9.11 Peripheral Nerves - Autonomic Sympathetic/Parasympathetic

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11. The Peripheral Nerves, it includes Autonomic Nervous System (ANS).
 Update: 06 Septr 2021 The following column has important headings in order of appearance in text. Use search & find or scroll to.

The peripheral nerves

Signals in the peripheral nerves

The Autonomic Nervous System (ANS)
The Sympathetic Division
 The Parasympathetic Division
The ANS is basically a reflex auto system
"fight-or-flight"
"rest-and-digest"
Autonomic and Endocrine Function Is coordinated by a Central Autonomic Network Centered in the Hypothalamus 

The spinal peripheral nerves are body-segment nerves

Nerve fibers need myelin fiber sheathing to function best

Peripheral Neuropathies

Symptoms of Peripheral Neuropathy

After I got a vitamin B12 shot

 Other types of peripheral neuropathy

Motor Nerve & Motor Related Sensory Neuropathies

Guillain-Barre Syndrome

For a more complete discussion of peripheral neuropathies see

(This is one of several chapters that, because packed with much new information, is best read slowly, in short segments, and re-read as needed, and supplemented by internet when further understanding needed.)

The peripheral nerves are outside the central nervous system, meaning they extend out from the CNS into the body beyond the spinal cord and brain stem. If you could peel the skin off a human body, you would see these nerve cords as white strings in the yellow fat. The nerves penetrate depths and go to all organs and tissues. The nerve cords we see are actually bundles of millions of individually-insulated-by-myelin-sheath, single neuron fibers transmitting signals either going into or out of the central nervous system (CNS is brain and spinal cord) and also have feeding blood vessels and fat. The fibers grew out of each one of millions of neurons in spinal cord or brain stem. 

The peripheral nervous system (PNS) includes spinal (into or out of spinal cord) nerves, cranial (into or out of brain stem inside the cranium) nerves and other autonomic nerves exiting the base of the brain and the spinal cord. The PNS should be looked at as an anatomic subsystem that is the final interface between an organism (You, a person) and its environment (the periphery and the outside) with its internal milieu (Your body organs and tissues). The sensory nerves (afferent fibers that carry signals into or out of  the CNS) are a part of the PNS and pass information on the environment and on the body's internal state back up into the CNS, while the PNS's motor nerves (efferent) pass commands down to the muscles and glands. In all cases, the most distal neuron source of the peripheral nerve fiber is either outside the CNS or at its outer edge.

 Signals in the peripheral nerves are either incoming (sensory input; afferent) or outgoing (motor output; efferent). The input transmits signals of touch, position, smell, taste, sound, light, or visceral sensations (autonomic). The outputs are either voluntary motor or autonomic nervous system (ANS) that includes involuntary motor (heart and other) and the glands. The peripheral nerves are the 3 types: somatic sensory (incoming), voluntary motor (outgoing) and incoming or outgoing ANS (visceral, motor and glands). 

The Autonomic Nervous System (ANS) Autonomic is used because, in contrast to the other peripheral nerves which are directly controlled or served by neurons in the brain or spinal cord, the Autonomic nerves are fibers from neurons either outside the spinal cord or just inside it, or in the lower brain. And these neurons outside the CNS are gathered into ganglia (singular: ganglion). In other words, the autonomic nerves in their ganglia are relatively free of central control; they are unconscious, automatic, reflexive.

Autonomic motor neurons lie outside the central nervous system in clusters called ganglia or nuclei (other meanings) and are controlled by preganglionic neurons in the spinal cord and brain stem. Specialized neurons regulate specific types of cells, such as smooth muscle, gland cells, and cardiac muscle.

The ANS has 2 functionally opposing yet complementary parts - the sympathetic nervous system which shows its effects in fight or flight stress situations, and the parasympathetic, whose effects have been summed up as the rest and digest functions. 

The sympathetic division originates from neurons in the spinal cord gray matter, Thoracic 1-12 and Lumbar 1-3, which send preganglionic nerve fibers to right and left sympathetic ganglia, mostly alongside (paravertebral) and in a few cases in front (prevertebral) of the thoracic and upper lumbar spinal cord. The nerve fibers from these sympathetic neurons connect by synapse with secondary neurons in the ganglia, and the fibers from these postganglionic sympathetic neurons and also connect to the left and right adrenal medulla which are specialized sympathetic nerve ganglia. At its destination, the sympathetic postganglionic fiber carries signals that release packets of norepinephrine or epinephrine (aka "noradrenalin” or “adrenalin") neurotransmitters that produce the sympathetic nervous system effect, preparing a person or animal for fight or flight. (Dilate eye pupils, speed and increase power in heart, dilate bronchial tubes and increase respiratory rate, increase muscle power.) Also the ANS receives sensory fibers from all the body organs (visceral sensory), allowing it to monitor their state (blood pressure, heart rate, pain from loss of blood flow).

The sympathetic ganglia lie close to the spinal column and supply almost every tissue in the body. In contrast the parasympathetic ganglia are found far from the spinal column in close to their targets, which do not include the skin or skeletal muscle.

The Sympathetic Division Neurotransmitters

The sympathetic neurotransmitter/hormone, epinephrine (aka adrenaline), speeds the heart and widens the bronchial tubes, and its structurally close norepinephrine contracts the small blood vessels and ups blood pressure, The sympathetic effect widens the eye pupils. The sympathetic system protects against acute stressful situations; but with excessive continued stress it can be a cause of disease (degenerative cardio- and cerebro-vascular diseases) from chronic stress. 

The parasympathetic division has 2 parts - one inside the cranium in the brainstem and the other at the end of the spinal cord (Sacral outflow). The parasympathetic is more pinpointed to organs than the sympathetic. Its main neurotransmitter is acetylcholine and its effect is to slow the heartbeat, to lower the arterial blood pressure and to stimulate contractions of intestines. Also it constricts the eye pupils. 

The ANS is basically a reflex auto system. It responds to body cues and internal changes to protect the body. 

The ANS's underlying tone (its gain or throttle down) is set in the CNS by the brain's hypothalamus which itself is reacting to signals from the cerebral cortex. In the natural state the sympathetic and parasympathetic should complement each other, smoothing out the various organ functions and reflexes; most importantly, heart rate and blood pressure and intestinal functions. But under various body stresses or states, one division may dominate and become a cause of disease. (Stress makes sympathetic dominance and high BP & heart disease; on the other hand, sleep or sedation makes parasympathetic dominance causing respiratory and cardiovascular failure.)

To survive, animals and humans must have a "fight-or-flight" response that prepares an animal to stand and fight or to run away, while the parasympathetic enhances rest-and-digest.

Two important ideas are, first, that the sympathetic and parasympathetic systems play complementary but opposite roles; the sympathetic system promotes arousal, defense, and escape, whereas the parasympathetic system promotes eating and procreation. Second, that actions of the sympathetic system are diffuse; they affect all parts of the body and once turned on can persist for some time while parasympathetic actions are more pinpointed.

We now know that extreme sympathetic responses such as the "fight-or-flight" can have long-term bad consequences resulting in the syndrome known as post-traumatic stress disorder. This disorder was first recognized in soldiers during the U.S. Civil War in the 1860s when it was referred to as DaCosta Syndrome (After the Jewish-American physician, Jacob DaCosta) and then in World Wars I & II as shell shock.. Life-threatening experiences, ranging from sexual abuse and domestic violence to aircraft disasters, can be flash-backed to consciousness decades later and that can induce post-traumatic stress disorder. It affects millions of people in the United States alone.

Because the fight-flight or rest-digest model assumes antagonistic roles for the sympathetic and parasympathetic divisions, it led to an overemphasis on the antagonism of autonomic behavior. Actually during everyday life the divisions of the autonomic system are smoothly meshed. In addition, the sympathetic division is less diffusely organized. Subsets of neurons even within the sympathetic division control specific targets, and these pathways can be activated independently.

Baroreceptor Reflex

The baroreceptor reflex is one of the simplest ANS mechanisms for regulating blood pressure. It prevents orthostatic hypotension (From suddenly standing up) and fainting by compensating for rapid effects produced by changes in posture.

When a recumbent person stands up, the sudden elevation of the head above the heart causes a transient decrease of blood pressure that is rapidly sensed in the carotid sinus in the neck. Other important pressure sensors are located in the aortic arch and in the pulmonary circulation. When neurons in the brain's medulla detect the decrease in activity produced by the low blood pressure, they produce a reflex suppression of parasympathetic activity and stimulation of sympathetic activity. These changes in autonomic tone restore blood pressure by increasing heart rate, increasing the strength of cardiac contractions, and increasing the overall vascular resistance to blood flow by arterial vasoconstriction.

Under the opposite of elevated arterial pressure, the baroreceptor activity enhances parasympathetic inhibition of the heart and decreases sympathetic stimulation of cardiac function and vascular resistance

Body control of blood pressure and body temperature often involves negative feedback loops. In the baroreceptor reflex, for example, the firing of sensory neurons conveys information about arterial pressure that brain medulla circuits use as feedback to control descending commands and thereby regulate preganglionic sympathetic neurons. This feedback is said to be negative because of the inverse relation between sensory input and functional motor output: An increase in blood pressure decreases sympathetic motor tone, which then reduces pressure.

Set point and gain are critical in this type of control. The set point is the target for regulation and is like thermostat settings, e.g., in home heating systems. Gain is the amplification generated by the feedback loop. The mathematics of control theory shows that the accuracy and speed of regulation through negative feedback are established by the gain of the loop. At a higher gain the reflex loop will control pressure more effectively and more quickly but may over- or under-shoot. 

Autonomic and Endocrine Function Is Coordinated by a Central Autonomic Network Centered in the Hypothalamus

Sympathetic and parasympathetic response are coordinated by a central autonomic network in the lower brain hypothalamus (Not shown in above I’ll, it is located just above in the diencephalon below the paired thalami.  This coordination interacts with other brain systems to support homeostasis, to enhance the "fight-or-flight" response, and to promote the `rest-and-digest* and reproduction which control endocrine responses, especially from the pituitary gland, and eating and drinking behavior and defensive and reproductive (sexual and parental) behaviors common to all animals.

The  spinal peripheral nerves are body-segment nerves. At each spinal-cord body level, peripheral nerves exit the CNS as right and left major nerve trunks, each contains all the incoming and outgoing signal fibers for that body level and then, as the major nerve trunk extends out from the CNS, its fibers separate into smaller nerve branches that become pure incoming-signal sensory or pure outgoing-signal motor nerves.  

Nerve fibers need myelin fiber sheathing to function best. This process is “myelin(iz)ation” and it is only partly complete at birth, in some cases taking up to age 20s to be completed. A major nerve disease caused by a patient's immune system attacking his own myelin nerve sheath in the CNS is Multiple Sclerosis; and, in the PNS, the Guillain-Barre Syndrome. These are so-called demyelinating diseases.

Peripheral Neuropathies: When a peripheral nerve (or nerves) get(s) damaged, irritated or diseased it causes a peripheral neuropathy. In my 60-year experience as a physician, peripheral neuropathies are the most common, most bothersome, least known nerve condition. I have experienced them much.

Symptoms of Peripheral Neuropathy are localized loss of motor power (if a motor nerve), loss or distortion of sensory (touch, pain, pleasure, vision, hearing, etc.) or loss or distortion of ANS functions. (Feelings of internal organs, motor function of glands and vital organs and especially blood pressure and heart rate controls.) The extent of the symptoms depends on whether only one nerve is affected (a mononeuropathy), which only affects the tissues served by the single nerve (patch of skin, part of an extremity like a finger) or many nerves (a polyneuropathy), which often affect large, symmetrical areas (both feet, both hands) or important body functions (breathing). Mononeuropathy damage comes from cutting or pressure on a nerve. The latter, or so called pressure palsies, are so common that almost everyone has suffered one or more, e.g., the arm "going to sleep" because of pressure on an elbow. I got a more serious pressure palsy once from carrying a too-heavy shoulder bag for too long on my right shoulder causing me to lose the use of my right arm for 6 weeks. Luckily, time healed it. Some of the worst neuropathies are nutritional (vitamin B12, diabetes) or toxic (metal ions like lead, Pb, or mercury, Hg, or Thallium, Th, poisonings that afflict painters). The nutritional neuropathies are almost always symmetrical involving large areas or many nerves. Lead (Plumbum) poisoning (cf. the 2015 Flint Michigan tap water lead poisonings), for example, will prevent you from waving bye-bye because a first symptom is loss of ability to flex and extend hands at the wrists. If you get a peripheral neuropathy involving more than one nerve it is important to check your blood tests for nutritional deficiency or toxic substance. The most serious polyneuropathies are immunologic where one's body starts attacking the substance in one's nerves. The worst example of that is the Guillain-Barre Syndrome made famous by the Catch-22 novelist Joseph Heller.

Several years ago I got a vitamin B12 shot into my right outer arm and quickly I began to notice an intermittently numb discomfort of right pinkie finger and its descending area. Because I knew the pinkie is inside the C8 dermatome of the ulnar nerve, I was not worried that I was getting the first symptom of multiple sclerosis. I guessed correctly I was experiencing a peripheral neuropathy of the mononeuropathy type which should be temporary from the shot affecting a branch of the ulnar nerve. (It lasted 8 months and is a reason not to get a shot in the arm or, if no choice, to be sure it is given correctly to avoid this.) Thus the usefulness of this knowledge to relieve anxiety.

 Other types of peripheral neuropathy are due to obstruction of arteries that supply the nerves and the basic cause is high cholesterol from overeating or high blood sugar from diabetes, or infections like syphilis or leprosy.  It causes loss of temperature sensation leading to bad burns. Victims of diabetes or high cholesterol neuropathies lose feeling of pressure and end up getting pressure ulcers and lose toes or feet or limbs. A bad habit for a diabetic is to test his bath water heat by dipping a toe into it and getting bad burn because he can't feel the scalding hot water. 

Motor Nerve & Motor Related Sensory Neuropathies in worst cases affect the motor nerves to cause weakness and paralysis of both lower or upper extremities or both, and the sensory nerves of position are damaged and cause unsteadiness and falls. To prevent, keep your cholesterol low, keep a normal blood sugar (glucose), a normal blood pressure and, if diabetic, keep the glucose normal by using insulin well, and, finally, avoid trauma or burns. And take care never to contact syphilis, leprosy or like infection and, if you do, to get them treated and cured quickly before the nerves get affected.

Guillain-Barre Syndrome A vignette: You get an upset stomach. Then, 3 weeks later you wake up with a numb tingling at tips of toes and each day you note clumsiness and weakness that worsens going progressively from feet to legs to thighs. You see the doctor, he refers you to neurology and a spinal tap. The spinal fluid shows high protein but no increase in white blood cells. Next day you are in hospital and each day the paralysis rises higher and then your breathing gets bad, your eyes become weird and your facial expression gets paralyzed, and then you’re in Intensive Care Unit on respirator with endotracheal tube and fighting for life: It is Guillain-Barre Syndrome (GBS), a demyelinating polyneuropathy, (the myelin cover of the nerve gets destroyed segmentally in many nerves all over the body) that kills 5% of its victims and needs respirator support – a non-infectious polio that almost killed the author of Catch-22, Joseph Heller.
  The importance of learning about GBS is that early diagnosis and getting to a hospital with top ICU care and with plasma-exchange treatment will strikingly improve chances of survival, which, if it can be achieved, is usually complete with return of all nerve functions. (Heller published several novels after recovery.)
  GBS is due to hyper-immune reaction to viruses or bacteria or to certain immunizations. The 1976 A/New Jersey (swine) flu vaccine caused a GBS upward blip in its incidence. A recent flu vaccine caused another. The older Rabies vaccine that contained brain proteins (No longer used in the USA but still used overseas.) was a frequent cause. The most common infection that leads to GBS is an intestinal one from the bacteria Campylobacter jejuni. (Good reason to treat GI infections with the combination antibiotic, Augmentin.) The victims develop antibodies and lymphocytes that attack the myelin in the sheaths of peripheral nerves. Autoimmune reactions to these proteins produce the Guillain-Barré syndrome. It is a reason for treating even minor intestinal infections with a brief high-dose of a strong antibiotic.

For a more complete discussion of peripheral neuropathies see Ropper, Samuels & Klein's "Adams and Victor's Principles of Neurology", 11^th ed., 2019 (or latest edition).

 END OF CHAPTER. To read next click 9.12 The Cranial Nerves - Smell/Vision/Eye/Face/Vi...