Have you ever stopped to wonder why some human activities require focused concentration while others don’t? For example, if you don’t pay attention while driving your car, you might crash. But are thinking about digesting the last food you ate?
Unless your tummy aches or you are experiencing some gastrointestinal distress, it is unlikely you spend any time considering all the hard work your body is doing to keep you alive.
That’s because of our body’s autonomic nervous system (ANS). This network of nerves regulates the functions of our internal organs (heart, stomach, intestines), breathing, and certain muscles without our having to think about it. The ANS is always on. If it switched off, we would die in short order.
Consider how much more difficult life would be if you had to tell your body to keep inhaling and exhaling or to digest the food we’ve eaten. The ANS operates functions involuntarily (without using our thoughts or willpower) and reflexively (in response to external and internal stimulation).
Normally, the ANS rules our basic body functions during periods of low stress (when we rest and digest) or in an emergency (when we fight or flee).
Two nerve cells are involved in an autonomic nerve pathway. One cell is located in the brain stem or spinal cord. Nerve fibers connect it to the second cell, located in a cluster of nerve cells called an autonomic ganglion.
Ganglian nerve fibers, in turn, connect with the internal organs. Most of the ganglia for the sympathetic division are situated just outside the spinal cord on either side of it. The ganglia for the parasympathetic division are found near, or in the organs, they connect with.
The ANS is divided into two main parts (divisions), the sympathetic and parasympathetic systems. The ANS can be triggered by signals within our bodies or outside them from the environment.
Once a signal is received, the ANS responds by either stimulating a body process, usually through the sympathetic division, or inhibits a physiological response via the parasympathetic division.
The ANS controls both muscles, glands, and internal bodily functions, including:
- Blood pressure
- Heart and breathing rates
- Body temperature
- Metabolism (thus affecting body weight)
- The balance of water and electrolytes (such as sodium and calcium)
- The production of body fluids (saliva, sweat, and tears)
- Sexual response
Sympathetic vs. Parasympathetic
Typically, the sympathetic division prepares the body to deal with stress or an emergency situation. It activates our natural defense system: fight or flight. When we sense danger, our ANS leaps into action, either to combat the danger or run from it.
In the absence of life-threatening stressors, the parasympathetic division regulates all our ordinary, non-emergency, resting and digesting processes.
As you can see from the table below, the parasympathetic division tends to conserve and restore, while the sympathetic division spends and discharges. For example, the sympathetic division will constrict the pupil of the eye, making it smaller, while the parasympathetic division will dilate the eye’s pupil, making it larger.
|Body Structure||Sympathetic Stimulation||Parasympathetic Stimulation|
|Iris (eye muscle)||Pupil dilation||Pupil constriction|
|Salivary Glands||Saliva production reduced||Saliva production increased|
|Oral/Nasal Mucosa||Mucus production reduced||Mucus production increased|
|Heart||Heart rate and force increased||Heart rate and force decreased|
|Lung||Bronchial muscle relaxed||Bronchial muscle contracted|
|Stomach||Peristalsis reduced||Gastric juice secreted; motility increased|
|Small Intestine||Motility reduced||Digestion increased|
|Large Intestine||Motility reduced||Secretions and motility increased|
|Liver||Increased conversion of
glycogen to glucose
|Kidney||Decreased urine secretion||Increased urine secretion|
|Adrenal medulla||Norepinephrine and
The ANS work with the somatic nervous system to keep our internal body processes going without any conscious thought. The somatic nervous system is made up of nerves that connect the brain and spinal cord with muscles controlled by conscious effort (voluntary or skeletal muscles) and with sensory receptors in the skin.
Most of us can be trained to control certain ANS processes, including heart rate and blood pressure. This can come in handy when taking a lie detector test (polygraph) which measures heart rate, blood pressure, respiration, and skin conductivity.
The body uses two chemical messengers (neurotransmitters) to communicate within the autonomic nervous system:
Acetylcholine is “the chief neurotransmitter of the parasympathetic nervous system, the part of the autonomic nervous system (a branch of the peripheral nervous system) that contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows heart rate.” Acetylcholine also stimulates perspiration and makes hair stand on end.
Norepinephrine (also called noradrenaline) is a substance “that is released predominantly from the ends of sympathetic nerve fibres and that acts to increase the force of skeletal muscle contraction and the rate and force of contraction of the heart. The actions of norepinephrine are vital to the fight-or-flight response, whereby the body prepares to react to or retreat from an acute threat.”
Damage to autonomic nerves or the parts of the brain that help control body processes can result in an autonomic disorder. Other autonomic imbalances seem to arise on their own, with no clear cause.
It is known that these health conditions can create autonomic disorders. Some are mild, temporary, and reversible. Others are severe, chronic, and get worse over time:
- Diabetes (the most common cause)
- Peripheral nerve disorders
- Parkinson’s disease
- Other, less common causes include the following:
- Autonomic neuropathies
- Multiple system atrophy
- Pure autonomic failure
- Spinal cord disorders
- Certain drugs
- Disorders of the neuromuscular junction (where nerves connect with muscles), such as botulism and Lambert-Eaton syndrome
- Certain viral infections
- Injury to nerves in the neck, including that due to surgery
Symptoms of autonomic disorders include:
- Lowered blood pressure to the point where a person feels dizzy or light-headed after standing up.
- Reduced or no sweating
- Intolerance to heat
- Dry eyes and mouth
- Feeling prematurely full or vomiting
- Urinary incontinence (loss of bladder control because it is overactive)
- Urinary retention (due to an underactive bladder)
- Constipation or diarrhea (over- or under-performing bowels)
- Pupils that fail to respond (constrict or dilate) to light changes
- Erectile dysfunction (in men, this may be an early indicator of an autonomic disorder)
Anyone who thinks they have an autonomic disorder should seek professional help for testing and evaluation. One such test measures the patient’s blood pressure and heart rate while lying down or sitting and after standing to see how blood pressure changes when the body’s position is changed.
Such an autonomic disorder is called orthostatic hypotension. It is a type of orthostatic intolerance caused by a sudden drop in blood pressure after standing up.
Although there are prescription medicines available to treat the symptoms of autonomic disorders, there are many things we can do to help ourselves prevent and ease orthostatic hypotension – notably, adjusting the diet, increasing exercise, and making certain lifestyle changes:
- Elevate the head of the bed
- Drink enough fluids
- Add salt to the diet
- Wear compression stockings to prevent blood pooling in the legs
- Change physical positions slowly
- Take medications such as midodrine
Nerve damage is hard to cure and is hard for both the patient and family. My father died after 12 years of steady deterioration from Parkinson’s disease. At the end, his entire right side was shriveled and crippled. He could hear and think but no longer speak. He was fed through a tube and a nurse gave him drops of liquid morphine to reduce the pain.
The best thing we can do is get an early diagnosis and begin treatment of the underlying condition. This can help slow the progression of some autonomic diseases and reduce their symptoms.
Improved quality of life is the goal, no matter how severe the autonomic disorder.