LESSON 8

Spinal nerves. Nerve Plexuses

SYMPATHETIC NERVOUS SYSTEM

PARASYMPATHETIC NERVOUS SYSTEM

A series of connective tissue layers surrounds each spinal nerve and continues along all its peripheral branches. These layers, best seen in sectional view (Figure 13-5Описание: View a text illustration), are comparable to those associated with skeletal muscles. The outermost layer, or epineurium, consists of a dense network of collagen fibers. The fibers of the perineurium extend inward from the epineurium, dividing the nerve into a series of compartments that contain bundles of axons, or fascicles. Delicate connective tissue fibers of the endoneurium extend from the perineurium and surround individual axons.

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Arteries and veins penetrate the epineurium and branch within the perineurium. Capillaries leaving the perineurium branch in the endoneurium and supply the axons and Schwann cells of the nerve and the fibroblasts of the connective tissues.

Spinal cord and spinal nerves: gross anatomy

This chapter deals with the gross anatomy of the structures which lie within the vertebral canal and its extensions through the intervertebral foramina, the spinal nerve or radicular (‘root') canals. The spinal cord, its blood vessels and nerve roots lie within a meningeal sheath, the theca, which occupies the central zone of the vertebral canal and extends from the foramen magnum, where it is in continuity with the meningeal coverings of the brain, to the level of the second sacral vertebra in the adult. Distal to this level the dura extends as a fine cord, the filum terminale externum, which fuses with the posterior periosteum of the first coccygeal segment. Tubular prolongations of the dural sheath extend around the spinal roots and nerves into the lateral zones of the vertebral canal and out into the root canals, eventually fusing with the epineurium of the spinal nerves. Between the theca and the walls of the vertebral canal is the epidural (spinal extradural) space, which is loosely filled with fat, connective tissue containing small arteries and lymphatics, and an important venous plexus. Three-dimensional appreciation of the anatomy of the spinal theca and its surroundings is essential for the efficient management of spinal pain and of spinal injuries, tumours and infections. Equally significant clinically is the anatomy of the often precarious blood supply of the spinal cord and its associated structures. The increasing application and refinement of diagnostic imaging and endoscopic procedures lend a new importance to topographical detail here.

SPINAL CORD (MEDULLA)

The spinal cord occupies the superior two-thirds of the vertebral canal (Fig. 18.1, Fig. 43.1). It is continuous cranially with the medulla oblongata, and narrows caudally to the conus medullaris, from whose apex a connective tissue filament, the filum terminale, descends to the dorsum of the first coccygeal vertebral segment. The cord extends from the upper border of the atlas to the junction between the first and second lumbar vertebrae: its average length in European males is 45 cm, its weight approximately 30 g. (For dimensional data consult Barson & Sands 1977.)

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Fig. 43.1  A, Brain and spinal cord with attached spinal nerve roots and dorsal root ganglia, photographed from the dorsal aspect. Note the fusiform cervical and lumbar enlargements of the cord, and the changing obliquity of the spinal nerve roots as the cord is descended. The cauda equina is undisturbed on the right but has been spread out on the left to show its individual components. B–D, Formation of typical spinal nerve, ventral aspect. B, Cervical level; C, Thoracic level; D, Lumbar level. E, Lower end of spinal cord, filum terminale and cauda equina exposed from behind. The dura mater and the arachnoid have been opened and spread out. F, Spinal cord segment showing mode of formation of a typical spinal nerve and the gross relationships of the grey and white matter. (B–D, From Sobotta 2006.)
(Dissection by MCE Hutchinson, GKT School of Medicine; photograph by Kevin Fitzpatrick on behalf of GKT School of Medicine, London.)


During development, the vertebral column elongates more rapidly than the spinal cord, so that there is an increasing discrepancy between the anatomical level of spinal cord segments and their corresponding vertebrae. At stage 23, the vertebral column and spinal cord are the same length, and the cord ends at the last coccygeal vertebra: this arrangement continues until the third fetal month. At birth, the spinal cord terminates at the lower border of the second lumbar vertebra, and may sometimes reach the third lumbar vertebra. In the adult, the spinal cord is said to terminate at the level of the disc between the first and second lumbar vertebral bodies, which lies a little above the level of the elbow joint when the arm is by the side, and also lies approximately in the transpyloric plane (p. 1054). However, there is considerable variation in the level at which the spinal cord ends. It may end below this level in as many as 40% of subjects, or opposite the body of either the first or second lumbar vertebra: very occasionally it ends as high as the caudal third of twelfth thoracic or as low as the disc between the second and third lumbar vertebrae. Its position rises slightly in vertebral flexion, and there is some correlation with the length of the trunk, especially in females. The spinal cord varies in transverse width, gradually tapering craniocaudally, except at the levels of the enlargements. It is not cylindrical, being wider transversely at all levels, especially in the cervical segments.

The cervical enlargement is the source of the large spinal nerves that supply the upper limbs. It extends from the third cervical to the second thoracic segments, its maximum circumference (approximately 38 mm) is in the sixth cervical segment. (A spinal cord segment provides the attachment of the rootlets of a pair of spinal nerves.) The lumbar enlargement is the source of the large spinal nerves that supply the lower limbs, and extends from the first lumbar to the third sacral segments, the equivalent vertebral levels being the ninth to twelfth thoracic vertebrae. Its greatest circumference (approximately 35 mm) is near the lower part of the body of the twelfth thoracic vertebra, below which it rapidly dwindles into the conus medullaris.

Fissures and sulci extend along most of the external surface. An anterior median fissure and a posterior median sulcus and septum almost completely separate the cord into right and left halves, but they are joined by a commissural band of nervous tissue which contains a central canal.

The anterior median fissure extends along the whole ventral surface with an average depth of 3 mm, although it is deeper at caudal levels. It contains a reticulum of pia mater. Dorsal to it is the anterior white commissure. Perforating branches of the spinal vessels pass from the fissure to the commissure to supply the central spinal region. The posterior median sulcus is shallower, and from it a posterior median septum penetrates more than halfway into the cord, almost to the central canal. The septum varies in anteroposterior extent from 4 to 6 mm, and diminishes caudally as the canal becomes more dorsally placed and the cord contracts.

A posterolateral sulcus exists from 1.5 to 2.5 mm lateral to each side of the posterior median sulcus. Dorsal roots (strictly rootlets) of spinal nerves enter the cord along the sulcus. The white substance between the posterior median and posterolateral sulcus on each side is the posterior funiculus. In cervical and upper thoracic segments a longitudinal posterointermediate sulcus marks a septum dividing each posterior funiculus into two large tracts: the fasciculus gracilis (medial) and fasciculus cuneatus (lateral). Between the posterolateral sulcus and anterior median fissure is the anterolateral funiculus. This is subdivided into anterior and lateral funiculi by ventral spinal rootlets which pass through its substance to issue from the surface of the cord. The anterior funiculus is medial to, and includes, the emerging ventral rootlets, whilst the lateral funiculus lies between the roots and the posterolateral sulcus. In upper cervical segments, nerve rootlets emerge through each lateral funiculus to form the spinal accessory nerve which ascends in the vertebral canal lateral to the spinal cord and enters the posterior cranial fossa via the foramen magnum (Fig. 28.11).

The filum terminale, a filament of connective tissue approximately 20 cm long, descends from the apex of the conus medullaris. Its upper 15 cm, the filum terminale internum, is continued within extensions of the dural and arachnoid meninges and reaches the caudal border of the second sacral vertebra. Its final 5 cm, the filum terminale externum, fuses with the investing dura mater, and then descends to the dorsum of the first coccygeal vertebral segment. The filum is continuous above with the spinal pia mater. A few strands of nerve fibres which probably represent roots of rudimentary second and third coccygeal spinal nerves adhere to its upper part. The central canal is continued into the filum for 5–6 mm. A capacious part of the subarachnoid space surrounds the filum terminale internum, and is the site of election for access to the CSF (lumbar puncture).

DORSAL AND VENTRAL ROOTS

The paired dorsal and ventral roots of the spinal nerves are continuous with the spinal cord (Fig. 43.1F; see also p. 754). They cross the subarachnoid space and traverse the dura mater separately, uniting in or close to their intervertebral foramina to form the (mixed) spinal nerves. Since the spinal cord is shorter than the vertebral column, the more caudal spinal roots descend for varying distances around and beyond the cord to reach their corresponding foramina. In so doing they form a divergent sheaf of spinal nerve roots, the cauda equina, which is gathered round the filum terminale in the spinal theca, mostly distal to the apex of the cord.

Ventral spinal roots contain efferent somatic and, at some levels, preganglionic sympathetic, axons which extend from neuronal cell bodies in the ventral horns and intermediolateral columns respectively. There are also afferent nerve fibres in these roots. The rootlets comprising each ventral root emerge from the anterolateral sulcus in groups over an elongated vertical elliptical area (Fig. 43.1F). Dorsal spinal roots bear ovoid swellings, the spinal ganglia, one on each root proximal to its junction with a corresponding ventral root in an intervertebral foramen. Each root fans out into six to eight rootlets before entering the cord in a vertical row in the posterolateral sulcus. Dorsal roots are usually said to contain only afferent axons (both somatic and visceral) which are the central processes of unipolar neurones in the spinal root ganglia, but they may also contain a small number (3%) of efferent fibres and autonomic vasodilator fibres.

Each ganglionic neurone has a single short stem which divides into a medial (central) branch which enters the spinal cord via a dorsal root, and a lateral (peripheral) branch which passes peripherally to a sensory end organ. The central branch is an axon while the peripheral one is an elongated dendrite (but when traversing a peripheral nerve is, in general structural terms, indistinguishable from an axon). The region of spinal cord associated with the emergence of a pair of nerves is a spinal segment, but there is no actual surface indication of segmentation. Moreover, the deep neural sources or destinations of radicular fibres may lie far beyond the confines of the ‘segment’ so defined.

 

 

Peripheral Distribution of Spinal Nerves

Figures 13-6aОписание: View a text illustration, bОписание: View a text illustration show the distribution, or pathway, of a typical spinal nerve that originates from the thoracic or upper lumbar segments of the spinal cord. The spinal nerve forms just lateral to the intervertebral foramen, where the dorsal and ventral roots unite. Let us follow the distribution of that nerve in the periphery.

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In the thoracic and upper lumbar regions (segments T1-L2), the first branch from the spinal nerve carries visceral motor fibers to a nearby autonomic ganglion of the sympathetic division of the ANS. (The sympathetic division is, along with other functions, responsible for elevating the metabolic rate and for increasing alertness.) Because preganglionic axons are myelinated, this branch has a light color and is known as the white ramus ("branch"). Postganglionic fibers innervating glands and smooth muscles in the body wall or limbs rejoin the spinal nerve. These fibers are unmyelinated and have a darker color; they form the gray ramus. The gray ramus is typically proximal to the white ramus. The gray and white rami are collectively termed the rami communicantes, or "communicating branches." Postganglionic fibers innervating visceral organs in the thoracic cavity form a series of separate sympathetic nerves. In the abdominal region, most of the sympathetic neurons innervating visceral organs in the abdominopelvic cavity are located in ganglia anterior to the spinal column rather than close to the bases of the spinal nerves. The preganglionic fibers traveling to these ganglia form the sympathetic nerves known as splanchnic nerves.

The dorsal ramus of each spinal nerve provides sensory and motor innervation to the skin and muscles of the back. The relatively large ventral ramus supplies the ventrolateral body surface, structures in the body wall, and the limbs. Each pair of spinal nerves monitors a specific region of the body surface, an area known as a dermatome. Dermatomes (Figure 13-7Описание: View a text illustration) are clinically important because damage or infection of a spinal nerve or dorsal root ganglion will produce a characteristic loss of sensation in the skin. For example, shingles, a virus that infects dorsal root ganglia, causes a painful rash whose distribution corresponds to that of the affected sensory nerves.

SPINAL NERVES PROPER

Immediately distal to the spinal ganglia, ventral and dorsal roots unite to form spinal nerves (see Fig. 43.5, Fig. 15.15). These very soon divide into dorsal and ventral rami, both of which receive fibres from both roots. At all levels above the sacral, this division occurs within the intervertebral foramen. Division of the sacral spinal nerves occurs within the sacral vertebral canal, and the dorsal and ventral rami exit separately through posterior and anterior sacral foramina at each level. Spinal nerves trifurcate at some cervical and thoracic levels, in which case the third branch is called a ramus intermedius. At or distal to its origin each ventral ramus gives off recurrent meningeal (sinuvertebral) branches and receives a grey ramus communicans from the corresponding sympathetic ganglion. The thoracic and first and second lumbar ventral rami each contributes a white ramus communicans to the corresponding sympathetic ganglia. The second, third and fourth sacral nerves also supply visceral branches, unconnected with sympathetic ganglia, which carry a parasympathetic outflow direct to the pelvic plexuses.

Cervical spinal nerves enlarge from the first to the sixth nerve. The seventh and eighth cervical and the first thoracic nerve are similar in size to the sixth cervical nerve. The remaining thoracic nerves are relatively small. Lumbar nerves are large, increasing in size from the first to the fifth. The first sacral is the largest spinal nerve, thereafter the sacral nerves decrease in size. The coccygeal nerves are the smallest spinal nerves. The size of the spinal nerve and its associated structures within the intervertebral foramen is not in direct relation to the size of the foramen. At lumbar levels, though L5 is the largest nerve, its foramen is smaller than those of L1–4, which renders this nerve particularly liable to compression.

In the radicular (‘root') canal and intervertebral foramen, the spinal nerve is related to the spinal artery of that level and its radicular branch, and to a small plexus of veins. At the outer end of the foramen the nerve may lie above or below transforaminal ligaments.

Meningeal nerves

Recurrent meningeal (or sinuvertebral) nerves (Fig. 43.7) occur at all vertebral levels. They are mixed sensory and sympathetic nerves, represented by numerous fine filaments amongst which one, or two to four, larger trunks may be evident. At cervical levels the autonomic roots arise from the grey rami that form the vertebral nerve (p. 461). At thoracic and lumbar levels, each nerve is formed by a somatic root from the ventral ramus and by an autonomic root from the grey ramus communicans of that segment. Each nerve pursues a recurrent course through the intervertebral foramen, passing ventral to the spinal nerve, to enter the vertebral canal, where it divides into ascending, descending, and transverse branches. These branches communicate with corresponding branches from the segments above and below, and from the opposite side, forming arcades along the floor of the vertebral canal. Meningeal branches of the arcades form a plexus on the ventral surface of the dural sac and nerve root sleeves which attenuates laterally; the posterior paramedian dura is devoid of nerve endings. Skeletal branches are distributed to the posterior longitudinal ligament, the periosteum of the vertebral bodies, and to the posterior and posterolateral aspects of the intervertebral discs. Vascular branches accompany the veins and arteries of the vertebral canal and those of the vertebral bodies. The upper three cervical meningeal nerves ascend through the foramen magnum into the posterior cranial fossa, where they innervate the dura mater that covers the clivus. En route, they innervate the median atlanto-axial joint and its ligaments.

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Fig. 43.7  The course and skeletal distribution of the lumbar sinuvertebral nerves. Each nerve supplies the intervertebral disc at its level of entry into the vertebral canal, the disc above, and the intervening posterior longitudinal ligament. In about one-third of cases, the nerve at a particular level may be represented by more than one filament.


Functional components of spinal nerves

A typical spinal nerve contains somatic efferent fibres and somatic and visceral afferent fibres. Some, but not all, spinal nerves also contain preganglionic autonomic fibres.

Somatic components

Somatic efferent fibres innervate skeletal muscles and are axons of α, β and γ neurones in the spinal ventral grey column. Somatic afferent fibres convey impulses into the CNS from receptors in the skin, subcutaneous tissue, muscles, tendons, fasciae and joints: they are peripheral processes of unipolar neurones in the spinal ganglia.

Visceral components

Preganglionic visceral efferent sympathetic fibres are axons of neurones in the spinal intermediolateral grey column throughout the thoracic and upper two or three lumbar segments: they join the sympathetic trunk via corresponding white rami communicantes and synapse with postganglionic neurones which are distributed to smooth muscle, myocardium or exocrine glands. The preganglionic visceral efferent parasympathetic fibres are axons of neurones in the spinal lateral grey column of the second to fourth sacral segments: they leave the ventral rami of corresponding sacral nerves and synapse in pelvic ganglia. The postganglionic axons are distributed mainly to smooth muscle or glands in the walls of the pelvic viscera. Visceral afferent fibres have cell bodies in the spinal ganglia. Their peripheral processes pass through white rami communicantes and, without synapsing, through one or more sympathetic ganglia to end in the walls of the viscera. Some visceral afferent fibres may enter the spinal cord in the ventral roots.

Central processes of ganglionic unipolar neurones enter the spinal cord by dorsal roots and synapse on somatic or sympathetic efferent neurones, usually through interneurones, completing reflex paths. Alternatively, they may synapse with other neurones in the spinal or brain stem grey matter which give origin to a variety of ascending tracts.

 

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Nerve Plexuses

The simple distribution pattern of dorsal and ventral rami illustrated in Figure 13-6aОписание: View a text illustration, bОписание: View a text illustration applies to spinal nerves T2-T12. But in segments controlling the skeletal musculature of the neck, upper limbs, or lower limbs, the situation is more complicated. During development, small skeletal muscles innervated by different ventral rami typically fuse to form larger muscles with compound origins. Although the anatomical distinctions between the component muscles may disappear, separate ventral rami continue to provide sensory innervation and motor control to each portion of the compound muscle. As they converge, the ventral rami of adjacent spinal nerves blend their fibers, producing a series of compound nerve trunks. Such a complex interwoven network of nerves is a nerve plexus (plexus, braid). There are three major plexuses: the cervical plexus, the brachial plexus, and the lumbosacral plexus (Figure 13-8Описание: View a text illustration).

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In Chapter 11, we introduced the peripheral nerves that control the major axial and appendicular muscles. Look at the tables in that chapter as we proceed to review the innervation of the skeletal muscle groups.

Peripheral nerve palsies, or peripheral neuropathies, are characterized by regional losses of sensory and motor function as the result of nerve trauma or compression. You have experienced a mild, temporary palsy if your arm or leg has ever "fallen asleep" after you leaned or sat in an uncomfortable position.

Although dermatomes can provide clues to the location of injuries along the spinal cord, the loss of sensation at the skin does not provide sufficiently precise information about the site of injury because the boundaries of dermatomes are not exact, clearly defined lines. More exact conclusions can be drawn from the loss of motor control, on the basis of the origin and distribution of the peripheral nerves originating at nerve plexuses.

If a peripheral axon is damaged but not displaced, normal function may eventually return as the cut stump grows across the injury site away from the soma and along its former path.

Repairs made after damage to an entire peripheral nerve are generally incomplete, primarily because of problems with axon alignment and regrowth. A variety of technologically sophisticated procedures designed to improve nerve regeneration and repair are currently under evaluation. An entire family of nerve growth factors has been discovered in recent years. Their use alone or in combination with other therapies may ultimately revolutionize the treatment of damaged neural tissue inside and outside the CNS.

 

The Cervical Plexus

The cervical plexus (Figures 13-8,Описание: View a text illustration 13-9Описание: View a text illustration; Table 13-1) consists of the ventral rami of spinal nerves C1-C5. Its branches innervate the neck's muscles and extend into the thoracic cavity, where they control the diaphragmatic muscles. The phrenic nerve, the major nerve of this plexus, provides the entire nerve supply to the diaphragm, a key respiratory muscle. Other branches are distributed to the skin of the neck and the upper part of the chest.

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The Cervical Plexus

Spinal Segment

Nerve(s)

Distribution

C1-C4

Ansa cervicalis (superior and inferior branches)

Five of the extrinsic laryngeal muscles: sternothyroid, sternohyoid, omohyoid, geniohyoid, and thyrohyoid (via N XII)

C2-C3

Lesser occipital, transverse cervical, supraclavicular, and greater auricular nerves

Skin of upper chest, shoulder, neck, and ear

C3-C5

Phrenic nerve

Diaphragm

C1-C5

Cervical nerves

Levator scapulae, scalenes, sternocleidomastoid, and trapezius (with N XI)

 

 

The Brachial Plexus

The brachial plexus (Table 13-2) innervates the shoulder girdle and upper limb, with contributions from the ventral rami of spinal nerves C5-T1 (Figures 13-8Описание: View a text illustration and 13-10Описание: View a text illustration). The nerves that form this plexus originate from trunks and cords named according to their location. Axons from the spinal nerves pass through the superior, middle, and inferior trunks to reach the lateral, medial, and posterior cords, respectively. The major nerves of the lateral cord are the musculocutaneous nerve and the median nerve. The ulnar nerve is the major nerve of the medial cord. The axillary (circumflex) nerve and radial nerve are the major nerves of the posterior cord.

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The Lumbosacral Plexus

The lumbosacral plexus (Figures 13-8Описание: View a text illustration) arises from the lumbar and sacral segments of the spinal cord, and the ventral rami of these nerves supply the pelvic girdle and lower limbs. This plexus can be subdivided into a lumbar plexus (T12-L4) and a sacral plexus (L4-S4). The individual nerves that form the lumbosacral plexus and their distributions are detailed in Table 13-3

The Lumbosacral Plexus

Spinal Segment

Nerve(s)

Distribution

Lumbar plexus

T12, L1

Iliohypogastric nerve

Abdominal muscles (external and internal obliques, transversus abdominis)

Skin over lower abdomen and buttocks

L1

Ilioinguinal nerve

Abdominal muscles (with iliohypogastric)

Skin over medial upper thigh and portions of external genitalia

L1, L2

Genitofemoral nerve

Skin over anteromedial surface of thigh and portions of external genitalia

L2, L3

Lateral femoral cutaneous nerve

Skin over anterior, lateral, and posterior surfaces of thigh

L2-L4

Femoral nerve

Anterior muscles of thigh (sartorius and quadriceps)

Adductors of thigh (pectineus and iliopsoas)

Skin over anteromedial surface of thigh, medial surface of leg and foot

L2-L4

Obturator nerve

Adductors of thigh (adductor magnus, brevis, longus)

Gracilis muscle

Skin over medial surface of thigh

L2-L4

Saphenous nerve

Skin over medial surface of leg

Sacral plexus

L4-S2

Gluteal nerves: Superior

 

Abductors of thigh (gluteus minimus, gluteus medius, and tensor fasciae latae)

Inferior

Extensor of thigh (gluteus maximus)

Posterior femoral cutaneous nerve

Skin of perineum and posterior surface of thigh and leg

L4-S3

Sciatic nerve:

Two of the hamstrings (semimembranosus and semitendinosus)

Adductor magnus (with obturator nerve)

Tibial nerve

Flexors of leg and plantar flexors of foot (popliteus, gastrocnemius, soleus, tibialis posterior, long head of biceps femoris)

Flexors of toes

Skin over posterior surface of leg, plantar surface of foot

Peroneal nerve

Biceps femoris (short head)

Peroneus (brevis and longus) and tibialis anterior

Extensors of toes

Skin over anterior surface of leg and dorsal surface of foot

S2-S4

Pudendal nerve

Muscles of perineum, including urogenital diaphragm and external anal and urethral sphincters

Skin of external genitalia and related skeletal muscles (bulbospongiosus and ischiocavernosus)

 

The major nerves of the lumbar plexus are the genitofemoral nerve, lateral femoral cutaneous nerve, and femoral nerve. The major nerves of the sacral plexus are the sciatic nerve and the pudendal nerve. The sciatic nerve passes posterior to the femur, deep to the long head of the biceps femoris muscle. As it approaches the popliteal fossa, the sciatic nerve divides into two branches—the peroneal nerve and the tibial nerve.


CONCEPT CHECK QUESTIONS


1.               An anesthetic blocks the function of the dorsal rami of the cervical spinal nerves. What areas of the body will be affected?

2.               Injury to which of the nerve plexuses would interfere with the ability to breathe?

3.               Compression of which nerve produces the sensation that your leg has "fallen asleep"?

THE SYMPATHETIC DIVISION

The sympathetic division (Figure 16-3 Описание: View a text illustration) consists of preganglionic neurons located between segments T1 and L2 of the spinal cord and ganglionic neurons located in ganglia near the vertebral column. The preganglionic neurons are situated in the lateral gray horns, and their axons enter the ventral roots of these segments. The ganglionic neurons occur in three different locations:

1.     Sympathetic chain ganglia. Sympathetic chain ganglia, also called paravertebral ganglia or lateral ganglia, lie on either side of the vertebral column. Neurons in these ganglia control effectors in the body wall and inside the thoracic cavity (Figure 16-4a Описание: View a text illustration).

2.     Collateral ganglia. Collateral ganglia , also known as prevertebral ganglia, are anterior to the vertebral bodies (Figure 16-4b Описание: View a text illustration). Collateral ganglia contain ganglionic neurons that innervate tissues and organs in the abdominopelvic cavity.

3.     The adrenal medullae.The center of each adrenal gland, an area known as the adrenal medulla, is a modified sympathetic ganglion. The ganglionic neurons of the adrenal medullae have very short axons; when stimulated, they release their neurotransmitters into the bloodstream (Figure 16-4c Описание: View a text illustration). This change in the release site—from a synapse to a capillary—allows the neurotransmitters to function as hormones that affect target cells throughout the body.

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The preganglionic fibers are relatively short, because the ganglia are located relatively near the spinal cord. In contrast, the postganglionic fibers are relatively long, except at the adrenal medullae.

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The Sympathetic Chain

The ventral roots of spinal segments T1 to L2 contain sympathetic preganglionic fibers. The basic pattern of sympathetic innervation in these regions was described in Figure 13-6a Описание: View a text illustration. After passing through the intervertebral foramen, each ventral root gives rise to a myelinated white ramus, or white ramus communicans, that carries preganglionic fibers into a nearby sympathetic chain ganglion. These fibers may synapse within the sympathetic chain ganglia (Figure 16-4a Описание: View a text illustration), at one of the collateral ganglia (Figure 16-4b Описание: View a text illustration), or in the adrenal medullae (Figure 16-4c Описание: View a text illustration). Extensive divergence occurs, with one preganglionic fiber synapsing on two dozen or more ganglionic neurons. Preganglionic fibers running between the sympathetic chain ganglia interconnect them, making the chain resemble a string of beads. Each ganglion in the sympathetic chain innervates a particular body segment or group of segments.

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If a preganglionic fiber carries motor commands that target structures in the body wall or the thoracic cavity, it will synapse in one or more of the sympathetic chain ganglia (Figure 16-4a Описание: View a text illustration). The paths of the unmyelinated postganglionic fibers differ depending on whether their targets lie in the body wall or within the thoracic cavity. Postganglionic fibers that control visceral effectors in the body wall, such as the sweat glands of the skin or the smooth muscles in superficial blood vessels, enter the gray ramus (gray ramus communicans) and return to the spinal nerve for subsequent distribution. However, spinal nerves do not innervate structures in the ventral body cavities. Postganglionic fibers targeting structures in the thoracic cavity, such as the heart and lungs, form sympathetic nerves that proceed directly to their peripheral targets. Although Figure 16-4a Описание: View a text illustrationshows sympathetic nerves on the left side and spinal nerve distrib- ution on the right, in reality both innervation patterns are found on each side of the body.

Postganglionic fibers leaving the sympathetic chain reach their peripheral targets by way of spinal nerves and sympathetic nerves. Figure 16-4a Описание: View a text illustrationsummarizes the primary results of increased activity in the postganglionic fibers leaving the sympathetic chain ganglia in spinal nerves and sympathetic nerves. How these effects are brought about will be the focus of a later section.

Anatomy of the Sympathetic Chain

Figure 16-5 Описание: View a text illustrationprovides a more detailed diagram of the structure of the sympathetic division. The left side represents the distribution to the skin and to skeletal muscles and other tissues of the body wall; the right side depicts the innervation of visceral structures.

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In each chain, there are 3 cervical, 10-12 thoracic, 4-5 lumbar, and 4-5 sacral sympathetic ganglia and 1 coccygeal sympathetic ganglion. Preganglionic neurons are limited to spinal cord segments T1-L2, and these spinal nerves have both white rami (myelinated preganglionic fibers) and gray rami (unmyelinated postganglionic fibers). The neurons in the cervical, lower lumbar, and sacral sympathetic chain ganglia are innervated by preganglionic fibers that run along the axis of the chain. In turn, these chain ganglia provide postganglionic fibers, via gray rami, to the cervical, lumbar, and sacral spinal nerves. As a result, although only spinal nerves T1-L2 have white rami, every spinal nerve has a gray ramus that carries sympathetic postganglionic fibers for distribution in the body wall.

About 8 percent of the axons in each spinal nerve are sympathetic postganglionic fibers. As a result, the dorsal and ventral rami of the spinal nerves, which provide somatic motor innervation to skeletal muscles of the body wall and limbs, also distribute sympathetic postganglionic fibers. In the head, postganglionic sympathetic fibers leaving the cervical chain ganglia supply the regions and structures innervated by cranial nerves III, VII, IX , and X (Figure 16-5 Описание: View a text illustration).

In summary: (1) Only the thoracic and upper lumbar ganglia receive preganglionic fibers from white rami; (2) the cervical, lower lumbar, and sacral chain ganglia receive preganglionic innervation through collateral fibers; and (3) every spinal nerve receives a gray ramus from a ganglion of the sympathetic chain.

This anatomical arrangement means that if the ventral roots of thoracic spinal nerves are damaged, there will be no sympathetic motor function on the affected side of the head, neck, and trunk. Yet damage to the ventral roots of cervical spinal nerves will produce voluntary muscle paralysis on the affected side but will leave sympathetic function intact, because the preganglionic fibers innervating the cervical ganglia originate in the white rami of thoracic segments, which are undamaged.

In contrast, damage to the cervical ganglia or thoracic segments can eliminate sympathetic innervation to the face, although sensation and muscle control remain unaffected. The affected side of the face becomes flushed, although sweating does not occur, and the pupil constricts. This combination of symptoms is known as Horner's syndrome.

Collateral Ganglia

The abdominopelvic viscera receive sympathetic innervation by way of preganglionic fibers that pass through the sympathetic chain without synapsing. These fibers originate at preganglionic neurons in the lower thoracic and upper lumbar segments. They synapse within separate collateral ganglia (Figure 16-4b Описание: View a text illustration). Preganglionic fibers that innervate the collateral ganglia form the splanchnic nerves, which lie in the dorsal wall of the abdominal cavity. Splanchnic nerves from both sides of the body converge on these ganglia. Although there are two sympathetic chains, one on each side of the vertebral column, most collateral ganglia are single rather than paired.

Postganglionic fibers leaving the collateral ganglia extend throughout the abdominopelvic cavity, innervating a variety of visceral tissues and organs. A summary of the effects of increased sympathetic activity along these postganglionic fibers is included in Figure 16-4b Описание: View a text illustration. The general pattern is (1) a reduction of blood flow and energy use by visceral organs that are not important to short-term survival, such as the digestive tract, and (2) the release of stored energy reserves.

The splanchnic nerves innervate three collateral ganglia. Preganglionic fibers from the seven lower thoracic segments end at the celiac ganglion and the superior mesenteric ganglion. These ganglia are embedded in an extensive network of autonomic nerves. Preganglionic fibers from the lumbar segments form splanchnic nerves that end at the inferior mesenteric ganglion. These ganglia are diagrammed in Figure 16-5 Описание: View a text illustration.

The ganglia are named by their association with adjacent arteries. For example, the celiac ganglion is named after the celiac artery. The celiac ganglion most commonly consists of a pair of interconnected masses of gray matter situated at the base of that artery. The celiac ganglion may also form a single mass or many small, interwoven masses. Postganglionic fibers from this ganglion innervate the stomach, liver, pancreas, and spleen.

The superior mesenteric ganglion sits near the base of the superior mesenteric artery. Postganglionic fibers leaving the superior mesenteric ganglion innervate the small intestine and the initial segments of the large intestine. The inferior mesenteric ganglion is located near the base of the inferior mesenteric artery. Postganglionic fibers from this ganglion provide sympathetic innervation to the terminal portions of the large intestine, the kidney and bladder, and the sex organs.

The Adrenal Medullae

Preganglionic fibers entering an adrenal gland proceed to its center, a region called the adrenal medulla (Figures 16-4c Описание: View a text illustrationand 16-5 Описание: View a text illustration). The adrenal medulla is a modified sympathetic ganglion. Within the medulla, preganglionic fibers synapse on neuroendocrine cells, specialized neurons that release the neurotransmitters epinephrine (E) and norepinephrine (NE) into the general circulation. Epinephrine, also called adrenaline, accounts for 75-80 percent of the secretory output; the rest is NE.

The bloodstream then carries the neurotransmitters throughout the body, causing changes in the metabolic activities of many different cells. In general, these effects resemble those produced by the stimulation of sympathetic postganglionic fibers. They differ, however, in two respects: (1) Cells not innervated by sympathetic postganglionic fibers are affected as well, and (2) the effects last much longer than those produced by direct sympathetic innervation.

Sympathetic Activation

The sympathetic division can change tissue and organ activities by releasing NE at peripheral synapses and by distributing E and NE throughout the body in the bloodstream. The visceral motor fibers that target specific effectors, such as smooth muscle fibers in blood vessels of the skin, can be activated in reflexes that do not involve other peripheral effectors. In a crisis, however, the entire division responds. This event is called sympathetic activation. Sympathetic activation is controlled by sympathetic centers in the hypothalamus. The effects are not limited to peripheral tissues; sympathetic activation also alters CNS activity. When sympathetic activation occurs, an individual experiences the following:

These changes, plus the peripheral changes already noted, complete the preparations necessary for the individual to cope with stressful situations.

Neurotransmitters and Sympathetic Function

We have examined the distribution of sympathetic impulses and the general effects of sympathetic activation. We will now consider the cellular basis of these effects on peripheral organs. On stimulation, sympathetic preganglionic fibers release ACh at synapses that innervate ganglionic neurons. Synapses that use ACh as a transmitter are called cholinergic. The effect on the ganglionic neurons is always excitatory.

Stimulation of the ganglionic neurons in the sympathetic division leads to the release of neurotransmitter at postganglionic neuroeffector junctions. (Recall from Chapter 12 that neuroeffector junctions are synapses between a neuron and another cell type.) Описание: Go to related contentThe synaptic terminals are typically different from the neuroeffector junctions of the somatic nervous system. Instead of forming individual synaptic knobs, the telodendria form a network or chain of varicosities, swollen segments that either contact the target cells or end in the adjacent connective tissues (Figure 16-6 Описание: View a text illustration).

Описание: TF_1606

Most sympathetic ganglionic neurons release NE at their varicosities. As we learned in Chapter 12, neurons that use NE as a neurotransmitter are called adrenergic. Описание: Go to related contentThe sympathetic division also contains a small but significant number of ganglionic neurons that release ACh rather than NE. The varicosities involved with ACh release are located in the body wall, the skin, and in skeletal muscles.

The NE released by varicosities affects its targets for just a few seconds before it is inactivated by enzymes. (As usual, the specific effects on the target cells vary with the nature of the receptor on the postsynaptic membrane.) Because your bloodstream does not contain the enzymes that break down NE or E, and because most tissues contain relatively low concentrations of those enzymes, the effects of E or NE released by the adrenal medullae last much longer. Tissue concentrations of epinephrine throughout the body may remain elevated for as long as 30 seconds, and the effects may persist for several minutes.

Membrane Receptors

The effects of sympathetic stimulation result primarily from interactions with membrane receptors sensitive to NE and E. There are two classes of sympathetic receptors: alpha receptors and beta receptors. In general, norepinephrine stimulates alpha receptors more than it does beta receptors, whereas epinephrine stimulates both classes of receptors.

Alpha Receptors
Stimulation of
alpha (Описание: ALPHABLU) receptors activates enzymes on the inside of the cell membrane. There are two types of alpha receptors: alpha-1 ( Описание: ALPHA1) and alpha-2 ( Описание: ALPHA2) (Figure 16-7a Описание: View a text illustration). The result for the most common type of alpha receptor, Описание: ALPHA1, is the release of intracellular calcium ions from reserves in the endoplasmic reticulum. This response follows the release of second messengers inside the target cell. Описание: Go to related contentThe release of calcium ions generally has an excitatory effect on the target cell. For example, the stimulation of Описание: ALPHA1 receptors on the surfaces of smooth muscle cells is responsible for the constriction of peripheral blood vessels and the closure of sphincters along the digestive tract. Alpha-2 receptors are less common; their stimulation results in a lowering of cyclic-AMP (cAMP) levels in the cytoplasm. This reduction generally has an inhibitory effect on the cell. The presence of Описание: ALPHA2 receptors within the parasympathetic division helps coordinate sympathetic and parasympathetic activities. When the sympathetic division is active, the NE released binds to parasympathetic neuroeffector junctions and inhibits their activity.

Описание: TF_1607

Beta Receptors
Beta (ß) receptors are located in many organs, including skeletal muscles, the lungs, the heart, and the liver. Stimulation of beta receptors at these sites triggers changes in the metabolic activity of the target cell. These alterations occur indirectly, as the beta receptor causes the formation of a second messenger, cAMP, which activates or inactivates key enzymes (Figure 16-7b
Описание: View a text illustration).

There are two major types of beta receptors: beta-1 (ß1) and beta-2 (ß2). Stimulation of ß1 receptors leads to an increase in metabolic activity. For example, stimulation of ß1 receptors on skeletal muscles accelerates the metabolic activities of the muscles. Stimulation of ß1 receptors in the heart causes an increase in heart rate and in the force of contraction. Stimulation of ß2 receptors causes inhibition. When stimulated, these receptors trigger a relaxation of smooth muscles along the respiratory tract, increasing the diameter of the respiratory passageways and making breathing easier. This response accounts for the effectiveness of the inhalers used to treat asthma.

The effects of NE on the postsynaptic membrane last longer than those of ACh, because the NE is removed relatively slowly. From 50 to 80 percent of the NE is re-absorbed by the varicosities and either re-used or broken down by the enzyme monoamine oxidase (MAO). The rest of the NE diffuses out of the area or is broken down by the enzyme catechol-O-methyltransferase (COMT) in surrounding tissues.

Sympathetic Stimulation, ACh, and NO

Although the vast majority of sympathetic postganglionic fibers are adrenergic, releasing NE, a few postganglionic fibers are cholinergic. These postganglionic fibers innervate sweat glands of the skin and the blood vessels to skeletal muscles and the brain. Activation of these sympathetic fibers stimulates sweat gland secretion and dilates the blood vessels.

It may seem strange that sympathetic terminals release ACh, which is the neurotransmitter used by the parasympathetic nervous system. However, (1) ACh stimulates sweat gland secretion much more than does NE; (2) NE release causes constriction of most peripheral arteries; and (3) neither the body wall nor skeletal muscles are innervated by the parasympathetic division. The distribution of cholinergic fibers via the sympathetic division provides a method of stimulating sweat gland secretion and selectively enhancing blood flow to skeletal muscles while the adrenergic terminals reduce the blood flow to other tissues in the body wall.

The sympathetic division also includes nitroxidergic synapses, which release nitric oxide (NO) as a neurotransmitter. As we mentioned in Chapter 12, such synapses occur where neurons innervate smooth muscles in the walls of blood vessels in many regions, notably in skeletal muscles and the brain. Описание: Go to related contentActivity of these synapses promotes immediate vasodilation and increased blood flow through the region.

A Summary of the Sympathetic Division

To summarize our discussion of the sympathetic division:

1.     The sympathetic division of the ANS includes two segmentally arranged sympathetic chains, one on each side of the spinal column; three collateral ganglia anterior to the spinal column; and two adrenal medullae.

2.     The preganglionic fibers are short, because the ganglia are close to the spinal cord. The postganglionic fibers are relatively long and extend a considerable distance before reaching their target organs. (In the case of the adrenal medullae, very short axons end at capillaries that carry their secretions to the bloodstream.)

3.     The sympathetic division shows extensive divergence, and a single preganglionic fiber may innervate two dozen or more ganglionic neurons in different ganglia. As a result, a single sympathetic motor neuron inside the CNS can control a variety of peripheral effectors and produce a complex and coordinated response.

4.     All preganglionic neurons release ACh at their synapses with ganglionic neurons. Most postganglionic fibers release NE, but a few release ACh or NO.

5.     The effector response depends on the nature of the channels or enzymes activated when NE or E binds to alpha or beta receptors.


CONCEPT CHECK QUESTIONS


1.     Where do the nerves that synapse in the collateral ganglia originate?

2.     How would a drug that stimulates acetylcholine receptors affect the sympathetic nervous system?

3.     An individual with high blood pressure may be given a medication that blocks beta receptors. How would this medication help that person's condition?

 

THE PARASYMPATHETIC DIVISION

The parasympathetic division of the ANS (Figure 16-8 Описание: View a text illustration) consists of the following:

Описание: TF_1608

1.     Preganglionic neurons in the brain stem and in sacral segments of the spinal cord. In the brain, the mesencephalon, pons, and medulla oblongata contain autonomic nuclei associated with cranial nerves III, VII, IX, and X. In the sacral segments of the spinal cord, the autonomic nuclei lie in the lateral gray horns of spinal segments S2-S4.

2.     Ganglionic neurons in peripheral ganglia located within or adjacent to the target organs. The preganglionic fibers of the parasympathetic division do not diverge as extensively as do those of the sympathetic division. A typical preganglionic fiber synapses on six to eight ganglionic neurons. In contrast to the pattern in the sympathetic division, all these ganglionic neurons are located in the same ganglion and their postganglionic fibers influence the same target organ. As a result, the effects of parasympathetic stimulation are more specific and localized than those of the sympathetic division.

Organization and Anatomy of the Parasympathetic Division

Parasympathetic preganglionic fibers leave the brain as components of cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus) (Figure 16-9 Описание: View a text illustration). These fibers carry the cranial parasympathetic output. Parasympathetic fibers in the oculomotor, facial, and glossopharyngeal nerves control visceral structures in the head. These fibers synapse in the ciliary, sphenopalatine, submandibular, and otic ganglia (Figure 14-23 Описание: View a text illustration) Short postganglionic fibers then continue to their peripheral targets. The vagus nerve provides preganglionic parasympathetic innervation to structures in the thoracic and abdominopelvic cavity as distant as the last segments of the large intestine. The vagus nerve alone provides roughly 75 percent of all parasympathetic outflow.

Описание: TF_1609

The preganglionic fibers in the sacral segments of the spinal cord carry the sacral parasympathetic output. These fibers do not join the ventral roots of the spinal nerves. Instead, the preganglionic fibers form distinct pelvic nerves, which innervate intramural ganglia in the kidney and urinary bladder, the terminal portions of the large intestine, and the sex organs.

Описание: TF_1423

General Functions of the Parasympathetic Division

The following is a partial listing of the major effects produced by the parasympathetic division:

These functions center on relaxation, food processing, and energy absorption. The parasympathetic division has been called the anabolic system because its stimulation leads to a general increase in the nutrient content of the blood. (Anabolic comes from the Greek word anabole, which means "a rising up." Описание: Go to related content) Cells throughout the body respond to this increase by absorbing nutrients and using them to support growth, cell division, and the creation of energy reserves in the form of lipids or glycogen.

Parasympathetic Activation and Neurotransmitter Release

All the preganglionic and postganglionic fibers in the parasympathetic division release ACh at synapses and neuroeffector junctions. The neuroeffector junctions are small and have narrow synaptic clefts. The effects of stimulation are short-lived, because most of the ACh released is inactivated by acetylcholinesterase (AChE) within the synapse. Any ACh diffusing into the surrounding tissues will be inactivated by the enzyme tissue cholinesterase. As a result, the effects of parasympathetic stimulation are quite localized, and they last a few seconds at most.

Membrane Receptors and Responses

Although all the synapses (neuron to neuron) and neuroeffector junctions (neuron to effector) of the parasympathetic division use the same transmitter, ACh, two different types of ACh receptors occur on the postsynaptic membranes:

1.     Nicotinic receptors are located on the surfaces of ganglion cells of both the parasympathetic and sympathetic divisions as well as at neuromuscular junctions of the somatic motor system. Exposure to ACh always causes excitation of the ganglionic neuron or muscle fiber via the opening of membrane ion channels.

2.     Muscarinic receptors are located at cholinergic neuroeffector junctions in the parasympathetic division as well as at the few cholinergic neuroeffector junctions in the sympathetic division. Stimulation of muscarinic receptors produces longer-lasting effects than does stimulation of nicotinic receptors. The response, which reflects the activation or inactivation of specific enzymes, may be excitatory or inhibitory.

The names nicotinic and muscarinic indicate the chemical compounds that stimulate these receptor sites. Nicotinic receptors bind nicotine, a powerful toxin that can be obtained from a variety of sources, including tobacco leaves. Muscarinic receptors are stimulated by muscarine, a toxin produced by some poisonous mushrooms.

These compounds have discrete actions, targeting either the autonomic ganglia and skeletal neuromuscular junctions (nicotine) or the parasympathetic neuroeffector junctions (muscarine). They produce dangerously exaggerated, uncontrolled responses that parallel those produced by normal receptor stimulation. For example, nicotine poisoning occurs if as little as 50 mg of the compound is ingested or absorbed through the skin. The symptoms reflect widespread autonomic activation—vomiting, diarrhea, high blood pressure, rapid heart rate, sweating, and profuse salivation. Because the neuromuscular junctions of the somatic motor system are stimulated, convulsions occur. In severe cases, stimulation of nicotinic receptors inside the CNS may lead to coma and death within minutes. The symptoms of muscarine poisoning are almost entirely restricted to the parasympathetic division: salivation, nausea, vomiting, diarrhea, constriction of respiratory passages, low blood pressure, and an abnormally slow heart rate. Table 16-1 summarizes details about the adrenergic and cholinergic receptors of the ANS.

A Summary of the Parasympathetic Division

To summarize our discussion of the parasympathetic division:

1.     The parasympathetic division includes visceral motor nuclei associated with four cranial nerves (III, VII, IX, and X) and with sacral segments S2-S4.

2.     The ganglionic neurons are located within or next to their target organs.

3.     The parasympathetic division innervates areas serviced by the cranial nerves and organs in the thoracic and abdominopelvic cavities.

4.     All parasympathetic neurons are cholinergic. Ganglionic neurons have nicotinic receptors, which are excited by ACh. Muscarinic receptors present at neuroeffector junctions may produce either excitation or inhibition, depending on the nature of the enzymes activated when ACh binds to the receptor.

5.     The effects of parasympathetic stimulation are generally brief and restricted to specific organs and sites.


CONCEPT CHECK QUESTIONS


1.     Which nerve is responsible for parasympathetic innervation of the lungs, heart, stomach, liver, pancreas, and parts of the small and large intestines?

2.     What effect would stimulation of muscarinic receptors in cardiac muscle have on the heart?

3.     Why is the parasympathetic division of the ANS sometimes referred to as the anabolic system?