Spinal Cord & Nerve (Tortora).pdf

JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 492 13 THE SPINAL CORD AND SPINAL NERVES THE SPINAL CORD AND SPINAL NERVES AND HOMEOSTASIS The spinal cord and spinal nerves...

JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 492 13 THE SPINAL CORD AND SPINAL NERVES THE SPINAL CORD AND SPINAL NERVES AND HOMEOSTASIS The spinal cord and spinal nerves contribute to homeostasis by providing quick, reflexive responses to many stimuli. The spinal cord is the pathway for sensory input to the brain and motor output from the brain. About 100 million neurons and even more neu- roglia compose the spinal cord, the part of the central nervous system that extends from the brain. The spinal cord and its associated spinal nerves contain neural circuits that control some of your most rapid reactions to environmental changes. If you pick up something hot, the grasping muscles may relax and you may drop the hot object even before you are consciously aware of the extreme heat or pain. This is an example of a spinal cord reflex—a quick, automatic response to certain kinds of stimuli that involves neurons only in the spinal nerves and spinal cord. Besides processing reflexes, the gray matter of the spinal cord also is a site for integration (summing) of excitatory post- synaptic potentials (EPSPs) and inhibitory postsy- naptic potentials (IPSPs), which you learned about in Chapter 12. These graded potentials arise as neurotransmitter molecules interact with their receptors at synapses in the spinal cord. The white matter of the spinal cord contains a dozen major sensory and motor tracts, which function as the “highways” along which sensory input travels to the brain and motor output travels from the brain to skeletal muscles and other effectors. Recall that the spinal cord is continuous with the brain and that together they make up the central nervous system (CNS). Did you ever wonder why 492 ? spinal cord injuries can have such widespread effects on the body? JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 493 13.1 SPINAL CORD ANATOMY 493 13.1 SPINAL CORD ANATOMY 2. Arachnoid mater (a-RAK-noyd MĀ-ter; arachn-  spider; -oid  similar to). This layer, the middle of the meningeal OBJECTIVES membranes, is a thin, avascular covering comprised of cells Describe the protective structures and the gross anatomical and thin, loosely arranged collagen and elastic fibers. It is features of the spinal cord. called the arachnoid mater because of its spider’s web arrange- Describe how spinal nerves are connected to the spinal ment of delicate collagen fibers and some elastic fibers. It is cord. deep to the dura mater and is continuous through the foramen magnum with the arachnoid mater of the brain. Between the Protective Structures dura mater and the arachnoid mater is a thin subdural space, Recall from the previous chapter that the nervous tissue of the cen- which contains interstitial fluid. tral nervous system is very delicate and does not respond well to in- 3. Pia mater (PĒ-a MĀ-ter; pia  delicate). This innermost jury or damage. Accordingly, nervous tissue requires considerable meninx is a thin transparent connective tissue layer that ad- protection. The first layer of protection for the central nervous sys- heres to the surface of the spinal cord and brain. It consists of tem is the hard bony skull and vertebral column. The skull encases thin squamous to cuboidal cells within interlacing bundles of the brain and the vertebral column surrounds the spinal cord, provid- collagen fibers and some fine elastic fibers. Within the pia mater ing strong protective defenses against damaging blows or bumps. are many blood vessels that supply oxygen and nutrients to the The second protective layer is the meninges, three membranes that spinal cord. Triangular-shaped membranous extensions of the lie between the bony encasement and the nervous tissue in both the pia mater suspend the spinal cord in the middle of its dural brain and spinal cord. Finally, a space between two of the meningeal sheath. These extensions, called denticulate ligaments (den- membranes contains cerebrospinal fluid, a buoyant liquid that sus- TIK-ū-lāt  small tooth), are thickenings of the pia mater. They pends the central nervous tissue in a weightless environment while project laterally and fuse with the arachnoid mater and inner surrounding it with a shock-absorbing, hydraulic cushion. surface of the dura mater between the anterior and posterior nerve roots of spinal nerves on either side (Figure 13.1a, b). Ex- tending along the entire length of the spinal cord, the denticu- Vertebral Column late ligaments protect the spinal cord against sudden displace- The spinal cord is located within the vertebral canal of the ver- ment that could result in shock. Between the arachnoid mater tebral column. As you learned in Chapter 7, the vertebral foram- and pia mater is a space, the subarachnoid space, which con- ina of all the vertebrae, stacked one on top of the other, form the tains shock-absorbing cerebrospinal fluid. vertebral canal. The surrounding vertebrae provide a sturdy shelter for the enclosed spinal cord (see Figure 13.1c). The CLINICAL CONNECTION | Spinal Tap vertebral ligaments, meninges, and cerebrospinal fluid provide additional protection. In a spinal tap (lumbar puncture), a local anesthetic is given, and a long hollow needle is inserted into the subarachnoid space to Meninges withdraw cerebrospinal fluid (CSF) for diagnostic purposes; to intro- duce antibiotics, contrast media for myelography, or anesthetics; to The meninges (me-NIN-jē-z; singular is meninx [ME-ninks]) are administer chemotherapy; to measure CSF pressure; and/or to evalu- three protective, connective tissue coverings that encircle the spinal ate the effects of treatment for diseases such as meningitis. During cord and brain. From superficial to deep they are the (1) dura mater, this procedure, the patient lies on his or her side with the vertebral (2) arachnoid mater, and (3) pia mater. The spinal meninges sur- column flexed. Flexion of the vertebral column increases the distance round the spinal cord (Figure 13.1a) and are continuous with the between the spinous processes of the vertebrae, which allows easy cranial meninges, which encircle the brain (shown in Figure access to the subarachnoid space. The spinal cord ends around the 14.2a). All three spinal meninges cover the spinal nerves up to the second lumbar vertebra (L2); however, the spinal meninges and cir- point where they exit the spinal column through the intervertebral culating cerebrospinal fluid extend to the second sacral vertebra (S2). foramina. The spinal cord is also protected by a cushion of fat and Between vertebrae L2 and S2 the spinal meninges are present, but connective tissue located in the epidural space (ep-i-DOO-ral), a the spinal cord is absent. Consequently, a spinal tap is normally per- space between the dura mater and the wall of the vertebral canal formed in adults between the L3 and L4 or L4 and L5 lumbar verte- brae because this region provides safe access to the subarachnoid (Figure 13.1c). Following is a description of each meningeal layer. space without the risk of damaging the spinal cord. (A line drawn 1. Dura mater (DOO-ra MĀ-ter  tough mother). The most su- across the highest points of the iliac crests, called the supracristal line, passes through the spinous process of the fourth lumbar vertebra and perficial of the three spinal meninges is a thick strong layer is used as a landmark for administering a spinal tap.) composed of dense, irregular connective tissue. The dura mater forms a sac from the level of the foramen magnum in the occipital bone, where it is continuous with the meningeal dura mater of the brain, to the second sacral vertebra. The dura External Anatomy of the Spinal Cord mater is also continuous with the epineurium, the outer cover- The spinal cord is roughly oval in shape, being flattened slightly ing of spinal and cranial nerves. anteriorly and posteriorly. In adults, it extends from the medulla JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 494 494 CHAPTER 13 THE SPINAL CORD AND SPINAL NERVES oblongata, the inferior part of the brain, to the superior border of the twelfth thoracic vertebra. Nerves to and from the lower limbs the second lumbar vertebra (Figure 13.2). In newborn infants, it arise from the lumbar enlargement. extends to the third or fourth lumbar vertebra. During early child- Inferior to the lumbar enlargement, the spinal cord terminates as hood, both the spinal cord and the vertebral column grow longer a tapering, conical structure called the conus medullaris (KŌ-nus as part of overall body growth. Elongation of the spinal cord stops med-ū-LAR-is; conus  cone), which ends at the level of the inter- around age 4 or 5, but growth of the vertebral column continues. vertebral disc between the first and second lumbar vertebrae Thus, the spinal cord does not extend the entire length of the adult (L1–L2) in adults. Arising from the conus medullaris is the filum vertebral column. The length of the adult spinal cord ranges from terminale (FĪ-lum ter-mi-NAL-ē  terminal filament), an extension 42 to 45 cm (16–18 in.). Its diameter is about 2 cm (0.75 in.) in the of the pia mater that extends inferiorly and fuses with the arachnoid midthoracic region, somewhat larger in the lower cervical and mater and dura mater and anchors the spinal cord to the coccyx. midlumbar regions, and smallest at the inferior tip. Spinal nerves are the paths of communication between the When the spinal cord is viewed externally, two conspicuous spinal cord and specific regions of the body. The spinal cord appears enlargements can be seen. The superior enlargement, the cervical to be segmented because the 31 pairs of spinal nerves emerge at reg- enlargement, extends from the fourth cervical vertebra (C4) to ular intervals from intervertebral foramina (Figure 13.2). Indeed, the first thoracic vertebra (T1). Nerves to and from the upper each pair of spinal nerves is said to arise from a spinal segment. limbs arise from the cervical enlargement. The inferior enlarge- Within the spinal cord there is no obvious segmentation but, for con- ment, called the lumbar enlargement, extends from the ninth to venience, the naming of spinal nerves is based on the segment in Figure 13.1 Gross anatomy of the spinal cord. The spinal meninges are evident in parts (a) and (c). Meninges are connective tissue coverings that surround the spinal cord and brain. Posterior SPINAL CORD: median sulcus Gray matter White matter Central canal Anterior median fissure Spinal nerve SPINAL MENINGES: Pia mater (inner) Denticulate Arachnoid ligament mater (middle) Subarachnoid space Subdural Dura mater space (outer) (a) Anterior view and transverse section through spinal cord JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 495 13.1 SPINAL CORD ANATOMY 495 SUPERIOR Fourth ventricle Cerebellum of brain (cut) Glossopharyngeal (IX) and vagus (X) nerves Accessory (XI) nerve Gracile fasciculus Occipital bone (cut) Posterior median sulcus Cuneate fasciculus Vertebral artery Denticulate ligament Posterior (dorsal) rootlets Dura mater of spinal nerve and arachnoid mater (cut) INFERIOR (b) Posterior view of cervical region of spinal cord POSTERIOR View Transverse plane Spinous Dura mater and process of arachnoid mater vertebra Subarachnoid Spinal cord space Pia mater Posterior (dorsal) root of Epidural space spinal nerve Denticulate Superior articular ligament facet of vertebra Posterior (dorsal) Anterior ramus of (ventral) root of spinal nerve spinal nerve Spinal nerve Transverse Anterior (ventral) foramen ramus of Body of spinal nerve vertebra Vertebral artery in transverse foramen ANTERIOR (c) Transverse section of the spinal cord within a cervical vertebra What are the superior and inferior boundaries of the spinal dura mater? JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 496 Figure 13.2 External anatomy of the spinal cord and the spinal nerves. The spinal cord extends from the medulla oblongata of the brain to the superior border of the second lumbar vertebra. CERVICAL PLEXUS (C1–C5): Medulla oblongata Lesser occipital nerve C1 Great auricular C2 Atlas (first cervical vertebra) Ansa cervicalis C3 Transverse cervical nerve CERVICAL NERVES (8 pairs) Supraclavicular nerve C4 Phrenic nerve Cervical enlargement C5 C6 BRACHIAL PLEXUS (C5–T1): C7 Musculocutaneous nerve C8 First thoracic vertebra Axillary nerve T1 Median nerve T2 Radial nerve Ulnar nerve T3 T4 T5 THORACIC NERVES (12 pairs) T6 T7 T8 Intercostal T9 (thoracic) nerves Lumbar enlargement T10 T11 T12 Subcostal nerve First lumbar vertebra (intercostal nerve 12) Conus medullaris LUMBAR PLEXUS (L1–L4): L1 Iliohypogastric nerve L2 Ilioinguinal nerve LUMBAR NERVES (5 pairs) Genitofemoral nerve L3 Cauda equina Lateral femoral cutaneous nerve L4 Femoral nerve Ilium of hip bone L5 Obturator nerve S1 Sacrum S2 SACRAL PLEXUS (L4–S4): Superior gluteal nerve S3 SACRAL NERVES (5 pairs) Inferior gluteal nerve S4 Sciatic nerve: S5 Common fibular COCCYGEAL NERVES (1 pair) nerve Filum terminale Tibial nerve Posterior cutaneous nerve of thigh Pudendal nerve Posterior view of entire spinal cord and portions of spinal nerves What portion of the spinal cord connects with nerves of the upper limbs? 496 JWCL316_c13_492-526.qxd 14/09/2010 01:23 Page 497 13.1 SPINAL CORD ANATOMY 497 which they are located. There are 8 pairs of cervical nerves (repre- Two bundles of axons, called roots, connect each spinal nerve sented in Figure 13.2 as C1–C8), 12 pairs of thoracic nerves to a segment of the cord by even smaller bundles of axons called (T1–T12), 5 pairs of lumbar nerves (L1–L5), 5 pairs of sacral rootlets (see Figure 13.3a). The posterior (dorsal) root and nerves (S1–S5), and 1 pair of coccygeal nerves (Co1). rootlets contain only sensory axons, which conduct nerve Figure 13.3 Internal anatomy of the spinal cord: the organization of gray matter and white matter. For simplicity, dendrites are not shown in this and several other illustrations of transverse sections of the spinal cord. Blue, red, and green arrows indicate the direction of nerve impulse propagation. The posterior gray horn contains axons of sensory neurons and cell bodies of interneurons; the lateral gray horn contains cell bodies of autonomic motor neurons; and the anterior gray horn contains cell bodies of somatic motor neurons. Posterior (dorsal) Posterior (dorsal) root of spinal nerve root ganglion Posterior rootlets Spinal nerve Posterior gray horn Lateral white column Posterior median sulcus Lateral gray horn Anterior (ventral) root Posterior white column of spinal nerve Central canal Anterior gray horn Axon of sensory neuron Gray commissure Cell body of interneuron Axon of interneuron Cell body of autonomic Anterior white commissure motor neuron Anterior white column Cell body of Cell body of somatic sensory neuron motor neuron Nerve impulses Anterior median fissure for sensations Anterior rootlets Nerve impulses to cardiac muscle, Axons of motor neurons smooth muscle, (a) Transverse section of thoracic spinal cord Nerve impulses to and glands skeletal muscles View Posterior median sulcus Posterior white column Posterior gray horn Lateral white column Transverse plane Gray commissure Lateral gray horn Central canal Anterior gray horn Anterior white column Anterior median fissure LM 5x (b) Transverse section of thoracic spinal cord What is the difference between a horn and a column in the spinal cord? JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 498 498 CHAPTER 13 THE SPINAL CORD AND SPINAL NERVES impulses from sensory receptors in the skin, muscles, and internal of autonomic motor neurons that regulate the activity of cardiac organs into the central nervous system. Each posterior root has a muscle, smooth muscle, and glands. swelling, the posterior (dorsal) root ganglion, which contains The white matter of the spinal cord, like the gray matter, is the cell bodies of sensory neurons. The anterior (ventral) root organized into regions. The anterior and posterior gray horns and rootlets contain axons of motor neurons, which conduct nerve divide the white matter on each side into three broad areas called impulses from the CNS to effectors (muscles and glands). columns: (1) anterior (ventral) white columns, (2) posterior As spinal nerves branch from the spinal cord, they pass later- (dorsal) white columns, and (3) lateral white columns (Fig- ally to exit the vertebral canal through the intervertebral foramina ure 13.3). Each column in turn contains distinct bundles of axons between adjacent vertebrae. However, because the spinal cord is having a common origin or destination and carrying similar infor- shorter than the vertebral column, nerves that arise from the lum- mation. These bundles, which may extend long distances up or bar, sacral, and coccygeal regions of the spinal cord do not leave the down the spinal cord, are called tracts. Recall that tracts are vertebral column at the same level they exit the cord. The roots of bundles of axons in the CNS, whereas nerves are bundles of axons these lower spinal nerves angle inferiorly alongside the filum termi- in the PNS. Sensory (ascending) tracts consist of axons that nale in the vertebral canal like wisps of hair. Accordingly, the roots conduct nerve impulses toward the brain. Tracts consisting of of these nerves are collectively named the cauda equina (KAW-da axons that carry nerve impulses from the brain are called motor ē-KWĪ-na), meaning “horse’s tail” (Figure 13.2). (descending) tracts. Sensory and motor tracts of the spinal cord are continuous with sensory and motor tracts in the brain. The internal organization of the spinal cord allows sensory in- Internal Anatomy of the Spinal Cord put and motor output to be processed by the spinal cord in the fol- A transverse section of the spinal cord reveals regions of white lowing way (Figure 13.4): matter that surround an inner core of gray matter (Figure 13.3). The white matter of the spinal cord consists primarily of bundles 1 Sensory receptors detect a sensory stimulus. of myelinated axons of neurons. Two grooves penetrate the white 2 Sensory neurons convey this sensory input in the form of nerve impulses along their axons, which extend from sensory matter of the spinal cord and divide it into right and left sides. The receptors into the spinal nerve and then into the posterior anterior median fissure is a wide groove on the anterior (ventral) root. From the posterior root, axons of sensory neurons may side. The posterior median sulcus is a narrow furrow on the pos- proceed along three possible paths (see steps 3 , 4 , and 5 ). terior (dorsal) side. The gray matter of the spinal cord is shaped like the letter H or a butterfly; it consists of dendrites and cell 3 Axons of sensory neurons may extend into the white matter of bodies of neurons, unmyelinated axons, and neuroglia. The gray the spinal cord and ascend to the brain as part of a sensory tract. commissure (KOM-mi-shur) forms the crossbar of the H. In the 4 Axons of sensory neurons may enter the posterior gray horn center of the gray commissure is a small space called the central and synapse with interneurons whose axons extend into the canal; it extends the entire length of the spinal cord and is filled white matter of the spinal cord and then ascend to the brain as with cerebrospinal fluid. At its superior end, the central canal is part of a sensory tract. continuous with the fourth ventricle (a space that contains cere- brospinal fluid) in the medulla oblongata of the brain. Anterior 5 Axons of sensory neurons may enter the posterior gray horn and synapse with interneurons that in turn synapse with to the gray commissure is the anterior (ventral) white commis- somatic motor neurons that are involved in spinal reflex sure, which connects the white matter of the right and left sides of pathways. Spinal cord reflexes are described in more detail the spinal cord. later in this chapter. In the gray matter of the spinal cord and brain, clusters of neu- ronal cell bodies form functional groups called nuclei. Sensory 6 Motor output from the spinal cord to skeletal muscles in- volves somatic motor neurons of the anterior gray horn. nuclei receive input from receptors via sensory neurons, and mo- Many somatic motor neurons are regulated by the brain. tor nuclei provide output to effector tissues via motor neurons. Axons from higher brain centers form motor tracts that The gray matter on each side of the spinal cord is subdivided into descend from the brain into the white matter of the spinal regions called horns (Figure 13.3). The posterior (dorsal) gray cord. There they synapse with somatic motor neurons either horns contain cell bodies and axons of interneurons as well as ax- directly or indirectly by first synapsing with interneurons that ons of incoming sensory neurons. Recall that cell bodies of sen- in turn synapse with somatic motor neurons. sory neurons are located in the posterior (dorsal) root ganglion of a spinal nerve. The anterior (ventral) gray horns contain so- 7 When activated, somatic motor neurons convey motor output matic motor nuclei, which are clusters of cell bodies of somatic in the form of nerve impulses along their axons, which motor neurons that provide nerve impulses for contraction of sequentially pass through the anterior gray horn and anterior skeletal muscles. Between the posterior and anterior gray horns root to enter the spinal nerve. From the spinal nerve, axons of are the lateral gray horns, which are present only in thoracic and somatic motor neurons extend to skeletal muscles of the body. upper lumbar segments of the spinal cord. The lateral gray horns 8 Motor output from the spinal cord to cardiac muscle, smooth contain autonomic motor nuclei, which are clusters of cell bodies muscle, and glands involves autonomic motor neurons of the JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 499 13.1 SPINAL CORD ANATOMY 499 Figure 13.4 Processing of sensory input and motor output by the spinal cord. Sensory input is conveyed from sensory receptors to the posterior gray horns of the spinal cord, whereas motor output is conveyed from the anterior and lateral gray horns of the spinal cord to effectors (muscles and glands). Sensory Motor Sensory (ascending) (descending) (ascending) tract tract tract 6 Posterior (dorsal) root ganglion Posterior (dorsal) root of spinal nerve Spinal nerve Posterior gray horn Lateral gray horn Anterior gray horn 3 Anterior (ventral) root of spinal nerve 4 Interneuron 5 Interneuron 8 Somatic motor neuron Autonomic motor neuron 7 Sensory neuron 2 1 Sensory receptors in the skin 9 Autonomic motor neuron Skeletal muscle Cardiac muscle, smooth muscle, and glands Lateral gray horns are found in which segments of the spinal cord? lateral gray horn. When activated, autonomic motor neurons 9 From the spinal nerve, axons of autonomic motor neurons convey motor output in the form of nerve impulses along their from the spinal cord synapse with another group of autonomic axons, which sequentially pass through the lateral gray horn, motor neurons located in the peripheral nervous system anterior gray horn, and anterior root to enter the spinal nerve. (PNS). The axons of this second group of autonomic motor JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 500 500 CHAPTER 13 THE SPINAL CORD AND SPINAL NERVES neurons in turn synapse with cardiac muscle, smooth muscle, tracts. (2) As the spinal cord descends from cervical to sacral seg- and glands. You will learn more about autonomic motor ments, the motor tracts decrease in thickness as more descending neurons when the autonomic nervous system is described axons leave the motor tracts to synapse with neurons in the gray in Chapter 15. matter of the spinal cord. Table 13.1 summarizes the variations in spinal cord segments. The various spinal cord segments vary in size, shape, relative amounts of gray and white matter, and distribution and shape of CHECKPOINT gray matter. For example, the amount of gray matter is largest in 1. Where are the spinal meninges located? Where are the the cervical and lumbar segments of the spinal cord because these epidural, subdural, and subarachnoid spaces located? segments are responsible for sensory and motor innervation of the 2. What are the cervical and lumbar enlargements? limbs. In addition, more sensory and motor tracts are present in 3. Define conus medullaris, filum terminale, and cauda the upper segments of the spinal cord than in the lower segments. equina. What is a spinal segment? How is the spinal cord Therefore, the amount of white matter decreases from cervical to partially divided into right and left sides? sacral segments of the spinal cord. There are two major reasons 4. What does each of the following terms mean? Gray commissure, central canal, anterior gray horn, lateral gray for this variation in spinal cord white matter: (1) As the spinal horn, posterior gray horn, anterior white column, lateral cord ascends from sacral to cervical segments, more ascending white column, posterior white column, ascending tract, axons are added to spinal cord white matter to form more sensory and descending tract. 13.2 SPINAL NERVES TABLE 13.1 OBJECTIVES Comparison of Various Spinal Cord Segments Describe the components, connective tissue coverings, and branching of a spinal nerve. SEGMENT DISTINGUISHING CHARACTERISTICS Define plexus, and identify the distribution of nerves of the Cervical Relatively large diameter, relatively cervical, brachial, lumbar, and sacral plexuses. large amounts of white matter, oval; in Describe the clinical significance of dermatomes. upper cervical segments (C1–C4), Spinal nerves are associated with the spinal cord and, like all posterior gray horn is large but anterior gray horn is relatively small; nerves of the peripheral nervous system (PNS), are parallel in lower cervical segments (C5 and bundles of axons and their associated neuroglial cells wrapped in below), posterior gray horns are several layers of connective tissue. Spinal nerves connect the (Segment C6) enlarged and anterior gray horns are CNS to sensory receptors, muscles, and glands in all parts of the well developed. body. The 31 pairs of spinal nerves are named and numbered Thoracic Small diameter due to relatively small according to the region and level of the vertebral column from amounts of gray matter; except for first which they emerge (see Figure 13.2). Not all spinal cord segments thoracic segment, anterior and posterior are aligned with their corresponding vertebrae. Recall that the gray horns are relatively small; spinal cord ends near the level of the superior border of the sec- small lateral gray horn is present. ond lumbar vertebra (L2), and that the roots of the lumbar, sacral, (Segment T5) and coccygeal nerves descend at an angle to reach their respective foramina before emerging from the vertebral column. This Lumbar Nearly circular; very large anterior arrangement constitutes the cauda equina. and posterior gray horns; small The first cervical pair of spinal nerves emerges from the spinal lateral gray horn is present in upper segments; relatively less white matter cord between the occipital bone and the atlas (first cervical verte- than cervical segments. bra, or C1). Most of the remaining spinal nerves emerge from the spinal cord through the intervertebral foramina between adjoining vertebrae. Spinal nerves C1–C7 exit the vertebral canal above (Segment L4) their corresponding vertebrae. Spinal nerve C8 exits the vertebral Sacral Relatively small, but relatively large canal between vertebrae C7 and T1. Spinal nerves T1–L5 exit amounts of gray matter; relatively the vertebral canal below their corresponding vertebrae. From the small amounts of white matter; anterior spinal cord, the roots of the sacral spinal nerves (S1–S5) and the and posterior gray horns are large and coccygeal spinal nerves (Co1) enter the sacral canal, the part of thick. (Segment S3) the vertebral canal in the sacrum (see Figure 7.21). Subsequently, spinal nerves S1–S4 exit the sacral canal via the four pairs of an- Coccygeal Resembles lower sacral spinal segments, terior and posterior sacral foramina, and spinal nerves S5 and Co1 but much smaller. exit the sacral canal via the sacral hiatus. JWCL316_c13_492-526.qxd 12/10/2010 10:32 Page 501 13.2 SPINAL NERVES 501 As noted earlier, a typical spinal nerve has two connections to sions of the epineurium also fill the spaces between fascicles. The the cord: a posterior root and an anterior root (see Figure 13.3a). dura mater of the spinal meninges fuses with the epineurium as The posterior and anterior roots unite to form a spinal nerve at the the nerve passes through the intervertebral foramen. Note the intervertebral foramen. Because the posterior root contains sensory presence of many blood vessels, which nourish the spinal axons and the anterior root contains motor axons, a spinal nerve is meninges (Figure 13.5b). You may recall from Chapter 10 that the classified as a mixed nerve. The posterior root contains a posterior connective tissue coverings of skeletal muscles—endomysium, root ganglion in which cell bodies of sensory neurons are located. perimysium, and epimysium—are similar in organization to those of nerves. Connective Tissue Coverings of Spinal Nerves Distribution of Spinal Nerves Each spinal nerve and cranial nerve consists of many individual axons and contains layers of protective connective tissue cover- Branches ings (Figure 13.5). Individual axons within a nerve, whether A short distance after passing through its intervertebral foramen, myelinated or unmyelinated, are wrapped in endoneurium (en⬘- a spinal nerve divides into several branches (Figure 13.6). These dō-NOO-rē-um; endo- ⫽ within or inner; -neurium ⫽ nerve), the branches are known as rami (RĀ-mı̄ ⫽ branches). The posterior innermost layer. The endoneurium consists of a mesh of collagen (dorsal) ramus (RĀ-mus; singular form) serves the deep mus- fibers, fibroblasts, and macrophages. Groups of axons with their cles and skin of the posterior surface of the trunk. The anterior endoneurium are held together in bundles called fascicles, each of (ventral) ramus serves the muscles and structures of the upper which is wrapped in perineurium (per⬘-i-NOO-rē-um; peri- ⫽ and lower limbs and the skin of the lateral and anterior surfaces of around), the middle layer. The perineurium is a thicker layer of the trunk. In addition to posterior and anterior rami, spinal nerves connective tissue. It consists of up to 15 layers of fibroblasts also give off a meningeal branch (me-NIN-jē⬘-al). This branch within a network of collagen fibers. The outermost covering over reenters the vertebral cavity through the intervertebral foramen the entire nerve is the epineurium (ep⬘-i-NOO-rē-um; epi- ⫽ and supplies the vertebrae, vertebral ligaments, blood vessels of over). It consists of fibroblasts and thick collagen fibers. Exten- the spinal cord, and meninges. Other branches of a spinal nerve Figure 13.5 Organization and connective tissue coverings of a spinal nerve. (Part b © Dr. Richard Kessel and Dr. Randy Kardon/Visuals Unlimited.) Three layers of connective tissue wrappings protect axons: Endoneurium surrounds individual axons, perineurium surrounds bundles of axons (fascicles), and epineurium surrounds an entire nerve. Blood EPINEURIUM ENDONEURIUM vessels PERINEURIUM Transverse plane Spinal nerve EPINEURIUM around entire nerve Fascicle PERINEURIUM around each fascicle Fascicle Myelin sheath Axons Blood vessels ENDONEURIUM around Axon each axon SEM 900x (a) Transverse section showing the coverings of a spinal nerve (b) Transverse section of 12 nerve fascicles Why are all spinal nerves classified as mixed nerves? JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 502 Figure 13.6 Branches of a typical spinal nerve, shown in transverse section through the thoracic portion of the spinal cord. (See also Figure 13.1c.) The branches of a spinal nerve are the posterior ramus, the anterior ramus, the meningeal branch, and the rami communicantes. View POSTERIOR Spinous process of vertebra Deep muscles of back Transverse plane POSTERIOR (DORSAL) Spinal cord RAMUS ANTERIOR (VENTRAL) RAMUS Posterior (dorsal) root MENINGEAL BRANCH Posterior (dorsal) root ganglion Anterior (ventral) root Denticulate ligament RAMI COMMUNICANTES Subarachnoid space (contains CSF) Dura mater and arachnoid mater Body of vertebra Sympathetic ganglion on sympathetic trunk Epidural space (contains fat and blood vessels) ANTERIOR (a) Superior view Spinal nerve Denticulate ligament ANTERIOR (VENTRAL) RAMUS POSTERIOR (DORSAL) RAMUS Anterior (ventral) rootlets Pedicle of vertebra Posterior (dorsal) rootlets (cut) Anterior (ventral) root Posterior (dorsal) root Dura mater and arachnoid mater (b) Anterior view and oblique section of spinal cord Which spinal nerve branches serve the upper and lower limbs? 502 JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 503 13.2 SPINAL NERVES 503 are the rami communicantes (kō-mū-ni-KAN-tēz), compo- Exhibits 13.A–13.D (Figures 13.7–13.10) summarize the prin- nents of the autonomic nervous system that will be discussed cipal plexuses. The anterior rami of spinal nerves T2–T12 are in Chapter 15. called intercostal nerves and will be discussed next. Plexuses Intercostal Nerves Axons from the anterior rami of spinal nerves, except for thoracic The anterior rami of spinal nerves T2–T12 do not enter into the nerves T2–T12, do not go directly to the body structures they sup- formation of plexuses and are known as intercostal or thoracic ply. Instead, they form networks on both the left and right sides of nerves. These nerves directly connect to the structures they sup- the body by joining with various numbers of axons from anterior ply in the intercostal spaces. After leaving its intervertebral fora- rami of adjacent nerves. Such a network of axons is called a men, the anterior ramus of nerve T2 innervates the intercostal plexus (PLEK-sus  braid or network). The principal plexuses muscles of the second intercostal space and supplies the skin of are the cervical plexus, brachial plexus, lumbar plexus, and the axilla and posteromedial aspect of the arm. Nerves T3–T6 ex- sacral plexus. A smaller coccygeal plexus is also present. Refer tend along the costal grooves of the ribs and then to the intercostal to Figure 13.2 to see their relationships to one another. Emerging muscles and skin of the anterior and lateral chest wall. Nerves from the plexuses are nerves bearing names that are often descrip- T7–T12 supply the intercostal muscles and abdominal muscles, tive of the general regions they serve or the course they take. Each along with the overlying skin. The posterior rami of the inter- of the nerves in turn may have several branches named for the costal nerves supply the deep back muscles and skin of the poste- specific structures they innervate. rior aspect of the thorax. EXHIBIT 13.A Cervical Plexus (Figure 13.7) OBJECTIVE The cervical plexus supplies the skin and muscles of the head, neck, Describe the origin and distribution of the cervical plexus. and superior part of the shoulders and chest. The phrenic nerves arise from the cervical plexuses and supply motor fibers to the diaphragm. The cervical plexus (SER-vi-kul) is formed by the roots (anterior rami) Branches of the cervical plexus also run parallel to two cranial nerves, of the first four cervical nerves (C1–C4), with contributions from C5 the accessory (XI) nerve and hypoglossal (XII) nerve. (Figure 13.7). There is one on each side of the neck alongside the first four cervical vertebrae. NERVE ORIGIN DISTRIBUTION SUPERFICIAL (SENSORY) BRANCHES Lesser occipital C2 Skin of scalp posterior and superior to ear. Great auricular (aw-RIK-ū-lar) C2–C3 Skin anterior, inferior, and over ear, and over parotid glands. Transverse cervical C2–C3 Skin over anterior aspect of neck. Supraclavicular C3–C4 Skin over superior portion of chest and shoulder. DEEP (LARGELY MOTOR) BRANCHES Ansa cervicalis (AN-sa ser-vi-KAL-is) Divides into superior and inferior roots. Superior root C1 Infrahyoid and geniohyoid muscles of neck. Inferior root C2–C3 Infrahyoid muscles of neck. Phrenic (FREN-ik) C3–C5 Diaphragm. Segmental branches C1–C5 Prevertebral (deep) muscles of neck, levator scapulae, and middle scalene muscles. E X H I B I T 13.A CONTINUES JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 504 EXHIBIT 13.A Cervical Plexus (Figure 13.7) CONTINUED Injuries to the CHECKPOINT CLINICAL CONNECTION | Which nerve from the cervical plexus causes contraction of Phrenic Nerves the diaphragm? The phrenic nerves originate from C3, C4, and C5 and supply the di- aphragm. Complete severing of the spinal cord above the origin of the phrenic nerves (C3, C4, and C5) causes respiratory arrest. In in- juries to the phrenic nerves, breathing stops because the phrenic nerves no longer send nerve impulses to the diaphragm. The phrenic nerves may also be damaged due to pressure from malignant tracheal or esophageal tumors in the mediastinum. Figure 13.7 Cervical plexus in anterior view. The cervical plexus supplies the skin and muscles of the head, neck, superior portion of the shoulders and chest, and diaphragm. Hypoglossal C1 (cranial nerve XII) Lesser Segmental branch occipital Great C2 auricular C3 Transverse cervical C4 Superior root of ansa cervicalis Cervical plexus projected to surface To brachial plexus Inferior root of C5 ansa cervicalis Supraclavicular Roots Nerves Phrenic Origin of cervical plexus Why does complete severing of the spinal cord at level C2 cause respiratory arrest? 504 EXHIBIT 13.A JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 505 EXHIBIT 13.B Brachial Plexus (Figure 13.8) OBJECTIVE muscles of the hand. (5) The ulnar nerve supplies the anteromedial Describe the origin, distribution, and effects of damage to muscles of the forearm and most of the muscles of the hand. the brachial plexus. The roots (anterior rami) of spinal nerves C5–C8 and T1 form the brachial plexus (BRĀ-kē-al), which extends inferiorly and laterally Injuries to Nerves on either side of the last four cervical and first thoracic vertebrae (Fig- CLINICAL CONNECTION | Emerging from the ure 13.8a). It passes above the first rib posterior to the clavicle and Brachial Plexus then enters the axilla. Since the brachial plexus is so complex, an explanation of its various Injury to the superior roots of the brachial plexus (C5–C6) may result parts is helpful. As with the cervical and other plexuses, the roots are the from forceful pulling away of the head from the shoulder, as might anterior rami of the spinal nerves. The roots of several spinal nerves unite occur from a heavy fall on the shoulder or excessive stretching of an to form trunks in the inferior part of the neck. These are the superior, infant’s neck during childbirth. The presentation of this injury is char- middle, and inferior trunks. Posterior to the clavicles, the trunks divide acterized by an upper limb in which the shoulder is adducted, the arm into divisions, called the anterior and posterior divisions. In the axillae, is medially rotated, the elbow is extended, the forearm is pronated, the divisions unite to form cords called the lateral, medial, and posterior and the wrist is flexed (Figure 13.8c). This condition is called Erb- cords. The cords are named for their relationship to the axillary artery, a Duchenne palsy or waiter’s tip position. There is loss of sensation large artery that supplies blood to the upper limb. The principal nerves along the lateral side of the arm. of the brachial plexus branch from the cords. Injury to the radial (and axillary) nerve can be caused by im- The brachial plexus provides almost the entire nerve supply of the properly administered intramuscular injections into the deltoid mus- shoulders and upper limbs (Figure 13.8b). Five important nerves arise cle. The radial nerve may also be injured when a cast is applied too from the brachial plexus: (1) The axillary nerve supplies the deltoid and tightly around the mid-humerus. Radial nerve injury is indicated by teres minor muscles. (2) The musculocutaneous nerve supplies the wrist drop, the inability to extend the wrist and fingers (Fig- anterior muscles of the arm. (3) The radial nerve supplies the muscles ure 13.8c). Sensory loss is minimal due to the overlap of sensory inner- on the posterior aspect of the arm and forearm. (4) The median nerve vation by adjacent nerves. supplies most of the muscles of the anterior forearm and some of the NERVE ORIGIN DISTRIBUTION Dorsal scapular (SKAP-ū-lar) C5 Levator scapulae, rhomboid major, and rhomboid minor muscles. Long thoracic (thor-RAS-ik) C5–C7 Serratus anterior muscle. Nerve to subclavius (sub-KLĀ-vē-us) C5–C6 Subclavius muscle. Suprascapular C5–C6 Supraspinatus and infraspinatus muscles. Musculocutaneous (mus-kū-lō-kū-TĀN-ē-us) C5–C7 Coracobrachialis, biceps brachii, and brachialis muscles. Lateral pectoral (PEK-to-ral) C5–C7 Pectoralis major muscle. Upper subscapular C5–C6 Subscapularis muscle. Thoracodorsal (tho-RĀ-kō-dor-sal) C6–C8 Latissimus dorsi muscle. Lower subscapular C5–C6 Subscapularis and teres major muscles. Axillary (AK-si-lar-ē) C5–C6 Deltoid and teres minor muscles; skin over deltoid and superior posterior aspect of arm. Median C5–T1 Flexors of forearm, except flexor carpi ulnaris; ulnar half of flexor digitorum profundus, and some muscles of hand (lateral palm); skin of lateral two-thirds of palm of hand and fingers. Radial Triceps brachii, anconeus, and extensor muscles of forearm; skin of posterior arm and forearm, lateral two-thirds of dorsum of hand, and fingers over proximal and middle phalanges. Medial pectoral C8–T1 Pectoralis major and pectoralis minor muscles. Medial cutaneous nerve of arm (kū-TĀ-nē-us) C8–T1 Skin of medial and posterior aspects of distal third of arm. Medial cutaneous nerve of forearm C8–T1 Skin of medial and posterior aspects of forearm. Ulnar C8–T1 Flexor carpi ulnaris, ulnar half of flexor digitorum profundus, and most muscles of hand; skin of medial side of hand, little finger, and medial half of ring finger. E X H I B I T 13.B CONTINUES EXHIBIT 13.B 505 JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 506 EXHIBIT 13.B Brachial Plexus (Figure 13.8) CONTINUED Injury to the median nerve may result in median nerve palsy, scapula to protrude; because the scapula looks like a wing, this condi- which is indicated by numbness, tingling, and pain in the palm and tion is called winged scapula (Figure 13.8c). The arm cannot be ab- fingers. There is also inability to pronate the forearm and flex the ducted beyond the horizontal position. proximal interphalangeal joints of all digits and the distal interpha- Compression of the brachial plexus on one or more of its nerves langeal joints of the second and third digits (Figure 13.8c). In addi- is sometimes known as thoracic outlet syndrome. The subclavian tion, wrist flexion is weak and is accompanied by adduction, and artery and subclavian vein may also be compressed. The compres- thumb movements are weak. sion may result from spasm of the scalene or pectoralis minor mus- Injury to the ulnar nerve may result in ulnar nerve palsy, cles, the presence of a cervical rib (an embryological anomaly), or which is indicated by an inability to abduct or adduct the fingers, at- misaligned ribs. The patient may experience pain, numbness, weak- rophy of the interosseous muscles of the hand, hyperextension of the ness, or tingling in the upper limb, across the upper thoracic area, metacarpophalangeal joints, and flexion of the interphalangeal and over the scapula on the affected side. The symptoms of thoracic joints, a condition called clawhand (Figure 13.8c). There is also loss of outlet syndrome are exaggerated during physical or emotional sensation over the little finger. stress because the added stress increases the contraction of the in- Injury to the long thoracic nerve results in paralysis of the ser- volved muscles. ratus anterior muscle. The medial border of the scapula protrudes, giving it the appearance of a wing. When the arm is raised, the verte- CHECKPOINT bral border and inferior angle of the scapula pull away from the tho- racic wall and protrude outward, causing the medial border of the Injury of which nerve could cause paralysis of the serratus anterior muscle? Figure 13.8 Brachial plexus in anterior view. The brachial plexus supplies the shoulders and upper limbs. From C4 Dorsal scapular C5 Suprascapular Nerve to subclavius C6 Superior trunk Lateral pectoral To phrenic nerve C7 cord dl e trunk al M id ter La Musculocutaneous rd C8 Axillary co r or trunk te rio Inferi s rd Lateral head Po l co T1 ia M ed Brachial plexus projected to surface Median T2 Radial Long thoracic Medial pectoral Medial head Ulnar Upper subscapular Medial antebrachial cutaneous nerve of forearm Thoracodorsal Medial brachial cutaneous Lower subscapular Roots nerve of arm Roots Trunks MNEMONIC for subunits of the brachial plexus: (a) Origin of brachial plexus Trunks Anterior division Risk Takers Don't Cautiously Behave. Anterior division Posterior division Roots, Trunks, Divisions, Cords, Branches Posterior division Cords Branches 506 EXHIBIT 13.B JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 507 Dorsal scapular nerve Nerve to subclavius Suprascapular nerve Superior trunk From C4 Middle trunk C5 C6 Inferior trunk C7 C8 T1 Clavicle Long thoracic nerve Lateral cord Medial pectoral Posterior cord Medial brachial Lateral pectoral nerve cutaneous Medial antebrachial Axillary nerve cutaneous Medial cord Scapula Musculocutaneous nerve Ulnar nerve Radial nerve Median nerve Humerus Wrist drop Deep branch of radial nerve Radius Superficial branch of radial nerve Erb-Duchenne palsy Ulna (waiter's tip) Median nerve palsy Median nerve Ulnar nerve Radial nerve Superficial branch of ulnar nerve Digital branch of median nerve Digital branch of ulnar nerve Ulnar nerve palsy Winging of right scapula (b) Distribution of nerves from the brachial plexus (c) Injuries to the brachial plexus What five important nerves arise from the brachial plexus? EXHIBIT 13.B 507 JWCL316_c13_492-526.qxd 8/19/10 9:44 AM Page 508 EXHIBIT 13.C Lumbar Plexus (Figure 13.9) OBJECTIVE the first four lumbar vertebrae, the lumbar plexus passes obliquely out- Describe the origin and distribution of the lumbar plexus. ward, between the superficial and deep heads of the psoas major muscle and anterior to the quadratus lumborum muscle. It then gives rise to its The roots (anterior rami) of spinal nerves L1–L4 form the lumbar peripheral nerves. plexus (LUM-bar) (Figure 13.9). Unlike the brachial plexus, there is The lumbar plexus supplies the anterolateral abdominal wall, external minimal intermingling of fibers in the lumbar plexus. On either side of genitals, and part of the lower limbs. NERVE ORIGIN DISTRIBUTION Iliohypogastric (il-ē-ō-hı̄-pō-GAS-trik) L1 Muscles of anterolateral abdominal wall; skin of inferior abdomen and buttock. Ilioinguinal (il-ē-ō-IN-gwi-nal) L1 Muscles of anterolateral abdominal wall; skin of superior and medial aspect of thigh, root of penis and scrotum in male, and labia majora and mons pubis in female. Genitofemoral (jen-i-tō -FEM-or-al) L1–L2 Cremaster muscle; skin over middle anterior surface of thigh, scrotum in male, and labia majora in female. Lateral cutaneous nerve of thigh L2–L3 Skin over lateral, anterior, and posterior aspects of thigh. Femoral L2–L4 Largest nerve arising from lumbar plexus; distributed to flexor muscles of hip joint and extensor muscles of knee joint, skin over anterior and medial aspect of thigh and medial side of leg and foot. Obturator (OB-too-rā-tor) L2–L4 Adductor muscles of hip joint; skin over medial aspect of thigh. Figure 13.9 Lumbar plexus in anterior view. The lum