The autonomic nervous system is regulated by integrated reflexes through the brainstem to the spinal cord and organs. Autonomic functions include control of respiration, cardiac regulation (the cardiac control center), vasomotor activity (the vasomotor center), and certain reflex actions such as coughing, sneezing, swallowing and vomiting. Those are then subdivided into other areas and are also linked to autonomic subsystems and the peripheral nervous system. The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving autonomic regulatory input from the limbic system.

The autonomic nervous system has three branches: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system. Some textbooks do not include the enteric nervous system as part of this system.^[8] The sympathetic nervous system is often considered the „fight or flight“ system, while the parasympathetic nervous system is often considered the „rest and digest“ or „feed and breed“ system. In many cases, both of these systems have „opposite“ actions where one system activates a physiological response and the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as „excitatory“ and „inhibitory“ was overturned due to the many exceptions found. A more modern characterization is that the sympathetic nervous system is a „quick response mobilizing system“ and the parasympathetic is a „more slowly activated dampening system“, but even this has exceptions, such as in sexual arousal and orgasm, wherein both play a role.

There are inhibitory and excitatory synapses between neurons. A third subsystem of neurons has been named as non-noradrenergic, non-cholinergic transmitters (because they use nitric oxide as a neurotransmitter) and are integral in autonomic function, in particular in the gut and the lungs.

Although the ANS is also known as the visceral nervous system, the ANS is only connected with the motor side. Most autonomous functions are involuntary but they can often work in conjunction with the somatic nervous system which provides voluntary control.