CH 14 The Spinal Cord and Spinal Nerves

Organization of White Matter

Although the general pattern of gray matter and white matter is the same throughout the spinal cord, the amount of white matter decreases as you move caudally within the spinal cord. White matter is divided into regions, or columns. The *posterior white columns* are located between the posterior horns and the posterior median sulcus. The *anterior white columns* are located between the anterior horns and the anterior median fissue, and they are interconnected by the *anterior white commissure*. *Lateral white columns*, between the anterior and posterior columns, are composed of white matter. Each column contains *tracts* composed of axons sharing functional and structural characteristics. A specific tract carries either sensory information or motor commands, and the axons within a tract are uniform in diameter, myelination, and conduction speed. All the axons within a tract relay information in the same direction. Small commissural tracts carry sensory or motor signals between segments of the spinal cord; other, larger tracts connect the spinal cord with the brain. *Ascending tracts* carry sensory information toward the brain, and *descending tracts* carry motor commands into the spinal cord. Within each column, the tracts are separated according to the destination of the motor information of the source of the sensory information. As a result, the tracts show a regional organization similar to that found in the nuclei of the gray matter. (pg 365)

Peripheral Distribution of Spinal Nerves

As the dorsal and ventral roots of a spinal nerve pass through an intervertebral foramen, they unite to form the spinal nerve. Distally, the spinal nerve divides into several branches. All spinal nerves form two branches: a dorsal ramus and a ventral ramus. Spinal nerves T1 to L2 have four branches: (1) a white ramus communicans, (2) a gray ramus communicans, (3) a dorsal ramus, and (4) and ventral ramus. The rami communicantes carry visceral motor fibers to and from a nearby *autonomic ganglion* associated with the sympathetic division of the ANS. Because preganglionic axons are myelinated, the branch carrying those fibers to the ganglion has a light color, and it is known as the *white ramus communicans*. Two groups of unmyelinated postganglionic fibers leave the ganglion. Those innervating glands and smooth muscles in the body wall or limbs form a second branch, the *gray ramus communicans*, that rejoins the spinal nerve. The gray ramus is proximal to the white ramus. Preganglionic or postganglionic fibers innervating internal organs do not rejoin the spinal nerves. Instead, they form a series of separate autonomic nerves, such as the splanchnic nerves, involved with regulating the activities of organs in the abdominopelvic cavity. The *dorsal (posterior) ramus* of each spinal nerve receives sensory innervation from, and sends motor innervation to, the skeletal muscles of the back. The relatively large *ventral (anterior) ramus* supplies the ventrolateral body surface, structures in the body wall, and the limbs.

The Pia Mater

Deep to the subarachnoid space is the pia mater, the innermost meningeal layer. The elastic and collagen fibers of the pia mater are interwoven with those of the arachnoid trabeculae. The blood vessels supplying the spinal cord are found within the pia mater. The pia mater is firmly bound to the underlying neural tissue, conforming to its bulges and fissures. The surface of the spinal cord consists of a thin layer of astrocytes, and cytoplasmic extensions of these neuroglia lock the collagen fibers of the spinal pia mater in place.

The Arachnoid Mater

In a cadaver, a narrow subdural space separates the dura mater from the deeper meninges of the spinal cord. However, in a living person this space does not exist, and the inner surface of the dura is lined by the outer surface of the arachnoid mater. The subarachnoid space separates the arachnoid mater from the innermost layer, the pia mater. This space contains cerebrospinal fluid (CSF), which is a shock absorber and a diffusion medium for dissolved gasses, nutrients, chemical messengers, and waste products. Bundles of fibers known as arachnoid trabeculae extend from the inner surface of the arachnoid mater to the outer surface of the pia mater. The subarachnoid space of the spinal meninges is easily accessed between L3 and L4 for the clinical examination of CSF or for administering anesthetics.

Denticulate ligaments

Paired denticulate ligaments are located along the length of the spinal cord. These structures, which are found between the dorsal and ventral roots of the spinal nerves, are extensions of the spinal pia mater, and they connect the pia mater and arachnoid mater to the dura mater of the spinal cord. The denticulate ligaments begin at the foramen magnum of the skull, and they prevent side-to-side and downward movement of the spinal cord. At the inferior tip of the conus medullaris, the connective tissue fibers of the spinal pia mater form the filum terminale.

Classification of Reflexes

Reflexes are classified according to (1) their development (innate or aquired); (2) their response (somatic or visceral); (3) the complexity of the circuit (monosynaptic or polysynaptic); and (4) their processing site (spinal or cranial). In the simplest reflex arc, a *monosynaptic reflex*, a sensory neuron synapses directly on a motor neuron. Transmission of information across a chemical synapse always involved a synaptic delay, but with only one synapse, the delay between stimulus and response is minimized. A *polysynaptic reflex* is more complex and has a longer delay between the stimulus and response. The length of the delay depends on the number of synapses involved. Polysynaptic reflexes produce far more complicated responses because the interneurons control several different muscle groups. The motor responses in a polysynaptic reflex is extremely complicated; for example, stepping on a sharp object not only causes withdrawal of the foot, but also initiates all the muscular adjustments needed to prevent a fall. Such complex responses result from the interactions between multiple interneuron pools.

Reflexes

Reflexes help preserve homeostasis by enabling us to respond rapidly to changes in the internal or external environment. Conditions inside or outside the body change unexpectedly. A *reflex* is an immediate involuntary motor response to a specific stimulus. Reflexes help preserve homeostasis by making rapid adjustments in the function of organs or organ systems. A reflexive response seldom varies - activation of a particular reflex always produces the same motor response. The neural "wiring" of a single reflex is called a *reflex arc*. A reflex arc begins at a receptor and ends at a peripheral effector, such as a muscle or gland cell. *Step 1:* Stimulation and activation of receptor - There are many types of sensory receptors, and each receptor has a characteristic range of sensitivity; some receptors, such as pain receptors, respond to almost any stimulus. These receptors, which are the dendrites of sensory neurons, are stimulated by pressure, temperature extremes, physical damage, or exposure to abnormal chemicals. Other receptors, such as visual, auditory, or taste receptors, are specialized cells that respond to only a limited range of stimuli. *Step 2:* Activation of a sensory neuron - info is carried in the form of an action potential along an afferent fiber. In this case, the axon conducts the action potential into the spinal cord through one of the dorsal roots. *Step 3:* Information processing in CNS - Information processing begins when the neurotransmitter released by the axon terminals of a sensory neuron reaches the postsynaptic membrane of a motor neuron or interneuron. In the simplest reflexes, this processing is performed by the motor neuron controlling the peripheral effectors. In more complex reflexes, one or more interneurons are located between the sensory and motor neurons, and both serial and parallel processing occur. This type of information processing selects the appropriate motor response through the activation of specific motor neurons. *Step 4:* Activation of a motor neuron - When a motor neuron is stimulated to threshold, it conducts an action potential through the ventral root of a spinal nerve to the peripheral effector organ. *Step 5:* Response by effector - Activation of the motor neuron causes a response by a peripheral effector, such as a skeletal muscle or gland. Reflexes play an important role in opposing potentially harmful changes in the internal or external environment.

Higher Center and Integration of Reflexes

Reflexive motor activities occur automatically, without instructions from the higher centers in the brain. However, higher centers can have a profound effect on reflex performance. For example, higher centers within the brain enhance or suppress spinal reflexes by modifying the information carried in descending tracts that synapse on interneurons and motor neurons throughout the spinal cord. Motor control therefore involves a series of interacting levels. At the lowest level are monosynaptic reflexes that are rapid and seldom change. At the highest level are centers in the brain that can modify reflexive motor patterns.

The Central Nervous System

The CNS consists of the spinal cord and brain. The spinal cord and brain are anatomically connected but have a significant degree of functional independence. The spinal cord is more than just a pathway for information traveling to and from the brain, it also integrates and processes information on its own.

Spinal Reflexes

The best-known spinal reflex is the *stretch reflex.* It is a simple monosynaptic reflex providing the autonomic regulation of skeletal muscle length. The stimulus stretches a relaxed muscle, activating a sensory neuron and triggering the contraction of that muscle. The stretch reflex also adjusts autonomic muscle tone, increasing or decreasing it in response to information provided by the stretch receptors of muscle spindles. The most familiar stretch reflex is the *patellar reflex* (also known as the knee jerk reflex). In this reflex, a sharp tap on the patellar ligament stretches muscle spindles in the quadriceps femoris. Because the stimulus is brief, the reflexive contraction is unopposed and produces a noticeable kick. clinicians use this reflex to check the status of the lower segments of the spinal cord. A normal patellar reflex indicates that spinal nerves and spinal segments L2-L4 are undamaged. The stretch reflex is an example of a *postural reflex*, a reflex that maintains normal upright posture. Postural muscles usually have a firm muscle tone and extremely sensitive stretch receptors. As a result, very fine adjustments are continually being made; you are not aware of the cycles of contraction and relaxation that occur.

The Brachial Plexus

The brachial plexus is larger and more complex than the cervical plexus. It originates from the ventral rami of spinal nerves C5-T1 and innervates the pectoral girdle and upper limb. The ventral rami converge to form the superior, middle, and inferior trunks. 1) *Ventral Rami (Roots):* The roots of the brachial plexus originate from the ventral rami of the spinal nerves C5-T1. 2) *Trunks:* The C5 & C6 ventral rami form the *superior trunk*; the C7 ventral ramus continues as the *middle trunk*, and the C8 & T1 ventral rami form the *inferior trunk.* 3) *Divisions:* Each of these then divide into an *anterior division* and a *posterior division.* 4) *Cords:* All three posterior divisions unite to form the *posterior cord*, while the anterior divisions of the superior and middle trunks unite to form the *lateral cord*. The *medial cord* is formed by a continuation of the anterior division of the inferior trunk. 5) *Nerves:* The nerves of the brachial plexus arise from one or more trunks or cords whose names indicate their positions relative to the axillary artery, a large artery supplying the upper limb. The lateral cord forms the musculocutaneous nerve exclusively and, together with the medial cord, contributes to the median nerve. The ulnar nerve is the other major nerve of the medial cord. The posterior cord forms the axillary nerve and the radial nerve.

Organization of Gray Matter

The cell bodies of neurons within the gray matter of the spinal cord are organized into groups called *nuclei* that have specific functions. *Sensory nuclei* receive and relay sensory information from peripheral receptors, such as touch receptors in the skin. *Motor nuclei* send motor commands to the peripheral effectors, such as skeletal muscle. Sensory nuclei and motor nuclei within the central gray matter of the spinal cord extend for a considereable distance along the length of the spinal cord. A frontal section along the axis of the central canal separates the sensory (dorsal) nuclei from the motor (ventral) nuclei. The *posterior (dorsal) horns* contain somatic and visceral sensory nuclei, and the *anterior (ventral) horns* contain somatic motor neurons. *Lateral horns* (intermediate horns) are found only between segments T1 and L1 and contain visceral motor neurons. The *gray commisures* contain axons decussating (crossing) from one side of the cord to the other. There are two gray commissures, one posterior to the central canal and one anterior to it. The motor nuclei within each horn are highly organized. Nerves innervating skeletal muscles of more proximal structures (such as the trunk and shoulder) are located more medially within the gray matter than nuclei innervating the skeletal muscles of more distal structures (forearm and hand). The size of the anterior horns varies depending on the number of skeletal muscles innervated by that segment. Therefore, the anterior horns are largest in cervical and lumbar regions of the spinal cord, regions that control the muscles of the upper and lower limbs.

The Cervical Plexus

The cervical plexus consists of cutaneous branches that innervate areas of the head, neck, and chest. These are the largest branches of the cervical plexus. It also includes smaller muscular branches that innervate the muscles of the larynx. The cervical plexus is composed of cutaneous and muscular branches of the ventral rami of spinal nerves C1-C4, and some nerve fibers from C5. The plexus lies deep to the sternocleidomastoid and anterior to the middle scalene and levator scapulae. The cutaneous branches of this plexus innervate areas on the head, neck, and chest. - Nerve Roots of the Cranial Plexus: the cervical plexus originates from the ventral rami of the second, third, fourth, and fifth cervical nerves. - Cranial Nerves: The cervical plexus supplies small branches to the hypoglossal nerve *(XII)* and the accessory nerve *(XI)*. - Lesser Occipital Nerve: the lesser occipital nerve originates from C2 and receives sensory information from the skin of the neck and the scalp posterior & superior to the ear. - Great Auricular Nerve: The great auricular nerve arises from C2 and C3 and crosses anteriorly to the sternocleidomastoid and travels toward the parotid gland, where it divides. This nerve receives sensory information from the skin over the gland, the posterior aspect of the ear, and skin of the neck. - Transverse Cervical Nerve: the transverse cervical nerve originates from C3 & C4 and receives sensory input from the skin of the anterior triangle of the neck. - Ansa Cervicalis: originates from branches C1-C3 (and sometimes C4) and travels inferiorly with fibers from cranial nerve *XII*. The ansa cervicalis innervates five of the extrinsic laryngeal muscles. - Nerves to Rhombiods and Serratus Anterior: Motor fibers originating at C5 innervate the rhombiods (major and minor) and a portion of the serratus anterior. - Supreclavicular Nerves: originate from C3 and C4 as a common trunk. This trunk receives sensory input from the skin of the neck and shoulder. - Phrenic Nerve: the phrenic nerve, which provides sensory information from and motor innervation to the diaphragm, originates from C4, with minor contributions from C3 & C5. (pg 370)

Dermatomes

The distribution of the sensory fibers within the dorsal and ventral rami illustrates the segmental division of labor along the length of the spinal cord. Each pair of psinal nerves supplies a specific region of the skin, an area known as a dermatome. Dermatomes are clinically important because damage to either a spinal nerve or dorsal root ganglion will produce a characteristic loss of sensation in specific areas of the skin.

Summary of Spinal Meninges

The dura mater is a tough, fibrous layer that forms the outermost covering of the spinal cord and brain. The arachnoid mater, the middle meningeal layer, is composed of a simple squamous epithelium. The arachnoid mater lines the inner surface of the dura mater. The surface of the CNS is covered with a connective tissue membrane, the pia mater. The pia mater closely follows the contours of the spinal cord. The spinal meninges surround the dorsal and ventral roots of the spinal nerves. The meningeal membranes are continuous with the connective tissues surrounding the spinal nerves and their peripheral branches.

The Dura Mater

The dura mater of the spinal cord consists of a layer of dense irregular connective tissue. A simple squamous epithelium covers the inner and outer surfaces of the dura mater. The outer epithelium is not attached to the body walls of the vertebral canal, and the resulting space is called the epidural space. The epidural space contains areolar tissue, blood vessels, and adipose tissue. **The dura mater attaches to the edge of the foramen magnum of the skull, C2 and C3, the sacrum, and the posterior longitudinal ligament. These attachments stabilize the spinal cord within the vertebral canal. Caudally, the spinal dura mater tapers and forms a dense cord of collagen fibers that blend with the filum terminale, forming the coccygeal ligament. The coccygeal ligament extends the length of the sacral canal and fuses with the periosteum of the sacrum and coccyx. Lateral support of the spinal cord is provided by the connective tissues within the epidural space and by the extensions of the dura mater accompanying the spinal nerve roots as they pass through the intervertebral foramina.

The Lumbosacral Plexus

The lumbar plexus and sacral plexus originate from the limbar and sacral segments of the spinal cord, respectively. The nerves originating from these plexuses innervate the pelvic girdle and lower limbs. Because the ventral rami of both plexuses are distributed to the lower limbs and spinal nerves L4 and L5 are involved, these two plexuses are collectively referred to as the lumbosacral plexus.

The Lumbar Plexus

The lumbar plexus originates from the ventral rami of T12-L4. The major nerves of the plexus are the lateral femoral cutaneous nerve, the genitofemoral nerve, and the femoral nerve. - *Nerve Roots of Lumbar Plexus:* The lumbar plexus is formed by the ventral rami of T12-L4. - *Iliohypogastric Nerve:* originates from the ventral rami of T12 and L1. It innervates the external and internal oblique and transverse abdominis. It receives sensory information from the skin over the abdomen and the buttocks. - *Ilio-inguinal Nerve:* originates from the ventral ramus of L1. It innervates the external and internal oblique an transverse abdominis. It receives sensory information from the skin over the superior and medial thigh and portions of the external genitalia. - *Lateral Femoral Cutaneous Nerve:* originates from the ventral rami of L2 & L3. It receives sensory information from the skin over the anterior, lateral, and posterior thigh. - *Genitofemoral Nerve:* originates from the ventral rami of L1 & L2. It receives sensory information from the skin over the anteromedial surface of the thigh and portions of the external genitalia. Branches of the genitofemoral nerve: femoral branch & genital branch. - *Femoral Nerve:* originates from the ventral rami of L2-L4. It innervates the quadriceps femoris, sartorius, pectineus, and iliopsoas. It receives sensory information from the skin of the anteromedial surface of the thigh and the medial surface of the leg and foot. - *Obturator Nerve:* originates from the ventral rami of L2-L4. It innervates the gracilis and obturator externus, and the adductor magnus, brevis, and longus. It receives sensory information from the medial surface of the thigh.

The Sacral Plexus

The sacral plexus is formed by the ventral rami of L4-S4. Part of the ventral ramus of L4 and the ventral ramus of L5 form the lumbosacral trunk, which joins the sacral plexus. The five major nerves of the sacral plexus are: - *Superior Gluteal Nerve:* originates from the ventral rami of L4-S1. It innervates the gluteus minimus, gluteus medius, and tensor fasciae latae. - *Inferior Gluteal Nerve:* originates from the ventral rami of L5-S2. IT innervates the gluteus maximus. - *Sciatic Nerve:* the largest nerve in the body! It originates from the ventral rami of L4-S3 and innervates the semimembranosus, semitendinosus, and adductor magnus. *** Branches of the Sciatic Nerve: it branches into the tibial and common fibular nerves near the popliteal fossa. The common fibular nerve then divides into the superifical and deep fibular nerves. 1) Tibial Nerve: innervates the flexors of the knee and plantar flexors of the ankle; flexors of the toes; and skin over the posterior surface of the leg and the plantar surface of the foot. 2) Fibular Nerves: innervate the short head of the bicceps femoris (common fibular), tibialis anterior and extensors of the toes (deep fibular), and the fibularis brevis and longus (superficial fibular). The common fibular nerve receives information from the anterior surface of the leg and skin over the lateral portion of the foot (through the sural nerve). - *Posterior Femoral Cutaneous Nerve:* originates from the ventral rami of S1-S3. It receives sensory information from the perineum and the posterior surface of the thigh and leg. - *Pudendal Nerve:* originates from ventral rami of S2-S4. It innervates muscles of the perineum, including the urogenital diaphragm adn the external anal and urethral sphincters. It receives sensory information from the external genitalia and related skeletal muscles (the bulbospongiosus and ischiocavernosus).

Sectional Anatomy of the Spinal Cord

The spinal cord contains a central mass of gray matter containing the cell bodies of neuroglia and the cell bodies of neurons, & a peripheral region of white matter containing myelinated and unmyelinated axons. The anterior median fissure and the posterior median sulcus divide the spinal cord into left and right halves. There is a central, H-shaped mass of gray matter containing cell bodies of neuroglia and neurons. The gray matter surrounds the narrow central canal, which is located in the horizontal bar of the H. Gray matter called horns project toward the outer surface of the spinal cord. The peripheral white matter contains myelinated and unmyelinated axons organized into tracts and columns.

Gross Anatomy of the Spinal Cord

The spinal cord is continuous with the brain and ends at the conus medullaris. The diameter of the spinal cord is largest in the cervical region and smallest in the sacral and coccygeal regions. **The adult spinal cord extends from the foramen magnum of the skull to the inferior border of the first lumbar vertebrae (L1). The posterior surface of the spinal cord has a shallow longitudinal groove, the posterior median sulcus. The anterior median fissure is a deep crease on the anterior surface of the cord. Each region of the spinal cord (cervical, thoracic, lumbar, and sacral) contains several tracts, which are bundles of axons sharing functional and structural characteristics. **The amount of gray matter increases substantially in those segments of the spinal cord that are concerned with the sensory and motor innervation of the limbs. These areas contain interneurons that are responsible for (1) relaying arriving sensory information and (2) coordinating the activities of the somatic motor neurons that control the complex muscles of the limbs. These expanded areas of the spinal cord form the cervical enlargement, which supplies nerves to the pectoral girdle and upper limbs, and the lumbosacral enlargement, which supplies nerves to the pelvis and lower limbs. Caudal to the lumbosacral enlargement, the spinal cord tapers and forms a cone-shaped tip called the conus medullaris, which is located at or inferior to the level of the first lumbar vertebra (L1). Extending within the vertebral canal from the inferior tip of the conus medullaris is the filum terminale. The filum terminale extends from L1 to the dorsum of the coccyx, where it connects the spinal cord to the first coccygeal vertebra. **The entire spinal cord is divided into 31 segments. A letter and number designation identifies each segment. For example, C3 is the third cerical segment. ** Every spinal segment is associated with a pair of dorsal root ganglia that contain cell bodies of sensory neurons. ***The only exceptions are the C1 and the first coccygeal vertebra Co1, where some people lack dorsal roots and the associated dorsal root ganglia.*** These sensory ganglia lie between the pedicles of adjacent vertebrae. On both sides of the spinal cord the dorsal roots contain the afferent axons of the sensory neurons in the dorsal root ganglion. Anterior to the dorsal root, a ventral root leaves the spinal cord. The ventral root contains the efferent visceral motor neurons that control peripheral effectors. The dorsal and ventral roots of each segment enter and leave the vertebral canal between adjacent vertebrae at the intervretebral foramina. The dorsal roots are thicker than the ventral roots. ** Distal to each dorsal root ganglion, the sensory and motor fibers form a single spinal nerve that exits from the intervertebral formina. Spinal nerves are classified as mixed nerves because they contain both afferent (sensory) and efferent (motor) fibers. **The spinal cord continues to grow until about age 4. Until then, the growth of the spinal cord keeps pace with the growth of the vertebral column, and the segments of the spinal cord are aligned with the corresponding vertebrae. The ventral and dorsal roots are short and leave the vertebral canal through adjacent intervertebral foramina. After age 4, the vertebral column continues to grow, bu the spinal cord does not. This vertebral growth carries the dorsal root ganglia and spinal nerves farther and farther away from their original position. As a result, the dorsal and ventral roots gradually elongate. The adult spinal cord extends only to the level of the first or second lumbar vertebra; thus spinal cord segment S2 lies at the level of vertebra L1. ** When seen in gross dissection, the filum terminale and the long ventral and dorsal roots are called the cauda equina because this structure reminded early scientists of a horse's tail.

Spinal Meninges

The vertebral column isolates the spinal cord from the external environment. The delicate neural tissue also must be protected from the surrounding vertebral canal. Specialized membranes known as the spinal meninges provide protection, physical stability, and shock absorption for the spinal cord. **The spinal meninges cover and protect the spinal cord and spinal nerve roots. Blood vessels branching within the meninges deliver oxygen and nutrients to the spinal cord. At the foramen magnum of the skull, the spinal meninges are continuous with the cranial meninges surrounding the brain. There are three meningeal layers: the dura mater, arachnoid mater, and pia mater.

Spinal Nerves

There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal spinal nerve. We identify spinal nerves by their association with adjacent vertebrae. In the cervical region, the first pair of spinal nerves, C1, exits between the skull and the first cervical vertebra. For this reason, *cervical nerves take their names from the vertebra immediately inferior to them.* In other words, cervical nerve C2 exits from the vertebral column superior to vertebra C2, and the same system is used for the rest of the cervical spinal nerves. The transition from this identification method occurs between the last cervical and first thoracic vertebrae. The spinal nerve lying between these two vertebrae is designated C8. Thus, there are *seven* cervical vertebrae but *eight* cervical nerves. Spinal nerves caudal to the first thoracic vertebra take their names from the vertebra immediately superior to them. Thus, spinal nerve T1 emerges immediately inferior to vertebra T1, spinal nerve T2 exits inferior to vertebra T2, and so forth.

Coverings of Spinal Nerves

Three layers of connective tissue surround each peripheral nerve: an outer epineurium, a central perineurium, and an inner endoneurium. The arrangement of these layers is comparable to the connective tissue layers in the skeletal muscles. The *epineurium* is a tough fibrous sheath forming the outermost layer of the peripheral nerve. It consists of dense irregular connective tissue mainly composed of collagen fibers and fibrocytes. At each intervertebral foramen, the epineurium of a spinal nerve is continuous with the dura mater of the spinal cord. The *perineurium* is composed of collagenous fibers, elastic fibers, and fibrocytes. The perineurium divides the nerve into a series of compartments that contain bundles of axons. A single bundle of axons is known as a fascicle, or fasciculus. Peripheral nerves must be isolated and protected from the chemical components of the interstitial fluid and the general circulation. The connective tissue fibers and fibrocytes of the perineurium serve this function, forming the blood-nerve barrier. The *endoneurium* surrounding each individual axon is composed of loose, irregularly arranged connective tissue containing delicate collagen and elastic connective tissue fibers and a few isolated fibrocytes. Capillaries pierce the epineurium and perineurium and branch in the endoneurium, providing oxygen and nutrients to the axons and Schwann cells of the nerve.

Nerve Plexuses

While white ramus communicans are only found in areas T1-L2, gray rami communicantes, dorsal rami, and ventral rami are characteristic of all spinal nerves. The dorsal rami provide roughly segmental sensory innervation, as evidenced by the pattern of dermatomes. The segmental alignment isn't exact, because the boundaries are imprecise, and there is some overlap between adjacent dermatomes. In adult spinal cord segments controlling skeletal musculature of the neck and the upper and lower limbs, the ventral rami do not proceed directly to their peripheral targets. Instead, during embryonic development the ventral rami of adjacent spinal nerves blend their fibers and produce a series of compound nerve trunks. Such complex, interwoven network of nerves is called a *nerve plexus.* Nerve plexuses form during embryonic development as small skeletal muscles fuse with their neighbors to form larger muscles. Although the anatomical boundaries between the embryonic muscles disappear, the original pattern of innervation remains intact. Therefore, "nerves" innervating these muscles in the adult contain sensory and motor fibers from the ventral rami that innervated the embryonic muscles, forming nerve plexuses. These plexuses are cervical plexus, brachical plexus, lumbar plexus, and sacral plexus.