Fundamentals of Neurology: An Illustrated Guide

15. Diseases of the Autonomic Nervous System

Anatomy

Normal and Pathological Function of the Autonomic Nervous System

Image  Anatomy

The autonomic nervous system is responsible for the neural control of all of the organs and tissues of the body whose function is involuntary. It thus innervates the internal organs of the throat, thorax, and abdomen, the blood vessels, and the lacrimal, salivary, and sweat glands (among other organs). It can be divided on structural and functional grounds into a sympathetic and a parasympathetic nervous system. These two systems largely exert mutually antagonistic effects on their target organs. The fundamental structural unit of each system is a two-neuron chain, in which the first neuron has its cell body within the central nervous system, i.e., in the brainstem or spinal cord (the preganglionic neuron) and the second neuron has its cell body in an autonomic ganglion or plexus (the postganglionic neuron). The hypothalamus is the “command center” of the autonomic nervous system: it exerts a major degree of control on both sympathetic and parasympathetic activity. Diseases of the autonomic nervous system very often manifest themselves in the form of disturbances of sweating, impairment of bladder, bowel, and sexual function, orthostatic hypotension, and Horner syndrome.

Sympathetic nervous system. The cell bodies of the preganglionic neurons lie in the lateral horns of the spinal cord at levels T1 to L2/3 (the intermediolateral nucleus; the entire system is thus sometimes called the thoracolumbar system). These cell bodies receive neural input from the hypothalamus, whose efferent projection (the central sympathetic pathway) descends through the brainstem and down the spinal cord to the sympathetic nuclei within the cord. The axons of the preganglionic neurons exit the spinal cord in the anterior roots and then travel by way of the rami communi cantes into the sympathetic chain, which lies lateral to

Image

Fig. 15.1 Anatomy of the sympathetic efferent fibers leaving the spinal cord. The sudomotor fibers accompany the spinal nerves (dorsal and ventral rami) to their areas of cutaneous distribution, while the autonomic fibers to the blood vessels and internal organs follow their own paths to their respective targets (vascular ramus, splanchnic ramus).

Image

Fig. 15.2 Anatomy of the sympathetic nervous system.

the spinal cord and consists of a chainlike arrangement of ganglia and the fibrous connections between them (interganglionic branches). Some of the fibers form a synapse with a postganglionic neuron inside the sympathetic chain, while others ascend the entire sympathetic chain without a synapse, not meeting their corresponding postganglionic neuron till they have arrived in the vicinity of the target organ (either in an autonomic plexus or in an intramural ganglion, i. e., a ganglion located within the wall of the organ in question). The postganglionic neurons project efferent fibers to the target tissue, e. g., the smooth muscle of the internal organs and blood vessels, and various glands. The relationship of the sympathetic fibers exiting the spinal cord to the

Image

Fig. 15.3 Anatomy of the parasympathetic nervous system.

nerve roots, sympathetic chain, and peripheral nerves is depicted in Fig. 15.1, while Fig. 15.2 provides an overview of the anatomy of the sympathetic nervous system.

Parasympathetic nervous system. The preganglionic neurons of the parasympathetic nervous system, unlike those of the sympathetic nervous system, are located in two parts of the central nervous system that lie at a considerable distance from each other. Some of the preganglionic neurons lie in the visceral motor and visceral sensory brainstem nuclei, the remainder in the lateral horns of spinal cord segments S2-S4 (the craniosacral system). The axons of the cranial preganglionic neurons exit the brainstem in cranial nerves III, VII, IX, and X and then travel onward to parasympathetic ganglia in the periphery, some of which are intramural, i. e., already located in the wall of the target organ; inside the parasympathetic ganglia, the preganglionic fibers form synapses onto the postganglionic neurons. The parasympathetic fibers of cranial nerves III, VII, and IX innervate the smooth musculature and glands of the head, while those of the vagus n. descend in an extensively branched fiber system to innervate the viscera of the throat, thorax, and abdomen, all the way down to the level of the left colic flexure. Beyond this point (the so-called point of Cannon and Böhm), the abdominal and pelvic viscera are innervated by the sacral portion of the parasympathetic nervous system. The axons of the preganglionic neurons whose cell bodies lie in the lateral horns of the sacral spinal cord reach the periphery by way of the anterior roots or the pelvic nerves. They form synapses onto the postganglionic neurons in the pelvic plexus (= inferior hypogastric plexus) and in the intramural ganglia of the abdominal and pelvic viscera. The anatomy of the parasympathetic nervous system is shown in Fig. 15.3.

Image  Normal and Pathological Function of the Autonomic Nervous System

The sympathetic and parasympathetic nervous systems regulate the functions of the internal organs and are the substrate for all of the autonomic reflexes of the body. The neurotransmitter used at the synapses of the parasympathetic nervous system is acetylcholine, while the sympathetic nervous system uses acetylcholine at the synapse onto the preganglionic neuron, norepinephrine at the synapse onto the postganglionic neuron, and epinephrine in the adrenal cortex (whence the name, epinephrine). An overview of the major functions of the two halves of the autonomic nervous system is provided in Table 15.1.

In the following paragraphs, we will describe just a few, clinically relevant functional disturbances and diseases of the autonomic nervous system.

Sweating

The autonomic fibers innervating the sweat glands are exclusively sympathetic. They run in the peripheral nerves in close association with the somatosensory fibers innervating the same area of skin. A lesion of a peripheral nerve, therefore, always impairs sweating in the sensory distribution of the nerve. The impairment of sweating can be demonstrated with various tests, such as the ninhydrin test (Fig. 15.4).

Image

Fig. 15.4 Ninhydrin test in a right median nerve lesion. The patient lays his hand on a piece of paper that is subsequently “developed” with several applications of a 1 ninhydrin solution, followed by drying in a hot air chamber. Sweating is diminished or absent in the cutaneous sensory distribution of the median n.

Table 15.1 Functions of the sympathetic and parasympathetic nervous systems

Effect of the sympathetic nervous system

Organ

Effect of the parasympathetic nervous

system

 
 

eye:

 

+ pupillary dilation

Image dilator pupillae m.

 
 

Image sphincter pupillae m.

+ pupillary constriction

+ vasoconstriction in certain areas of the

body, e. g., the skin

vascular smooth muscle

- vasodilatation in certain areas of the

body, e. g., gastrointestinal tract

- diminished secretion

salivary glands

+ increased secretion

+ increased secretion

sweat glands

 
 

lacrimal glands

+ increased secretion

 

glands of the Gl tract

+ increased secretion

- diminished motility and peristalsis

smooth muscle of the Gl tract

+ increased motility and peristalsis

- bronchodilation

bronchial smooth muscle

+ bronchoconstriction

+ increased heart rate

heart

- decreased heart rate

- urinary retention

smooth muscle of the vesical wall

+ micturition

+ urinary retention

sphincters

- micturition

Image

Fig. 15.5 Spastic neurogenic bladder. The bladder is cut off from the influence of the CNS above the level of the lesion, but the spinal reflex arc controlling micturition is intact. It is automatically activated whenever the bladder is filled to a certain volume.

Bladder, Bowel, and Sexual Function

Anatomy. The neural elements controlling bladder, bowel, and sexual function are the following:

Image Sympathetic fibers from spinal cord segments T12-L2 and parasympathetic fibers from spinal cord segments S2-S4 innervate the smooth muscle of the urinary bladder, rectum, and internal genitalia, including the corpora cavernosa. The sympathetic fibers travel to their target organs after a synaptic relay in the superior hypogastric plexus, while the parasympathetic fibers do so after a synaptic relay in the inferior hypogastric plexus. There are ganglion cells and synapses not just in the plexuses, but also within the walls of the target organs. Visceral sensory (afferent) fibers return to the spinal cord from the urinary bladder, genitalia, and rectum.

Image The spinal center for micturition and defecation receives supranuclear input from multiple higher cortical areas (paracentral lobule → voluntary initiation of micturition and defecation) through a number of different pathways in the spinal cord, and it also conveys afferent information back upward to the brain (→ conscious perception of bladder filling and of noxious and thermal stimuli). These mechanisms are the basis of the voluntary control of micturition and defecation.

Image The striated skeletal muscle of the pelvic floor and of the external sphincters of the bladder and rectum, which are under voluntary control, is innervated by the pudendal n., whose fibers are derived from spinal cord segments S2-S4. This nerve also conveys afferent impulses arising in the urethra, prostate gland, anal canal, and external genitalia.

Disturbances of bladder, bowel, and sexual function.The clinical manifestations depend on the site of the lesion (peripheral/central, unilateral/bilateral):

Image Spinal cord transection above the sacral level cuts off the bladder and bowel from the supraspinally derived (cortical) impulses subserving the voluntary control of micturition and defecation, but all of the afferent and efferent nerve pathways of the bladder remain intact, including the spinal reflex arc for bladder emptying. The result is a spastic (automatic) neurogenic bladder, which empties itself reflexively whenever it is filled to a certain volume (Fig. 15.5). Penile erection remains possible, though there may be retrograde ejaculation into the bladder.

Lesions of the conus medullaris, cauda equina, sacral plexus, and pelvic plexus.Lesions of these structures inactivate the sacral centers for micturition and defecation. The result is atony of the bladder and bowel musculature,leading to severe impairment of emptying. Bladder filling can no longer be perceived, either consciously or unconsciously. Tone is preserved in the sympathetically elevated vesical sphincter; the bladder,

Image

Fig. 15.6 Flaccid neurogenic bladder. The sacral micturition center is destroyed, and the vesical musculature can no longer be induced to contract. The bladder fills until the intravesical pressure exceeds that of the external vesical sphincter; thereafter, urine is released in small quantities at shorter than normal intervals. Complete bladder emptying is no longer possible.

therefore, continues to fill until the passive intravesical pressure overcomes the closing force of the sphincter. The continually overfilled bladder lets out small amounts of urine at short intervals (overflow incontinence,Fig. 15.6). Defecation, meanwhile, occurs passively and in uncontrolled fashion through a patulous anal sphincter. In the male, lesions of these structures cause erectile impotence.Psychosexually mediated arousal remains possible in rare cases because of the preserved sympathetic efferent innervation through the hypogastric plexus. Thus, a small number of affected men are still able to have an emission of semen, but without ejaculation, and without rhythmic contraction of the pelvic floor muscles.

Lesions of the pudendal n. An isolated lesion of the pudendal n., which contains parasympathetic fibers from segments S2-S4, causes erectile dysfunction: the sacral erection center can no longer be activated because its somatosensory afferent input has been interrupted. Moreover, because the somatic efferent impulses to the bulbocavernosus and ischiocavernosus mm. no longer reach their targets, the maximal tumescence of the corpora cavernosa mediated by these muscles also fails to occur.

Impairment of the sympathetic innervation of the pelvic organs can be caused, for example, by tumor infiltration or by surgical procedures. Bilateral lesions of the sympathetic chain and lesions of the superior hypogastric plexus abolish seminal emission into the proximal urethra; if ejaculation does occur, then the semen goes into the bladder, in retrograde fashion. As long as the parasympathetic innervation of the genital organs by the pelvic plexus and their somatic sensory and motor innervation by the pudendal n. remain intact, the affected men are still able to have erections, and affected persons of both sexes can still experience pelvic floor contractions and orgasm. This constellation of symptoms (preserved ability to experience orgasm, in the absence of seminal emission) is seen in about half of all men who have undergone bilateral sympathectomy. It does not occur after unilateral lumbar sympathectomy.

The Cervical Sympathetic Pathway and Horner Syndrome

Anatomy. As already discussed at the beginning of this chapter, the spinal cord nuclei in which sympathetic impulses originate are present only from the T2 level downward. Thus, the sympathetic fibers innervating the head must ascend from the thoracic spinal cord and the thoracic segments of the sympathetic chain, by way of the interganglionic branches, to the cervical sympathetic chain, where they make a synaptic relay onto the second neuron in one of the three cervical ganglia (including the stellate ganglion). From these ganglia, the sympathetic fibers continue upward in periarterial nerve plexuses until they reach their destinations. Sympathetic fibers in the head innervate the walls of the blood vessels, the sweat glands, and the salivary, lacrimal, nasal, and palatal glands, as well as the smooth muscle of the dilator pupillae m. See also Fig. 15.2, p. 280.

Lesions of the cervical sympathetic pathway. Destruction of the stellate ganglion or of the cervical sympathetic chain causes Horner syndrome: the pupil is (unilaterally) narrow and, when the patient looks slightly downward, ptosis is evident (p. 192). Horner syndrome is usually seen in conjunction with loss of sweating on the ipsilateral upper quadrant of the body, particularly on the neck and face. Depending on the level of the lesion, the arm, hand, and axilla may be affected as well. If the sympathetic chain is interrupted immediately below the stellate ganglion, anhidrosis of the upper quadrant of the body results, but without Horner syndrome. On the other hand, isolated Horner syndrome without anhidrosis can occur as the result of a lesion of the C8-T2 nerve roots between the spinal cord and the sympathetic chain.

Generalized Autonomic Dysfunction

Polyneuropathy. Damage to autonomic fibers is often a component of polyneuropathy. Affected persons suffer from impaired regulation of blood pressure and sweating, as well as from diarrhea, urinary disturbances, and erectile dysfunction. Autonomic manifestations of these kinds are particularly common in diabetic polyneuropathy.

Acute pandysautonomia. This condition is due to a neuropathy affecting either preganglionic or postganglionic autonomic nerve fibers. Patients suffer from orthostatic hypotension, an invariant heart rate, a lack of sweating and lacrimation, nonreactive midsized pupils, impotence, and an atonic bladder. The etiology of this condition is not known; it gradually resolves spontaneously over the course of a few months.

Familial dysautonomia (Riley). This autosomal recessive disease is probably due to a disturbance of norepinephrine synthesis. Its manifestations, which are already evident in infancy, include dysphagia, lack of tears when the infant cries, abnormally intense sweating, diminished sensitivity to pain, and impaired temperature regulation. The prognosis is poor.

Other generalized autonomic disturbances. A number of degenerative conditions of the basal ganglia can impair autonomic function; further diseases that can affect the autonomic nervous system include orthostatic hypotension of Shy-Drager type, botulinus intoxication, and congenital sensory neuropathy with anhidrosis.