The Anatomy of the Autonomic Nervous System

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The nervous system is one of the most incredible parts of the human body. Your nervous system takes in all the information in the world around you and sends a message to your muscles, allowing you to make your way through the world. Your autonomic nervous system also controls all of your vital functions, many of which you aren't consciously aware of. In short, it keeps you alive.

Human nerve cells
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While it might feel like a disservice that such an important part of your body is under-recognized by design, 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 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. In order to fully understand disease and health, it's important to know how the autonomic nervous system works.

Autonomic Nervous System Anatomy

Your 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 parasympathetic is associated with relaxation, digestion, and generally taking it easy. The sympathetic is responsible for the "fight or flight" 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.

Function of the Autonomic Nervous System

Since the autonomic nervous system is divided into two parts, its function varies based upon which part of the system you are looking at. The parasympathetic system performs basic housekeeping and controls things when you are at rest. The sympathetic system is the emergency system and performs life saving flight or fight responses.

The Parasympathetic

Many nerves of the parasympathetic 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 are what causes your eyes to tear and your mouth to salivate. Other parasympathetic terminate in the walls of thoracic and abdominal organs like the esophagus, gastrointestinal tract, pharynx, heart, pancreas, gallbladder, kidney, and ureter. The sacral parasympathetic 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 part of your 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 your eyes in response to darkness or a threat 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. These supply nerves to the abdominal and pelvic visceral organs, as well as hair follicles, sweat glands, and more.

Autonomic Neurotransmitters

The nervous systems communicate by chemical messengers called neurotransmitters. Neurotransmitters like acetylcholine and norepinephrine are primarily responsible for communication in your 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 has subcategories with their own unique properties and can be manipulated by different types of 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 your pumping heart, and the narrowness of the blood vessels in your body. When the sympathetic nervous system dominates, your heart pumps hard and quickly, your peripheral blood vessels are narrow and tight, and your 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 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. 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.

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 ANS

For most of us, the autonomic nervous system is generally out of our conscious control. However, the cortex of your brain, normally associated with conscious thought, can change your 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 your autonomic nervous system. 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.

Because the 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. 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.

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