cated 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.

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).

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.

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Diseases of the Autonomic Nervous System

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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 (in-

cluding 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 sweat-

ing, 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.