The autonomic nervous system is a segment of the peripheral nervous system that controls unconscious actions. The autonomic nervous system controls the cardiovascular system, digestive system, urinary system, and reproductive system.

Among many processes, the functions of the autonomic nervous system include the regulation of heart rate, body temperature, blood pressure, breathing, and respiration. The autonomic nervous system also facilitates digestion, and controls urination and defecation. It’s responsible for stimulating sexual responses and hormone secretion.

The autonomic nervous system works with the central nervous system to control homeostasis. The autonomic nervous system largely works with primitive parts of the brain in the brainstem, including the medulla. Synapses between neurons and nerve fibers in the brainstem connect with those in the spinal cord to send autonomic messages through the body. They also interact with sensory neurons, or afferent neurons, to send information back to the brain.

Neuroscience shows that the autonomic nervous system uses acetylcholine (ACh) as its primary neurotransmitter. This neurotransmitter targets receptors on the axons of smooth muscle, like cardiac muscle, and follow vasoconstrictor pathways to various parts of the body.

The autonomic nervous system consists of three parts. Each division comprises ganglion, groups of ganglia or nerve bodies that work together to perform a function. The three divisions include:

• Sympathetic nervous system: The sympathetic nervous system regulates the body’s fight-or-flight responses. These responses might include increasing heart rate and blood flow, dilation of blood vessels, activation of sweat glands, urination, and secretion of hormones, such as epinephrine or adrenaline and norepinephrine. Its outflow to the body is located in the thoracic and lumbar areas of the spinal cord.
• Parasympathetic nervous system: The parasympathetic division handles regular body functions and maintains homeostasis. This system includes the sacral spinal cord and cranial nerves that connect the brain to organs throughout the body. The vagus nerve, a cranial nerve, branches into vagal trunks and connects to many organs, including the heart, lungs, kidneys, esophagus, and gallbladder. Other parasympathetic outflows include the oculomotor nerve, facial nerve, and glossopharyngeal nerve.
• Enteric nervous system: The enteric nervous system controls gastrointestinal function. It regulates the viscera or internal organs of the digestive tract that run from the mouth through the anus. The enteric division works closely with the sympathetic and parasympathetic divisions to control all aspects of digestion.

Autonomic dysfunction, also known as dysautonomia or autonomic neuropathy, is a condition in which basic functions of the autonomic nervous system are unable to work properly. Nerve damage interferes with messages between the brain and the autonomic nervous system, leading to the inability to regulate functions like body temperature and heart rate. Conditions and diseases including hypertension, diabetes, Lupus, Sjogren’s syndrome, and Chron’s disease can cause dysautonomia, and some symptoms may be chronic.