Diseases of the Autonomic Nervous System

16.2 Normal and Pathologic Function of the Autonomic Nervous System

Bad News

The patient, a 54-year-old bank manager, had been feeling “worn out” for months; she was always tired and listless. She also complained of recurrent abdominal pain and a feeling of pressure in the pelvic area. She had gained 3 kg of weight (“mostly belly fat”) in recent months, despite a lack of appetite. She had repeatedly formed the intention of seeing her doctor as soon as possible, but then put it off again because her symptoms were so diffuse and because she had too much work to do.

Finally, she developed deep pain in the low back radiating down the back of the right leg down to the sole of the foot, and the sole felt numb. She decided she could not wait any longer and went to see the doctor.

On examination, the doctor found weakness of plantar flexion of the right foot, with an absent ankle-jerk reflex and diminished sensation in dermatomes S1 to S3. On the basis of these findings and the patient’s complaint of low back pain radiating down the right leg, the doctor suspected a right L5–S1 intervertebral disk herniation with compression of the S1 nerve root. He ordered a lumbar spinal magnetic resonance imaging scan to confirm the diagnosis and was surprised to find that it was negative.

The doctor had not paid adequate attention to the autonomic nervous system; if he had done so, he would have noted that the sole of the right foot, as opposed to the left, was entirely dry because of the absence of sweating (anhidrosis).

The sympathetic fibers that innervate the sweat glands arise from the intermediolateral cell column of the spinal cord at levels T1 to L2/L3. These spinal segments are the only ones at which sympathetic fibers exit the spinal cord together with the anterior roots; the fibers then join the sympathetic chain and proceed onward to the autonomic nerve plexuses, the viscera, or (in the case of the sudomotor fibers) the skin, by way of the peripheral nerve plexuses and sensory peripheral nerves. The nerve roots above T1 and below L3 contain no autonomic fibers, and thus root lesions below L3 are not associated with autonomic deficits of any kind, including a loss of sweating.

This patient, however, had anhidrosis of the sole of the right foot, which is not consistent with an S1 nerve root lesion. The site of the disturbance had to be further distal. On the other hand, the spatial extent of the sensory disturbance and the sweating abnormality was too large for a lesion of a single peripheral nerve; the hypesthetic skin area corresponded to multiple dermatomes (S1–S3). It follows that the lesion was most likely located in the sacral plexus.

A computed tomography scan of the pelvis revealed a tumor of the right ovary invading the sacral plexus. The patient’s more diffuse symptoms, which she had noted long before the neurologic symptoms arose, were also consistent with the diagnosis of ovarian cancer. Her weight gain and increase in girth were probably due to ascites. In retrospect, the physician should have listened more carefully to her full story before formulating his presumptive diagnosis.

16.1 Anatomy

Key Point

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 the sympathetic and parasympathetic nervous systems, which mainly exert mutually antagonistic effects on their target organs. The fundamental structural unit in both systems is a two-neuron chain, in which the first neuron has its cell body within the central nervous system, that is, 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.

16.1.1 Sympathetic Nervous System

The cell bodies of the preganglionic neurons lie in the lateral horns of the spinal cord at levels T1 to L2/L3 (the intermediolateral cell column or intermediolateral nucleus; the sympathetic nervous 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 communicantes into the sympathetic chain, which lies lateral to the spinal cord and consists of a chain of ganglia connected by communicating nerve bundles (interganglionic branches). Some of the preganglionic axons form a synapse onto a postganglionic neuron within the sympathetic chain, while others travel through the entire sympathetic chain without a synapse, only meeting their postganglionic neuron when they have reached 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 their axons to the target tissue—for example, 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 nerve roots, sympathetic chain, and peripheral nerves is shown in ▶ Fig. 16.1, while ▶ Fig. 16.2 is an overview of the anatomy of the sympathetic nervous system.

Fig. 16.1 Anatomy of the sympathetic efferent fibers that exit 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).

Fig. 16.2 Anatomy of the sympathetic nervous system.

16.1.2 Parasympathetic Nervous System

The preganglionic neurons of the parasympathetic nervous system, unlike those of the sympathetic nervous system, are located in two different parts of the central nervous system that lie at a considerable distance from each other: some of them lie in the visceromotor and sensory brainstem nuclei, while others lie 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, that is, 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 nerve 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) or in the intramural ganglia of the abdominal and pelvic viscera. The anatomy of the parasympathetic nervous system is shown in ▶ Fig. 16.3.

Fig. 16.3 Anatomy of the parasympathetic nervous system.

16.2 Normal and Pathologic Function of the Autonomic Nervous System

Key Point

The sympathetic and parasympathetic nervous systems regulate the functions of the internal organs and are responsible 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 (epinephrine = adrenaline). An overview of the major functions of the two components of the autonomic nervous system is provided in ▶ Table 16.1. Diseases of the autonomic nervous system commonly express themselves in disorders of sweating, impairment of bladder, bowel, and sexual function, orthostatic hypotension, and Horner syndrome.

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