The Anatomy of the Autonomic Nervous System

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Your nervous system helps you regulate your voluntary and involuntary actions, as well as thinking, communicating, and memory. Your autonomic nervous system is the aspect of the nervous system that controls all of your vital functions, like breathing, digestion, and heart rate—many of which you aren't consciously aware of. In short, it keeps you alive.

Human nerve cells
Ian Cuming / Getty Images

It's probably a good thing that your autonomic nervous system is out of your conscious control. If you fall when learning to walk, you may temporarily injure yourself, but you generally learn how to pick yourself up and start again. Can you imagine if you had to learn how to speed up your heart whenever you needed to? Or if you stopped breathing every time you forgot to breathe?

While few diseases attack the autonomic nervous system alone, almost all medical disorders have some impact on autonomic functions.

Autonomic Nervous System Anatomy

Your autonomic nervous system includes a craniosacral parasympathetic portion and a thoracolumbar part sympathetic portion. These are sometimes thought of as being opposite to each other, ultimately striking a balance within the body.

The sympathetic and parasympathetic functions:

  • The parasympathetic is associated with rest and digestion. Its main function is to conserve the body’s energy and to help you sleep or break down and absorb the food you eat.
  • The sympathetic is responsible for the "fight or flight" response that helps you quickly use your body's energy in an emergency situation—like running away from danger.

The nerves of the autonomic nervous system synapse in a clump of nerves called a ganglion before the message is transmitted to the target organ, such as a salivary gland. This allows for another level of communication and control.

Function of the Autonomic Nervous System

The autonomic nervous system has many functions. The parasympathetic system performs basic housekeeping and controls things when you are at rest. The sympathetic system is the emergency system and helps you carry out life-saving flight or fight responses.

The Parasympathetic

Many nerves of the parasympathetic portion of the autonomic nervous system begin in the nuclei in your brainstem. From there, they travel through cranial nerves such as the vagus nerve, which slows the heart rate, or the oculomotor nerve, which constricts the pupil of the eye. Parasympathetic stimulation also causes your eyes to tear and your mouth to salivate.

Other parasympathetic nerves terminate in the walls of thoracic and abdominal organs like the esophagus, gastrointestinal tract, pharynx, heart, pancreas, gallbladder, kidney, and ureter. The sacral parasympathetic nerves synapse in ganglia in the walls of the colon, bladder, and other pelvic organs.

The Sympathetic

Sympathetic fibers of the autonomic nervous system exit the lateral (side) part of your spinal cord. They receive information from parts of the brain such as the brainstem and the hypothalamus.

Fibers run from synapses in ganglia just outside the spinal column to their targets, usually along blood vessels. For example, the sympathetic nerves that dilate your pupils exit the spinal cord in your neck and synapse in the ganglion called the superior sympathetic ganglion, they then run along the carotid artery to your face and eye. The sympathetic nervous system supplies nerves to the abdominal and pelvic visceral organs, as well as hair follicles, sweat glands, and more.

Autonomic Neurotransmitters

The nervous system communicates through chemical messengers called neurotransmitters. Neurotransmitters like acetylcholine and norepinephrine are primarily responsible for communication in your autonomic nervous system.

  • In both the parasympathetic and sympathetic parts of the autonomic system, acetylcholine is released at the level of the ganglia.
  • Acetylcholine receptors in ganglia are nicotinic and may be blocked by drugs such as curare.
  • In the parasympathetic nervous system, postganglionic receptors in organs such as the gastrointestinal tract are called muscarinic and are susceptible to drugs such as atropine.
  • The post-ganglionic sympathetic neurons release norepinephrine. The norepinephrine released by the post-ganglionic neurons binds to adrenergic receptors. There are two main categories of adrenergic receptors, alpha, and beta, each of which has subcategories with their own unique properties and can be manipulated by different types of medication.

Neurotransmitters of the autonomic nervous system mediate important functions of the body, and these actions can also be regulated by medications that inhibit or stimulate these actions.

Blood Pressure Control

Blood pressure is a good example of how the sympathetic and parasympathetic components of the nervous system work together within the body. In general, there are two main things that cause blood pressure to go up: The speed and force of your pumping heart, and the narrowness of the blood vessels in your body.

  • When sympathetic nervous system activity dominates, your heart pumps hard and quickly, your peripheral blood vessels are narrow and tight, and your blood pressure will be high.
  • The parasympathetic system slows the heart and widens peripheral blood vessels, causing the blood pressure to fall.

Imagine that you stand suddenly after having been in a seated position for a long time. Receptors in blood pressure walls at the carotid sinus and aortic arch sense the change in pressure and send messages to the brainstem, which responds appropriately by increasing your blood pressure.

In other cases, you may need your blood pressure to rise because you are, say, terrified by an angry bear and you need quick energy to be able to run away. Even before you start to run, your brain has recognized the bear and sent messages to your hypothalamus to prepare your body to spring into action. Sympathetics are activated, the heart starts pounding, and the blood pressure begins to rise, providing you with oxygen and glucose to power your muscles so you can run as fast as possible.

While there are other systems that can control blood pressure, such as hormones, these tend to be gradual and slow, not immediate like those controlled directly by your autonomic nervous system.

Control of the Autonomic Nervous System

Most of the time, your autonomic nervous system works very well. However, the fight or flight response may become activated with small everyday stresses, releasing a lot of the stress hormone cortisol and driving your blood pressure and heart rate up unnecessarily.

For most of us, the autonomic nervous system is generally out of our conscious control. In the brainstem, the nucleus tractus solitarius is the main command center for the autonomic nervous system, sending input largely through cranial nerves IX and X.

The cerebral cortex of your brain, normally associated with conscious thought, can change your autonomic nervous system to some degree—usually involuntarily, but sometimes voluntarily. In the cerebral cortex, the insula, anterior cingulate cortex, substantia innominata, amygdala, and ventromedial prefrontal cortex are areas that help you understand the events that are going on around you, as well as your emotions. These regions communicate with your hypothalamus to impact the actions of your autonomic nervous system.

Voluntary Control of the Autonomic Nervous System

Because the cerebral cortex is linked to the autonomic nervous system, you may be able to control your autonomic nervous system through conscious effort, especially with some practice. Practices like yoga, mindfulness, and meditation can help you manage your physical autonomic nervous system activity.

Highly trained people, such as advanced yoga practitioners, may be able to intentionally slow their heart rate or even control their body temperature. Mindfulness and meditation can have similar effects.

For most of us, though, focusing on things that are relaxing rather than stressful, or just taking a slow, deep breath when you notice that you're feeling anxious or your heart is racing can bring your autonomic nervous system back into a degree of control.

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  • Adams and Victor's Principles of Neurology, 9th ed: The McGraw-Hill Companies, Inc., 2009.
  • Blumenfeld H, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002.

By Peter Pressman, MD
Peter Pressman, MD, is a board-certified neurologist developing new ways to diagnose and care for people with neurocognitive disorders.