At some level, most people recognize that the nervous system takes in all the information in the world around us, and sends messages to our muscles that allow us to make our way through that world. If this first springs to mind when thinking of the nervous system, it's because those are the parts about which we are most consciously aware. But the truth is that our nervous systems do even more than that: the autonomic nervous system also controls vital functions every single second (of which we are generally not aware), and it keeps us alive.
While it somehow feels like a disservice that such an important part of the body should be under-recognized by design, it's probably a good thing that the autonomic nervous system is out of our 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 fell asleep?
Like many things taken for granted, the significance of the autonomic nervous system is suddenly recognized when something goes wrong. While few diseases attack the autonomic nervous system alone, almost all medical disorders have some impact on the autonomics. It's impossible for us to fully understand disease or health without a basic grasp of how the autonomic nervous system works.
Autonomic Nervous System Anatomy
The autonomic nervous system lies almost entirely outside of the central nervous system and involves two main parts: the craniosacral part (parasympathetic), and the thoracolumbar part (sympathetic). These are sometimes thought of as being opposite to each other, ultimately striking a balance within the body. The parasympathetics are associated with relaxation, digestion, and generally taking it easy. The sympathetics is responsible for the " response.
One of the interesting things about the autonomic nervous system is that, almost without exception, the nerves 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.
Many nerves of the parasympathetic autonomic nervous system begin in nuclei in the 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. Parasympathetics also cause tearing in the eye and salivation in the mouth. Other parasympathetics terminate in the walls of thoracic and abdominal organs like the esophagus, gastrointestinal tract, pharynx, heart, pancreas, gallbladder, kidney and ureter. The sacral parasympathetics synapse in ganglia in the walls of the colon, bladder and other pelvic organs.
Sympathetic fibers of the autonomic nervous system exit the lateral part of the spinal cord where 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 the eye in response to darkness or a threat exit the spinal cord in the neck and synapse in the ganglion called the superior sympathetic ganglion, then run along the carotid artery to the face and eye. These innervate (supply nerves to) the abdominal and pelvic visceral organs, and also hair follicles, sweat glands, and more.
The nervous systems communicate by chemical messengers called neurotransmitters. Neurotransmitters like acetylcholine and norepinephrine are primarily responsible for communication in the autonomic nervous system. For both parasympathetic and sympathetic parts of the autonomic system, acetylcholine is released at the level of the ganglia. The acetylcholine receptors in ganglia are nicotinic and may be blocked by drugs such as curare. Neurotransmitters differ, though, when the nerve cells reach their targets.
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.
In contrast, the post-ganglionic sympathetic neurons only release norepinephrine, with the exception of sweat glands and some smooth muscle on blood vessels, in which acetylcholine is still used. The norepinephrine released by the post-ganglionic neurons hit a group of receptors called the adrenergic family of receptors. There are two main categories of adrenergic receptors, alpha and beta, each of which have subcategories with their own unique properties, each of which can be manipulated by different types of prescribed medication.
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 the pumping heart, and the narrowness of the blood vessels in the body. When the sympathetic nervous system dominates, the heart pumps hard and quickly, peripheral blood vessels are narrow and tight, and the blood pressure will be high. In contrast, the parasympathetic system slows the heart and opens 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. Two receptors sense pressure in blood pressure walls at the carotid sinus and aortic arch, and send messages to the brainstem, which responds appropriately with an autonomic reflex that increases the blood pressure.
In other cases, you may need your blood pressure to rise because you are, say, terrified by an angry bear. Even before you start to run, your brain has recognized the bear and sent messages to the hypothalamus to prepare the body to spring into action. Symapthetics are activated, the heart starts pounding, and the blood pressure begins to rise.
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 the autonomic nervous system.
How to Control Your Autonomic Nervous System
While for most of us the autonomic nervous system is generally out of our conscious control, the truth is that the cortex of the brain, normally associated with conscious thought, is able to change the autonomic nervous system to some degree. In the cerebrum, the insula, anterior cingulate cortex, substantia innominata, amygdala and ventromedial prefrontal cortex communicate with the hypothalamus to impact the ANS. In the brainstem, the nucleus tractus solitarius is the main command center for the ANS, sending input largely through cranial nerves IX and X.
Because the cortex is linked to the ANS, the autonomic nervous system may be controlled through conscious effort, especially with some practice. Highly trained people, such as advanced yoga practitioners, may be able to intentionally slow their heart rate or even control their body temperature through meditative practices. For most of us, though, focusing on things that are relaxing rather than stressful, or just taking a large breath when you notice your sympathetic nervous system is causing a fast pulse or anxious feeling, can bring your parasympathetic nervous system back into a degree of control.
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.