Autonomic nervous system, in vertebrates, the part of the nervous system that controls and regulates the internal organs without any conscious recognition or effort by the organism. The autonomic nervous system regulates certain body processes, such as blood pressure and the rate of breathing. This system works automatically (autonomously).

The autonomic nervous system has two main divisions:

● Sympathetic

● Parasympathetic

After the autonomic nervous system receives information about the body and external environment, it responds by stimulating body processes, usually through the sympathetic division, or inhibiting them, usually through the parasympathetic division.

Functions of Sympathetic nervous system:

● It is responsible for the response commonly referred to as "fight or flight.

● It prepares the body to react to stresses such as threat or injury.

● It causes muscles to contract and heart rate to increase.

Functions of Para-sympathetic nervous system:

● It is referred to as "rest and digest."

● It controls functions of the body at rest.

● It helps maintain homeostasis in the body.

● It causes muscles to relax and heart rate to decrease.

Para-sympathetic nervous system can be controlled by factors (such as exercise and experience) that can help people exert a level of control over the sympathetic responses. The two systems generally do not function well at the same time. That is, when faced with imminent danger, the body will divert blood flow from parasympathetic nerve functions (such as digestion) to sympathetic nerve functions (such as muscle contraction and heavy breathing to run away).