nervous system does not only serve to calm down after a stressful situation. The system controls several areas of the human body and involuntary actions, which do not happen consciously.

  • Cardiovascular system;
  • Excretory system;
  • Digestion
  • Sexual excitation;
  • Breath.


It is said that the individual realized that a car was coming towards him, and then had the sympathetic nervous system activated in order to act to save his own life, escaping from the accident unharmed. That’s when the parasympathetic system will go into action.

After understanding that there is no more danger, the organism will trigger the parasympathetic system. This to make the heart rate return to normal, decrease the adrenaline and return the body to the State before the stress situation.

Without the autonomic nervous system (divided into sympathetic and parasympathetic nervous system), the human being would not be able to respond to dangerous situations. Fight and escape reactions are essential and have helped mankind for thousands of years, since the beginnings of evolution.