Chapter 14 Vertebral Subluxation and the Anatomic Relationships of the Autonomic Nervous System
|Question 1||How are the three components of the autonomic nervous system differentiated?|
|Question 2||How does the location of preganglionic axons differ between the sympathetic and parasympathetic nervous systems?|
|Question 3||How do the functions of the sympathetic and parasympathetic nervous systems differ?|
|Question 4||On what basis can the treatment of visceral disorders be included in the scope of chiropractic practice?|
The structure of the nervous system is commonly divided into the central nervous system (CNS), which consists of the brain, brainstem, and spinal cord, and the peripheral nervous system (PNS), which includes all neuronal processes outside of the CNS such as cranial nerves and spinal nerves, as well as ganglia associated with these nerves. Beyond pure anatomic description, however, such subdivisions are inconsequential because functional components of the nervous system extend beyond these artificial structural limits. Numerous examples can be identified in which single neurons extend axonal processes either centrally or peripherally into or out of the CNS, crossing the anatomic barrier between the CNS and PNS to form an integrated, functional system. The student of neuroanatomy should strive to understand both structural and functional relationships within the nervous system because this integrated knowledge of neuroscience is not only much more interesting and meaningful but also abundantly more useful in the clinical setting. At the same time, students should realize that we are now only beginning to understand the mechanisms whereby the nervous system monitors and modulates functional activities throughout body systems and, conversely, how stimuli remote from the CNS may be integrated into neuronal functions within the brain and spinal cord.
The autonomic nervous system (ANS) is defined traditionally as the self-regulating visceral motor (efferent) portion of the nervous system, although it is now recognized that the ANS has a major visceral afferent component. The ANS is an excellent example of structural and functional integration, encompassing the CNS, PNS, and numerous other body systems. The term self-regulating refers to the fact that in many ways the ANS functions independently of conscious control. Visceral motor (efferent) identifies the ANS as that part of the nervous system that functions largely to activate or regulate organ systems, including the heart, smooth muscle (e.g., respiratory, vascular, gastrointestinal), and glandular (exocrine and endocrine) tissues. The ANS functions to a large extent in response to environmental stimuli that may originate either outside the body or from within a specific organ or tissue. These sensory signals are carried to the CNS by afferent neuronal connections where they are integrated with other somatic or visceral sensations. An appropriate regulatory efferent response is then transmitted through the ANS to effect an alteration of visceral function if necessary. Therefore external or internal sensations such as pain, temperature, proprioception, touch, pressure, vibration, and stretch may act reflexive to elicit an autonomic response that functions to achieve and maintain homeostasis. Numerous examples of the effects of somatic or visceral sensations on visceral functions that are mediated and regulated through the ANS can be cited; they will become apparent in this chapter as the various functional systems are described. However, in many instances the neurophysiologic mechanisms involved remain poorly understood, primarily because the interneuronal connections that constitute the relevant neuronal pathways have not been described adequately.
It now appears that a more comprehensive definition of the ANS should include conscious control of external factors, such as somatic sensations, which influence the regulatory activity of the ANS. The ANS, although predominantly self-regulatory, is not limited to self-regulation. This is of particular clinical significance because therapeutic intervention that alters somatic or visceral function may have effects in body systems apparently remote from the site of applied therapy. A growing body of evidence suggests that there exists a close correlation between somatic (sensory and motor) functions and visceral (sensory and motor) functions. It appears that somatic and visceral functions are coordinated closely through somatovisceral and viscerosomatic reflex mechanisms involving the ANS, PNS, and CNS. Therefore therapeutic interventions such as vertebral manipulation, ingestion of analgesic or antiinflammatory agents, and even surgery, to name a few, can alter somatic sensations (proprioception, for example) in such a way that visceral functions may become altered. However, as stated previously, the underlying neuronal mechanisms require further elucidation through scientific investigation to understand the complexity of factors that interact to regulate organ function.1
The purpose of this chapter is to provide a clear, up-to-date description of the structural and functional anatomy of the ANS. The relevant neuroanatomy is discussed in some detail, focusing on neuronal centers involved with autonomic function within the PNS and CNS as they are currently understood. Throughout this chapter an integrated functional approach is employed in an effort to clarify the complex functional interactions that occur. It is hoped that a sound understanding of the ANS will stimulate students and practitioners to pursue scientific research into this basic and as yet poorly understood area of neuroscience, and that this chapter will provide a firm foundation for future research in chiropractic.
The autonomic nervous system (ANS) can be subdivided into three components: the sympathetic, parasympathetic, and enteric (intestinal) nervous systems. Each division of the ANS is composed of distinct neuronal populations interconnected by axonal processes that form an integrated functional unit. Neuron pools that make up the sympathetic and parasympathetic divisions are localized either within well-defined nuclei of the brainstem and spinal cord or within ganglia located in the periphery, whereas neurons of the enteric division are isolated in the wall of the gastrointestinal tract. Higher neuronal centers located in the hypothalamus, thalamus, hippocampus, and other areas of the cerebrum function to integrate ascending afferent stimuli from all regions of the body and are the source of descending efferent impulses that function to control and modulate autonomic activity. Like the nerve pathways of somatic portions of the nervous system, the ANS consists of visceral sensory axons that enter the CNS, ascending visceral sensory tracts within the brainstem and spinal cord, visceral reflex arcs, and descending visceral motor tracts that influence neural activity of the sympathetic, parasympathetic, and enteric divisions of the ANS.
The sympathetic and parasympathetic divisions are similar structurally, whereas the enteric division, modulated by other autonomic centers, is distinct structurally and functionally from other components of the nervous system. Sympathetic and parasympathetic divisions both originate from preganglionic neurons located within the CNS that extend thinly myelinated axonal processes that synapse with dendrites of postganglionic neurons located mostly in peripheral ganglia. All preganglionic neurons are similar functionally because, regardless of their location or which autonomic division they are part of, these neurons are cholinergic (Figure 14-1). In contrast, postganglionic neurons extend unmyelinated axonal processes that innervate specific viscera directly. In addition, unlike preganglionic neurons, which secrete the neurotransmitter acetylcholine, postganglionic neurons vary in the transmitters they synthesize and secrete. Sympathetic postganglionic neurons are largely catecholaminergic (those that innervate sweat glands are cholinergic), and parasympathetic postganglionic neurons are entirely cholinergic. Sympathetic postganglionic neurons are further subdivided functionally into α- and β-catecholaminergic neurons, which exert different influences on target tissues. Generally α-catecholaminergic innervation is excitatory to smooth muscle, and β-catecholaminergic stimulation is inhibitory.
Figure 14-1 Schematic diagram illustrating the general structural pattern of peripheral components of the autonomic nervous system. Note that the somata of preganglionic neurons are located within the CNS (brain and spinal cord), whereas postganglionic neuronal somata are in the peripheral ganglia. Generally, sympathetic postganglionic axons are long and course along arteries to reach target viscera (e.g., the eye), and parasympathetic postganglionic axons are short. Also note that all preganglionic neurons are cholinergic (red) as are parasympathetic postganglionic neurons, while sympathetic postganglionic neurons are catecholaminergic (blue).
The two-neuron chain (pre-postganglionic) pattern of innervation is unique to the ANS and is fundamentally different from the single-neuron innervation pattern that is characteristic of somatic neuronal systems. An anatomic feature that may assist in distinguishing conceptually between the sympathetic and parasympathetic divisions is the characteristic location of the ganglia, which contain postganglionic neurons. Sympathetic ganglia are localized near the CNS, indicating that sympathetic preganglionic axons are mostly short. Axons of parasympathetic preganglionic neurons are long because parasympathetic ganglia are located some distance away from the CNS near the viscus they innervate. (See Figure 14-1.)
The sympathetic and parasympathetic divisions of the ANS function to regulate and maintain the internal body environment. At times of emergency when there is a sudden change in external or internal body conditions such as during an argument, an examination, an athletic competition, or a drastic temperature change, the sympathetic division allows the body to cope with these stresses. The sympathetic response to external or internal stress has been referred to as the fight-or-flight reaction. This reaction, which is initiated by autonomic neuronal centers in the hypothalamus, results in increased cardiac output (rate and stroke volume), increased blood supply to appendicular muscles, increased blood glucose levels, and activation of sweat glands, erector pili muscles, and dilator pupillae. In this way, sympathetic activity is protective because it allows for a rapid response to potentially dangerous external factors.
In contrast, the parasympathetic division of the ANS serves a major role to store, conserve, and replenish body energy and can be said to be the vegetative component of the nervous system, functioning primarily at times of rest and digestion. Activation of parasympathetic neuronal centers causes increased secretion of saliva, mucus, and digestive enzymes into the gastrointestinal tract and functions to maintain basal cardiac, respiratory, and metabolic rates. Gut motility is initiated primarily through mechanical reflex activation of the enteric division of the ANS and is modulated by parasympathetic and sympathetic neuronal influences.
Although the functions of the ANS are extremely important for the maintenance of homeostasis and at times for the survival of the individual, hyperactive or hypoactive autonomics caused by pathology, trauma, aberrant physiology, or altered biomechanics can be detrimental and may give rise to characteristic symptomatology. To understand and accurately diagnose clinical presentations of abnormal autonomic function, a sound knowledge of the peripheral and central components of the ANS is necessary.
The peripheral components of the sympathetic division, as described by Warwick and Williams,2 include gray and white communicating rami, two bilaterally symmetric and ganglionated sympathetic trunks that house postganglionic neurons; clusters of prevertebral ganglia, which also house postganglionic neurons; splanchnic nerves, which innervate prevertebral ganglia; and vascular nerve plexuses, which conduct postganglionic axons to target viscera. In fact, most sympathetic postganglionic axons course along arterial vessels to reach viscera that they innervate. Many sympathetic axons within these vascular nerve plexuses penetrate the arterial or arteriolar wall along which they pass to supply smooth muscle in that wall and serve to regulate blood flow and blood pressure. Also in these vascular nerve plexuses are large numbers of parasympathetic axons that innervate viscera and numerous visceral afferent nerve fibers, which are sensory to the viscera and pass back to the CNS by autonomic nerves. In addition, vast numbers of sympathetic postganglionic axons course together with somatic nerve fibers within spinal nerves and their branches to reach target structures in the body wall and limbs. Still other postganglionic axons innervate viscera such as the heart and lungs as direct branches from the sympathetic trunks. The sympathetic division, which is the largest division of the ANS, therefore innervates all regions of the body by three different routes: (1) by arterial nerve plexuses, (2) as a component of somatic nerves, and (3) by direct nerve branches from the sympathetic trunks.
The sympathetic division is commonly referred to as the thoracolumbar portion of the ANS because all sympathetic preganglionic neurons are localized in the thoracic and upper lumbar (T1-L3) spinal cord segments. Within these cord segments, preganglionic neurons form a well-defined column of cells that is called the intermediolateral cell column because of its intermediate and lateral position between the posterior and anterior horns of gray matter. Axons of sympathetic preganglionic neurons, which are thinly myelinated, exit the spinal cord along with axons of somatic motoneurons through anterior roots of spinal nerves at the same spinal level as their soma of origin. On exiting the intervertebral foramen, preganglionic axons branch from the spinal nerve as a white communicating ramus (white because these axons are myelinated), which joins the paravertebral sympathetic trunk. Large numbers of preganglionic axons end by synapsing with dendritic branches of postganglionic neurons, the cell bodies of which are located in these ganglia. Many preganglionic axons innervate postganglionic cells in trunk ganglia located at the same vertebral level as the intervertebral foramen through which they emerge. Alternatively, numerous preganglionic axons or their collateral branches course along the length of the sympathetic trunk to innervate postganglionic neurons in trunk ganglia located more cranially or caudally than their spinal level of preganglionic origin. (See Figure 14-1.) Although all sympathetic axons enter the sympathetic trunk through white communicating rami, indicating that white rami are evident only at spinal nerve levels T1 through L3, not all of these axons terminate in sympathetic trunk ganglia as just described. Many preganglionic axons branch from the sympathetic trunk without having synapsed and course as splanchnic nerves that end in prevertebral ganglia, where additional postganglionic neuronal cell bodies are located. (See Figure 14-1.)
In summary, preganglionic axons terminate by synapsing with dendrites of postganglionic neurons located in one of two ganglionated structures, either ganglia of the sympathetic trunk or prevertebral ganglia. (Note: One exception to this general scheme that is described later is the medulla of suprarenal glands.) It is also important to realize that each preganglionic neuron normally innervates up to 20 postganglionic cells either within a single ganglion or distributed among a number of paravertebral or prevertebral ganglia. In one early study it was found that the ratio of preganglionic to postganglionic neurons in the superior cervical ganglion (described further) may be as high as 1 to 196.3 Functionally this innervation pattern allows for divergence of sympathetic activation and coordination of postganglionic response at several spinal levels.4 For this and other reasons that will become apparent, sympathetic activation results in a mass response, such as generalized constriction of cutaneous arteries, as compared with the more localized parasympathetic response.
The peripheral distribution of sympathetic postganglionic axons can be divided into two groups on the basis of the location of postganglionic somata. The first group consists of postganglionic neurons located in ganglia of the paravertebral sympathetic trunk. Postganglionic cells in these ganglia are concerned generally with innervation of viscera in the head, neck, and thorax, including lacrimal and salivary glands, pupillary dilator, heart, and lungs. These neurons also supply superficial structures in the head, neck, body wall, and limbs, including sweat glands, erector pili muscles, and smooth muscle in the walls of arteries and arterioles. The second group consists of postganglionic neurons located in abdominal and pelvic prevertebral ganglia. These postganglionic cells innervate organs associated with the gastrointestinal and urogenital systems, including the stomach, small and large intestines, pancreas, liver, gallbladder, kidneys, urinary bladder, and external genitalia. The following is a detailed description of the origin, course, and anatomic relationships of sympathetic postganglionic axons. The discussion focuses first on branches of the sympathetic trunk and subsequently on the distribution from prevertebral ganglia. A sound knowledge of the postganglionic distribution pattern is crucial to the understanding of autonomic function and clinical conditions that occur when this function is disturbed.
The paired sympathetic trunks and their ganglia extend the length of the vertebral column (paravertebral) and are closely related to the anterolateral aspect of vertebral bodies and intervertebral discs throughout their course. For this reason, conditions such as abnormal biomechanics, subluxations, bony spurs, and other pathologies at intervertebral and costovertebral joints, which are commonly seen in chiropractic offices, as well as more severe conditions such as ankylosing spondylitis and severe osteoporosis, may have a profound influence on sympathetic functions.5 The trunk courses through the cervical region posterior to the carotid sheath, where it is related closely to prevertebral muscles and fascia (Figure 14-2). In the thoracic region the trunk passes along the necks of upper ribs and is related directly to the fibrous capsules of costovertebral joints in the lower thoracic region (Figure 14-3). The sympathetic trunk continues into the abdominal cavity by coursing between the medial arcuate ligament of the diaphragm (anteriorly) and the psoas major muscle (posteriorly). As the trunk descends through the abdomen, it lies adjacent to lumbar vertebral bodies and intervertebral discs anterior to the psoas major muscle and posterior to the inferior vena cava (on the right) or posterolateral to the abdominal aorta (on the left; Figure 14-4). Near its caudal termination the trunk courses posterior to the common iliac vein, descends anterior to the ala of the sacrum just medial to the anterior sacral foramina, where it lies related directly to the origin of piriformis and terminates finally by joining the trunk of the opposite side anterior to the coccyx and coccygeus muscle as the single ganglion impar (Figure 14-5).
Figure 14-2 Cervical sympathetic trunk within the retropharyngeal space. The prevertebral fascia has been removed and the common carotid artery and vagus nerve are shown reflected to the left with the trachea and pharynx.
In the neck extensive fusion of sympathetic trunk ganglia takes place during embryonic development,2 resulting in three (superior, middle, and inferior) cervical ganglia that are joined by the cervical continuation of the sympathetic trunk. (See Figure 14-2.) The largest is the superior cervical ganglion, which is believed to have developed from the coalescence of the upper four cervical ganglia.2 This ganglion lies at the level of C1-C3 vertebrae interposed between the carotid sheath anteriorly and the longus capitis posteriorly and, like the lower cervical sympathetic trunk, is enveloped by prevertebral fascia. Preganglionic innervation of the superior cervical ganglion, as in the middle and inferior cervical ganglia, is derived from neurons in the upper three thoracic spinal cord segments.
Postganglionic axons of neurons in the superior cervical ganglion are distributed to target structures along branches of the internal carotid artery or through a number of direct nerve branches from the ganglion. In addition, the first four cervical spinal nerves, like all spinal nerves, receive gray communicating rami composed of unmyelinated axons of postganglionic neurons from the superior ganglion and its caudal connection with the middle ganglion. These postganglionic axons are distributed to blood vessels, erector pili muscles, and sweat glands in the territory of each of these spinal nerves. Extending from the superior limit of the superior cervical ganglion, the internal carotid nerve conducts postganglionic axons to the internal carotid artery, which lies immediately anterior to the ganglion within the carotid sheath. The internal carotid nerve in this way forms the internal carotid plexus, which supplies the artery and its branches to regulate cerebral blood flow, although this is now thought to be only a minor role of the sympathetic division. The internal carotid plexus enters the cranial cavity along the surface of the internal carotid artery as it passes through the carotid canal and provides the clinically important sympathetic innervation to arteries that supply the cerebrum, meninges of the anterior and middle cranial fossae, hypophysis, orbital contents, and the upper parts of the face and scalp. It is this portion of the sympathetic division that may be involved in the cause of migraine headache. As the internal carotid artery passes through the cavernous sinus, the nerve plexus on its surface extends branches that join the oculomotor, trochlear, abducens, and ophthalmic nerves through which sympathetic postganglionic axons are distributed. Thrombosis of the cavernous sinus, as may occur with infections of the orbit, nasal cavity, paranasal sinuses, and tympanic cavity, may therefore impinge on these cranial nerves, leading to characteristic cranial nerve signs and symptoms, including those associated with sympathetic blockage, as observed in a classic Horner’s syndrome. The signs associated with sympathetic nerve blockage at this site include ptosis caused by the loss of sympathetic innervation to the levator palpebrae superius through the oculomotor nerve and miosis caused by unopposed parasympathetic activation of the sphincter pupillae. Sympathetic postganglionic axons reach the eye to supply the dilator pupillae and arterial vessels of the eyeball along two routes. Some axons branch from the oculomotor nerve, pass through the ciliary ganglion without synapsing, and enter the eye with parasympathetic fibers in short ciliary nerves. Other postganglionic fibers continue along the nasociliary branch of the ophthalmic nerve (CNVI) to enter the eyeball as long ciliary branches.
The internal carotid plexus also supplies sympathetic innervation to arteries and mucous glands in the tympanic cavity through the caroticotympanic nerve, which joins the tympanic branch of the glossopharyngeal nerve (CNIX) to enter the tympanic plexus. In addition, arteries and mucous glands in the nasal cavity, nasopharynx, hard palate, and soft palate receive sympathetic innervation by way of the deep petrosal branch of the internal carotid plexus, which joins the greater petrosal nerve in the foramen lacerum.
The greater petrosal nerve traverses the pterygoid canal to reach the pterygopalatine ganglion through which sympathetic axons pass without synapsing and enters the infraorbital and nasopalatine branches of the maxillary nerve (CNV2). These sympathetic postganglionic axons supply the lacrimal gland and mucosa of the nasal cavity, paranasal sinuses, nasopharynx, hard palate, and soft palate.
In addition to the internal carotid plexus, other nerve branches of the superior cervical ganglion innervate structures within the posterior cranial fossa, oral cavity, neck, and thorax. Small branches from the lateral aspect of the ganglion join the vagus (CNX) and hypoglossal (CNXII) nerves in the carotid sheath and are distributed with these two cranial nerves to blood vessels and mucous glands in the mucosa of the oral cavity, oropharynx, pharynx, larynx, trachea, and esophagus, as well as the submandibular, sublingual, and intralingual salivary glands.
The jugular nerve branches from the superior cervical ganglion to connect with the glossopharyngeal (CNIX) and vagus (CNX) nerves to also innervate the oral cavity and oropharynx. In addition, the meninges in the posterior cranial fossa receive sympathetic innervation through a plexus of postganglionic axons that originate in the superior cervical ganglion and join the internal jugular vein to enter the cranial cavity by way of the jugular foramen.
The cervical sympathetic trunk also contributes to the cardiac plexus through cardiac branches that arise bilaterally from all three cervical ganglia. The cardiac branch of the superior cervical ganglion courses inferiorly along the anterior aspect of the longus cervicis muscle (see Figure 14-2) partly enveloped by prevertebral fascia, where it may be influenced by damage or increased tonicity of this muscle. On the right the cardiac branch passes most commonly posterior to the subclavian artery, where it is related directly to the cupula of parietal pleura and may be affected by pathology in the apical region of the lung. The nerve continues into the thorax on the posterolateral aspect of the brachiocephalic trunk to enter the cardiac plexus posterior to the arch of the aorta, although some axons may contribute to the anterior cardiac plexus. In contrast, the left cardiac branch of the superior cervical ganglion enters the thorax most commonly along the anterior aspect of the common carotid artery to reach the anterior portion of the cardiac plexus anterolateral to the arch of the aorta. Like the right cardiac branch, the left nerve may contribute to the posterior cardiac plexus as well. En route, both the right and left cardiac branches of the superior cervical ganglia commonly receive communications from the external laryngeal, recurrent laryngeal, and cardiac branches of the vagus (CNX) nerve, indicating that, on reaching the cardiac plexus, the cardiac nerves are mixed, having both sympathetic and parasympathetic components. In addition, cardiac nerve branches of the middle and inferior cervical ganglia commonly have communicating branches with that of the superior cervical ganglion.
The cardiac plexus also receives direct cardiac branches from the upper four or five thoracic sympathetic trunk ganglia (described further) and is divided into anterior (superficial) and posterior (deep) plexuses. The anterior cardiac plexus is dispersed along the anterior aspect of the right pulmonary artery inferior to the arch of the aorta. The posterior cardiac plexus receives input from the anterior plexus and is located near the bifurcation of the pulmonary trunk posterior to the arch of the aorta. Scattered amongst the nerve fibers of the cardiac plexus are small ganglia that contain parasympathetic postganglionic neurons that innervate the heart. (See the following discussion.) Sympathetic postganglionic axons course through the cardiac plexus along the right and left coronary arteries and their branches, which they innervate. They then penetrate the atrial and ventricular walls to supply cardiac muscle directly. Whereas the anterior cardiac plexus supplies some input to the right coronary and left pulmonary plexuses, most cardiac innervation reaches the heart by way of the posterior (deep) cardiac plexus, which can be divided into right and left halves. The right half of the posterior cardiac plexus supplies the right coronary plexus, right atrium and ventricle, and right pulmonary plexus and helps to form the left coronary plexus. The left half of the posterior cardiac plexus receives some input from the anterior (superficial) cardiac plexus and innervates the left atrium and ventricle, the left coronary plexus, and the left pulmonary plexus.
Most sympathetic postganglionic neurons are catecholaminergic including those that inner-vate the heart and coronary vessels, and their activation causes the release of norepinephrine (β-catecholaminergic). Interestingly, this neurotransmitter performs a dual role to control and regulate cardiac function. Cardiac myofibers are stimulated by norepinephrine to contract more forcefully and more rapidly. Concomitantly, smooth muscle in the walls of coronary arteries is inhibited, causing dilation of these vessels and increased blood flow to the heart, although Berne and Levy6 suggest that the coronary circulation responds primarily to the metabolic needs of the myocardium and is predominantly under nonneuronal control. In fact, the sympathetic division may serve its greatest regulatory role of cardiac function indirectly by affecting the release of norepinephrine and epinephrine into the bloodstream from medullary cells of the suprarenal glands.
In addition to sympathetic postganglionic axons, visceral sensory (afferent) axons are also present in all cardiac branches of the sympathetic trunk, except those arising from the superior cervical ganglia. Cardiac pain is transmitted through these sympathetic cardiac nerves to upper thoracic spinal cord segments and for this reason may be referred to the medial aspect of the arm and adjacent thoracic wall. These neuronal pathways also may provide a viscerosomatic reflex mechanism whereby cardiac pain provokes increased tonus and even spasm of muscles innervated by upper thoracic spinal cord segments, as observed in angina pectoris and cardiac arrest.
From the superior cervical ganglion the sympathetic trunk courses inferiorly along the anterior aspect of the longus capitis and longus cervicis muscles to connect with the middle cervical ganglion at the level of the sixth cervical vertebra. (See Figure 14-2.) Occasionally this ganglion is poorly defined or absent, in which case the postganglionic neurons normally present in the middle ganglion are dispersed along the length of the cervical sympathetic trunk. The middle cervical ganglion is formed most commonly by the coalescence of the fifth and sixth cervical ganglia and is related to the inferior thyroid artery, which is innervated by a sympathetic plexus derived from this ganglion. Sympathetic postganglionic axons are distributed to branches of this artery that supply deep posterior neck muscles, prevertebral muscles, and the external vertebral arterial plexus as well as the thyroid and parathyroid glands. Postganglionic axons derived from neurons predominantly in the middle and inferior cervical ganglia form a nerve plexus on the external carotid artery. This external carotid nerve plexus supplies the artery and follows its branches to innervate structures supplied by the artery in the neck and face.
Postganglionic axons from the middle ganglion commonly enter the fifth and sixth (occasionally the fourth and seventh as well) cervical spinal nerves by gray communicating rami and are distributed to the periphery through these nerves and their branches to innervate sweat glands, erector pili muscles, and cutaneous and muscular arteries in the shoulder and upper limb regions. Because of the cervical origin of postganglionic axons that control and regulate blood flow to the upper limb, any condition that interferes with this autonomic function, including trauma, pathology, spastic neck musculature, and abnormal somatovisceral reflexes, may compromise this blood supply and lead to numbness, tingling, and pain the upper limb, a condition commonly associated with thoracic outlet syndrome.
The cardiac plexus also receives a large cardiac branch from the middle cervical ganglion, which courses along the lateral border of the longus cervicis muscle. On the right, this cardiac nerve passes posterior to the common carotid artery; it is related directly to the trachea and enters the right half of the posterior cardiac plexus. The cardiac branch of the left middle cervical ganglion follows a similar course into the thorax but passes between the left common carotid and subclavian arteries to enter the left half of the posterior cardiac plexus.
A number of nerve cords that form the inferior continuation of the sympathetic trunk join the middle cervical ganglion to the inferior cervical (cervicothoracic) ganglion. Some nerve fibers of the cervical sympathetic trunk pass posterior and anterior to the vertebral artery to reach the inferior ganglion and may contribute to the nerve plexus around this vessel. A large nerve cord called the ansa subclavia passes inferiorly and anteriorly to the origin of the subclavian artery, loops around this vessel just medial to the internal thoracic artery, and joins the inferior cervical ganglion posterior to the subclavian artery, The ansa subclavia commonly contributes to the nerve plexuses surrounding both the subclavian and internal thoracic arteries. (See Figure 14-2.) The inferior cervical ganglion is considerably larger than the middle ganglion and is formed by the coalescence of the C7, C8, and T1 ganglia. Other names given to the inferior ganglion are the cervicothoracic ganglion, because of its embryonic origin, and the stellate ganglion, because of its shape. The inferior cervical ganglion is positioned anterior to the C7 transverse process. It extends inferiorly to the neck of the first rib just posterolateral to the origin of the vertebral artery. (See Figure 14-2.) The ganglion is also related directly to the cupula of parietal pleura and the lower vertebral attachment of the scalenus medius muscle. In some individuals the lateral border of the longus cervicis muscle is also related to the medial aspect of the inferior ganglion.
Postganglionic axons from neurons in the inferior cervical ganglion join the C7, C8, and T1 spinal nerves through gray communicating rami; they are distributed by these nerves to the periphery including cutaneous and muscular arteries of the forearm and hand. Other nerve branches of the inferior cervical ganglion are a cardiac branch, which joins the posterior cardiac plexus along with the cardiac branch from the middle cervical ganglion, and vascular branches, which form plexuses on the subclavian and vertebral arteries as well as on the thyrocervical and costocervical trunks. The subclavian plexus extends into the axilla along the first part of the axillary artery and its superior thoracic branch, but it rarely reaches arteries in the upper limb because these appendicular vessels receive direct sympathetic innervation from nerves of the brachial plexus. The large vertebral branch of the inferior cervical ganglion courses along the vertebral artery through foramina transversarii to form a vertebral nerve plexus, which supplies arteries to the cervical spinal cord, external and internal vertebral arterial plexuses, and deep muscles of the neck. The vertebral nerve plexus enters the cranial cavity with the vertebral arteries through the foramen magnum and courses along the length of the basilar artery and its branches as far as the posterior cerebral arteries. In this way postganglionic axons derived from the inferior cervical ganglion primarily supply sympathetic innervation that functions to regulate blood flow to occipital and temporal lobes of the cerebrum and cerebellum as well as vital neuronal centers in the brainstem and cervical spinal cord. It has been suggested recently, however, that neural control of cerebral blood flow may not be as important as once thought. Rather, it may be that the flow rate of blood in cerebral arterioles is regulated primarily by regional metabolic needs.6 Near the posterior portion of the circulus arteriosus, the vertebral sympathetic plexus meets that of the internal carotid plexus. If sympathetic function in the inferior cervical ganglion is compromised, as may happen in a variety of clinical conditions (for example, cervical rib; abnormal biomechanics of the lower cervical and upper thoracic spine), a number of signs and symptoms associated with the syndrome referred to as vertebrobasilar insufficiency are observed. Upper limb symptoms may be present, resulting in a classic thoracic outlet syndrome. Hyperactive sympathetics may give rise to tinnitus, hearing loss, dizziness, facial nerve (CNVII) palsy, blurred vision, nausea, and vomiting, as well as cardiac and respiratory arrhythmia,7 which result from insufficient blood flow to the brainstem and cerebellum. Migraine-type headache also has been reported to occur after whiplash injury to the cervical spine8 and may be the result of trauma or pressure on the cervical sympathetic trunks.
The thoracic sympathetic trunk consists of a series of small ganglia that vary somewhat in number. Generally there is one ganglion for each thoracic spinal nerve, although the first thoracic ganglion is most commonly fused with the inferior cervical ganglion. As the sympathetic trunk descends through the thorax, the ganglia come to lie in direct contact with the fibrous capsules of costovertebral joints, except in the lower thorax, where they lie more medially adjacent to T10-11 and T11-12 intervertebral discs. (See Figure 14-3.)
Numerous nerve branches arise from the thoracic sympathetic trunk, some of which are composed of postganglionic axons of neurons in thoracic trunk ganglia, and other branches that consist of preganglionic axons of neurons located in the thoracic spinal cord. The nerves composed of preganglionic axons that innervate postganglionic cells in prevertebral ganglia are referred to as splanchnic (visceral) nerves and are described after the following description of the distribution of postganglionic nerve branches of the thoracic trunk.
Each thoracic spinal nerve receives both white and gray communicating rami from the sympathetic trunk and its ganglia, with the white ramus joining the spinal nerve slightly more distal than the gray ramus. (See Figure 14-3.) Occasionally, the white communicating rami are fused and only one mixed white and gray ramus is present. Each spinal nerve conducts these postganglionic axons to sweat glands and erector pili muscles, as well as cutaneous and muscular arteries within the thoracic and abdominal walls. Of particular clinical importance is the sympathetic nerve plexus formed by postganglionic branches of the thoracic sympathetic trunk on the aorta and its intercostal, esophageal, and bronchial branches. Segmental arteries that originate from intercostal arteries carry sympathetic innervation to the external and internal vertebral arterial plexuses and its radicular branches. By this route the autonomic nervous system can function to regulate blood flow to bones, joints, and ligaments of the thoracic vertebral column and structures within the spinal canal, including the spinal cord, spinal nerve roots, and meninges.
The pulmonary plexus receives direct input of sympathetic postganglionic axons from branches of upper thoracic (T2-T5) sympathetic trunk ganglia as well as indirect input from the anterior and posterior cardiac plexuses. As mentioned previously, the posterior cardiac plexus is formed in part by sympathetic postganglionic axons that enter the plexus directly from upper thoracic (T2-T5) trunk ganglia. From the pulmonary and cardiac plexuses, a small network of sympathetic postganglionic axons branch away to supply the mucosa of the trachea and esophagus. Within the root of the lung bilaterally, sympathetic axons from the pulmonary plexus form a delicate nerve plexus along the surfaces of pulmonary and bronchial arteries as well as on the bronchial tree to supply smooth muscle in the walls of these structures. Activation of the sympathetic division causes release of norepinephrine, which is inhibitory to bronchial smooth muscle and results in bronchodilation. It is excitatory to arterial smooth muscle, causing constriction of pulmonary and bronchial arteries. However, neuronal control of bronchial smooth muscle has been shown to be relatively insignificant when compared with the bronchiolar response to local tissue factors.8 For example, circulating levels of norepinephrine and epinephrine secreted into the bloodstream by suprarenal glands during sympathetic activation act as potent β-catecholaminergic receptor stimulants that elicit rapid dilation of the bronchial tree.9,10 In contrast, other local factors such as histamine and the slow reactive substance of anaphylaxis that are released from mast cells into lung tissues after exposure to allergens act as bronchoconstrictors.11 It appears that local physiologic needs of the tissues serve as the primary regulatory control of pulmonary function, whereas direct ANS influences are limited.
Similarly, the ANS is believed to have little control over the pulmonary circulation during normal daily activity. Pulmonary vascular resistance, which is a measure of the freedom with which arterial blood flows through the pulmonary circulation, is known to be inversely proportional to cardiac output.12,13 That is, pulmonary and bronchiolar arteries expand or collapse passively in response to an increase or decrease, respectively, in blood pressure.14 However, a clinically significant role of the sympathetic division in the regulation of pulmonary circulation has been suggested by Fishman,15 who indicated that pulmonary obstruction may reflexly stimulate sympathetic vasomotor activity to cause generalized constriction of pulmonary vessels and increased arterial pressure in the lung. It is also thought that constriction of larger pulmonary veins in response to sympathetic stimulation may be the primary mechanism whereby blood is shunted from the pulmonary to the systemic circulation when needed. In summary, the principal mechanism whereby the sympathetic division regulates pulmonary function is humeral because, as in the control of cardiac function, it has its greatest influence through activation of the suprarenal glands.
As mentioned previously, there are three pairs of bilaterally symmetric splanchnic (visceral) nerves, called the greater, lesser, and least (lowest) splanchnic nerves, that arise from the thoracic sympathetic trunks. The splanchnic nerves are composed of preganglionic axons of neurons located in the intermediolateral cell column of the thoracolumbar spinal cord and do not synapse within sympathetic trunk ganglia. Instead, these axons pass through the sympathetic trunk, branch medially away as splanchnic nerves, and terminate finally by synapsing with dendrites of postganglionic neurons in prevertebral sympathetic ganglia. (See Figures 14-1 and 14-3.) The prevertebral ganglia are located anterior to the lumbar spine and sacrum in association with major branches of the abdominal aorta. The largest splanchnic nerve branch of the thoracic sympathetic trunk is the greater splanchnic nerve, which is formed by five roots arising from the fifth through ninth (sometimes tenth) thoracic trunk ganglia. (See Figure 14-3.) Most preganglionic axons within the roots of the greater splanchnic nerve originate in the thoracic spinal cord segments adjacent to these ganglia. However, a small proportion of preganglionic neurons in upper thoracic spinal segments also may contribute to this nerve. The lesser splanchnic nerve is somewhat smaller, having only two roots of origin normally from the ninth and tenth (sometimes tenth and eleventh) thoracic trunk ganglia. The least (lowest) splanchnic nerve is the smallest, arising singly from the last thoracic trunk ganglion. All three splanchnic nerves course anteromedially and inferiorly along vertebral bodies and intervertebral discs to reach the diaphragm. The greater and lesser splanchnic nerves normally gain access to the abdominal cavity by piercing the crus of the diaphragm, and the least splanchnic nerve enters the abdomen with the sympathetic trunk by passing between the medial arcuate ligament of the diaphragm and the psoas major muscle. Often an enlargement of the greater splanchnic nerve called the splanchnic ganglion is present just before the nerve pierces the diaphragm.
Although the innervation of prevertebral ganglia by splanchnic nerves is somewhat variable, a general pattern can be described. The greater splanchnic nerve terminates primarily by innervating postganglionic neurons in the celiac ganglion. This large prevertebral ganglion is located anterior to the crus of the diaphragm just lateral to the origin of the celiac trunk, a large branch of the abdominal aorta at the level of the twelfth thoracic vertebral body. To a lesser degree the greater splanchnic nerve also innervates the aorticorenal ganglion, a relatively large cluster of postganglionic neurons that is dispersed along the abdominal aorta near the origins of the superior mesenteric and renal arteries. The greater splanchnic nerve also innervates the suprarenal gland (described later). The lesser splanchnic nerve, which courses with the greater splanchnic nerve, supplies the celiac ganglion minimally and sends most of its axons to innervate the aorticorenal and renal ganglia. The innervation pattern of the least (lowest) splanchnic nerve is to small clusters of postganglionic neurons scattered along the renal artery and in the hilum of the kidney. For this reason the least (lowest) splanchnic nerve is often referred to as the renal nerve. The distribution of postganglionic axons of neurons in the prevertebral ganglia is described after consideration of the suprarenal glands.
The suprarenal glands are discussed in some detail because of the important functional role these glands play in the sympathetic division of the ANS. These glands are supplied by the greater splanchnic nerve, an innervation pattern that may appear unusual in that preganglionic axons innervate medullary cells of the gland directly. However, when one considers the embryonic origin of the gland, this innervation pattern can be better understood. Histologically the gland is composed of a layered cortex that develops from mesodermal cells near the developing dorsal mesentery of the embryo and a central medulla derived from neuroepithelial cells of the neural crest.16 The medullary cells of the gland in this way develop from the same primordium as all sympathetic postganglionic neurons and may be considered to be homologous to postganglionic neurons. Like most sympathetic postganglionic neurons, medullary cells synthesize and secrete the hormones (neurotransmitters) epinephrine and norepinephrine, which when released into the bloodstream evoke a generalized sympathetic (fight-or-flight) response.
The suprarenal nerve plexus is composed of axons of the greater splanchnic nerve, which reach the plexus on the anterior aspect of the crus of the diaphragm by passing through the celiac ganglion and plexus without synapsing. The suprarenal glands, located just lateral to the crus of the diaphragm adjacent to the superomedial pole of each kidney, are said to have the largest sympathetic innervation relative to size when compared with other organs. This is understandable considering the important role this gland serves during sympathetic activation. Preganglionic axons enter the gland and terminate on two types of medullary cells called chromaffin and ganglion cells. (Note: Cortical layers of the gland that function to synthesize and secrete glucocorticoid, mineralocorticoid, and corticosteroid hormones are not innervated.) The chromaffin cells, innervated by synapse-like junctions, synthesize the catecholamines epinephrine and norepinephrine. These products then are stored in separate cytoplasmic granules recognized at the electron microscopic level to be distinct for one specific hormone. On stimulation of chromaffin cells by preganglionic axons, the granules bind with the plasmalemma and release their contents into adjacent blood vessels. The second medullary cell type innervated by preganglionic axons is a multipolar neuron-like cell referred to as a ganglion cell. However, the axons of ganglion cells and their terminations have not been studied definitively. It is possible that ganglion cells may serve to magnify incoming excitatory sympathetic stimuli and disseminate the signals to chromaffin cells. After release into the bloodstream, these hormones circulate throughout the body and act to elicit the characteristic sympathetic response to stressful conditions that the individual may be facing.
The distribution of postganglionic axons from neurons in prevertebral ganglia follows a characteristic pattern; the axons course through delicate nerve plexuses formed on the external surfaces of large branches of the abdominal aorta near the ganglion in which they originate. Each arterial nerve plexus supplies smooth muscle in the wall of the vessels along which it passes and terminates by innervating viscera supplied by the vessel. Axons of neurons in the celiac ganglion are distributed along branches of the celiac trunk, which come off the aorta at the level of the twelfth thoracic vertebra near the aortic hiatus of the diaphragm. The right and left celiac ganglia are interconnected by a massive nerve plexus called the celiac plexus that extends inferiorly to the level of the first lumbar vertebra and surrounds both the celiac trunk and origin of the superior mesenteric artery. (See Figure 14-4.) Entering the plexus from each side are the greater and lesser splanchnic nerves and parasympathetic axons from the vagus (CNX) nerve (discussed further). Exiting the celiac plexus are sympathetic preganglionic axons, which supply the suprarenal glands, parasympathetic (vagal) axons, and sympathetic postganglionic axons that enter subsidiary plexuses to be distributed to abdominal viscera. These secondary nerve plexuses include the phrenic plexus, the left gastric, splenic, and hepatic plexuses, which course along branches of the celiac trunk, and the renal, gonadal, superior mesenteric, and inferior mesenteric plexuses, which follow lower branches of the abdominal aorta.
The phrenic plexus is small and sends nerve axons along the inferior phrenic arteries to the diaphragm and suprarenal glands. This sparse plexus contains some preganglionic axons that have traversed the celiac plexus without synapsing to aid in the innervation of the suprarenal glands. Also contained in the phrenic plexus are some postganglionic axons, which innervate smooth muscle in the inferior vena cava and then join the much larger hepatic nerve plexus. Axons that are sensory to the gallbladder also enter the hepatic plexus as branches from the phrenic nerve (a mixed somatic and visceral nerve).
Sympathetic postganglionic axons in the hepatic plexus reach the liver along branches of the hepatic artery and portal vein. The innervation that reaches the liver by this route is believed to function only to regulate blood flow to that organ because the characteristic glycogenolytic response of the liver to sympathetic activation is a result of increased circulating levels of epinephrine and norepinephrine derived from suprarenal glands. A small cystic plexus originating from the hepatic plexus contains sympathetic postganglionic axons that are inhibitory to smooth muscle in the wall of the gallbladder but excitatory to the sphincter of the common bile duct. The hepatic plexus also contributes to the nerve plexus surrounding the gastroduodenal branch of the hepatic artery to innervate the right side of the stomach, duodenum, head of the pancreas, and the most distal portion of the common bile duct. (Note: Sympathetic innervation of the gastrointestinal tract functions to inhibit neuronal activity in the enteric division of the ANS, described subsequently.)
Also extending from the celiac plexus are the left gastric and splenic plexuses, which follow the arteries of the same name and terminate by innervating the left side of the stomach, tail of the pancreas, and the spleen. The sympathetic innervation of the spleen is excitatory to smooth muscle in the capsule of this organ and causes expulsion of the relatively large reservoir of blood into the general circulation at times of need, such as during exercise or serious blood loss caused by injury. More importantly, sympathetic influences on the spleen and other lymphoid organs function to regulate the immune system.17 The immunoregulatory role of the sympathetic nervous system is elaborated in the final section of this chapter.
The renal plexus is derived from the more inferior portion of the celiac plexus as well as the aorticorenal and renal ganglia, thus receiving input from the lesser and least (lowest) splanchnic nerves. Ganglia within the renal plexus give rise to postganglionic axons, which follow and supply branches of the renal artery and innervate glomeruli and convoluted tubules in the renal cortex. Both afferent and efferent arterioles of the glomeruli are innervated; however, the influence of sympathetic innervation on the glomerular filtration rate is only minor because neuronal control is superseded by renal autoregulatory mechanisms.18,19 From the renal plexus, the upper part of the ureter and gonadal (testicular/ovarian) arteries also receive sympathetic innervation, which is excitatory to smooth muscle in their walls.
Finally, the superior and inferior mesenteric plexuses are located on the anterior aspect of the abdominal aorta surrounding the origin of the corresponding artery. The two plexuses contain postganglionic neurons in small ganglia dispersed along the first part of each vessel. Although some postganglionic axons from the celiac and aorticorenal ganglia enter the superior mesenteric plexus, its principle input is from the lesser splanchnic nerve, which terminates in ganglia within the plexus. Postganglionic axons from superior mesenteric ganglia follow all the branches of the superior mesenteric artery to supply inhibitory input to the enteric division of the ANS within the jejunum, ileum, vermiform appendix, cecum, ascending colon, and most of the transverse colon. However, sympathetic innervation of the ileocecal sphincter, which reaches this site by the same route, is excitatory. The inferior mesenteric ganglia, which receives only a minor innervation from the celiac plexus, as well as lesser and least splanchnic nerves are supplied primarily by lumbar splanchnic nerves and are described with these nerves in the next section of this chapter.