NERVOUS SYSTEM

We studied the nervous system at a cellular level in. In these next two chapters, we move up the structural hierarchy to study the nervous system at the organ and system levels of organization. The spinal cord is an “information highway” between your brain and your trunk and limbs. It is about as thick as a finger, and extends through the vertebral canal as far as your first lumbar vertebra. At regular intervals, it gives off a pair of spinal nerves that receive sensory input from the skin, muscles, bones, joints, and viscera, and that issue motor commands back to muscle and gland cells. The spinal cord is a component of the central nervous system and the spinal nerves a component of the peripheral nervous system, but these central and peripheral components are so closely linked structurally and functionally that it is appropriate that we consider them together in this chapter. The brain and cranial nerves will be discussed later.

The Spinal

Objectives

When you have completed this section, you should be able to
• name the two types of tissue in the central nervous system and state their locations;
• describe the gross and microscopic anatomy of the spinal cord; and
• name the major conduction pathways of the spinal cord and state their functions.

Functions

The spinal cord serves three principal functions:
1. Conduction. The spinal cord contains bundles of nerve fibers that conduct information up and down the cord, connecting different levels of the trunk with each other and with the brain. This enables sensory information to reach the brain, motor commands to reach the effectors, and input received at one level of the cord to affect output from another level.
2. Locomotion. Walking involves repetitive, coordinated contractions of several muscle groups in the limbs. Motor neurons in the brain initiate walking and determine its speed, distance, and direction, but the simple repetitive muscle contractions that put one foot in front of another, over and over, are coordinated by groups of neurons called central pattern generators in the cord. These neuronal circuits produce the sequence of outputs to the extensor and flexor muscles that cause alternating movements of the legs.
3. Reflexes. Reflexes are involuntary stereotyped responses to stimuli. They involve the brain, spinal cord, and peripheral nerves.

Gross Anatomy

The spinal cord is a cylinder of nervous tissue that begins at the foramen magnum and passes through the vertebral canal as far as the inferior margin of the first lumbar vertebra (L1). In adults, it averages about 1.8 cm thick and 45 cm long. Early in fetal development, the spinal cord extends for the full length of the vertebral column. However, the vertebral column grows faster than the spinal cord, so the cord extends only to L3 by the time of birth and to L1 in an adult. Thus, it occupies only the upper two-thirds of the vertebral canal; the lower onethird is described shortly. The cord gives rise to 31 pairs of spinal nerves that pass through the intervertebral foramina. Although the spinal cord is not visibly segmented, the part supplied by each pair of spinal nerves is called a segment. The cord exhibits longitudinal grooves on its ventral and dorsal sides—the ventral median fissure and dorsal median sulcus, respectively.<
The spinal cord is divided into cervical, thoracic, lumbar, and sacral regions. It may seem odd that it has a sacral region when the cord itself ends well above the sacrum. These regions, however, are named for the level of the vertebral column from which the spinal nerves emerge, not for the vertebrae that contain the cord itself
In the inferior cervical region, a cervical enlargement of the cord gives rise to nerves of the upper limbs.
In the lumbosacral region, there is a similar lumbar enlargementwhere nerves to the pelvic region and lower limbs arise. Inferior to the lumbar enlargement, the cord tapers to a point called the medullary cone. The lumbar enlargement and medullary cone give rise to a bundle of nerve roots that occupy the canal of vertebrae L2 to S5. This bundle, named the cauda equina1 (CAW-duh ee-KWY-nah) for its resemblance to a horse’s tail, innervates the pelvic organs and lower limbs.

Meninges of the Spinal Cord

The spinal cord and brain are enclosed in three fibrous membranes called meninges (meh-NIN-jeez)—singular, meninx2 (MEN-inks). These membranes separate the soft tissue of the central nervous system from the bones of the vertebrae and skull. From superficial to deep, they are the dura mater, arachnoid mater, and pia mater.

The dura mater (DOO-ruh MAH-tur) forms a loosefitting sleeve called the dural sheath around the spinal cord. It is a tough collagenous membrane with a thickness and texture similar to a rubber kitchen glove. The space between the sheath and vertebral bone, called the epidural space, is occupied by blood vessels, adipose tissue, and loose connective tissue . Anesthetics are sometimes introduced to this space to block pain signals during childbirth or surgery; this procedure is called epidural anesthesia.

The arachnoid (ah-RACK-noyd) mater adheres to the dural sheath. It consists of a simple squamous epithelium, the arachnoid membrane, adhering to the inside of the dura, and a loose mesh of collagenous and elastic fibers spanning the gap between the arachnoid membrane and the pia mater. This gap, called the subarachnoid space, is filled with cerebrospinal fluid (CSF), a clear liquid discussed in later.

The pia (PEE-uh) mater is a delicate, translucent membrane that closely follows the contours of the spinal cord. It continues beyond the medullary cone as a fibrous strand, the terminal filum, forming part of the coccygeal ligament that anchors the cord to vertebra L2. At regular intervals along the cord, extensions of the pia called denticulate ligaments extend through the arachnoid to the dura, anchoring the cord and preventing side-to-side movements

CLINICAL APPLICATION , SPINA BIFIDA

Cross-Sectional Anatomy

The spinal cord, like the brain, consists of two kinds of nervous tissue called gray and white matter. Gray matter has a relatively dull color because it contains little myelin. It contains the somas, dendrites, and proximal parts of the axons of neurons. It is the site of synaptic contact between neurons, and therefore the site of all synaptic integration (information processing) in the central nervous system. White matter contains an abundance of myelinated axons, which give it a bright, pearly white appearance. It is composed of bundles of axons, called tracts, that carry signals from one part of the CNS to another. In fixed and silver-stained nervous tissue, gray matter tends to have a darker brown or golden color and white matter a lighter tan to yellow color.

Gray Matter

The spinal cord has a central core of gray matter that looks somewhat butterfly- or H-shaped in cross sections. The core consists mainly of two dorsal (posterior) horns, which extend toward the dorsolateral surfaces of the cord, and two thicker ventral (anterior) horns, which extend toward the ventrolateral surfaces. The right and left sides are connected by a gray commissure. In the middle of the commissure is the central canal, which is collapsed in most areas of the adult spinal cord, but in some places (and in young children) remains open, lined with ependymal cells, and filled with CSF. As a spinal nerve approaches the cord, it branches into a dorsal root and ventral root. The dorsal root carries sensory nerve fibers, which enter the dorsal horn of the cord and sometimes synapse with an interneuron there. Such interneurons are especially numerous in the cervical and lumbar enlargements and are quite evident in histological sections at these levels. The ventral horns contain the large somas of the somatic motor neurons. Axons from these neurons exit by way of the ventral root of the spinal nerve and lead to the skeletal muscles. The spinal nerve roots are described more fully later in this chapter. In the thoracic and lumbar regions, an additional lateral horn is visible on each side of the gray matter. It contains neurons of the sympathetic nervous system, which send their axons out of the cord by way of the ventral root along with the somatic efferent fibers.

White Matter

The white matter of the spinal cord surrounds the gray matter and consists of bundles of axons that course up and down the cord and provides avenues of communication between different levels of the CNS. These bundles are arranged in three pairs called columns or funiculi8 (few-NIC-you-lie)—a dorsal (posterior), lateral, and ventral (anterior) column on each side. Each column consists of subdivisions called tracts or fasciculi

(fah-SIC-you-lye).

Spinal Tracts

Knowledge of the locations and functions of the spinal tracts is essential in diagnosing and managing spinal cord injuries. Ascending tracts carry sensory information up the cord and descending tracts conduct motor impulses down. All nerve fibers in a given tract have a similar origin, destination, and function. Several of these tracts undergo decussation (DEEcuh-SAY-shun) as they pass up or down the brainstem and spinal cord—meaning that they cross over from the left side of the body to the right, or vice versa. As a result, the left side of the brain receives sensory information from the right side of the body and sends its motor commands to that side, while the right side of the brain senses and controls the left side of the body. A stroke that damages motor centers of the right side of the brain can thus cause paralysis of the left limbs and vice versa. When the origin and destination of a tract are on opposite sides of the body, we say they are contralateral to each other. When a tract does not decussate, so the origin and destination of its fibers are on the same side of the body, we say they are ipsilateral.

The major spinal cord tracts are summarized in table 13.1 and figure 13.4. Bear in mind that each tract is repeated on the right and left sides of the spinal cord.

Ascending Tracts

Ascending tracts carry sensory signals up the spinal cord. Sensory signals typically travel across three neurons from their origin in the receptors to their destination in the sensory areas of the brain: a first-order neuron that detects a stimulus and transmits a signal to the spinal cord or brainstem; a second-order neuron that continues as far as a “gateway” called the thalamus at the upper end of the brainstem; and a third-order neuron that carries the signal the rest of the way to the sensory region of the cerebral cortex. The axons of these neurons are called the firstthrough third-order nerve fibers. Deviations from the pathway described here will be noted for some of the sensory systems to follow.

The major ascending tracts are as follows. The names of most ascending tracts consist of the prefix spino- followed by a root denoting the destination of its fibers in the brain.

• The gracile fasciculus (GRAS-el fah-SIC-you-lus) carries signals from the midthoracic and lower parts of the body. Below vertebra T6, it composes the entire dorsal column. At T6, it is joined by the cuneate fasciculus, discussed next. It consists of first-order nerve fibers that travel up the ipsilateral side of the spinal cord and terminate at the gracile nucleus in the medulla oblongata of the brainstem. These fibers carry signals for vibration, visceral pain, deep and discriminative touch (touch whose location one can precisely identify), and especially proprioception14 from the lower limbs and lower trunk. (Proprioception is a nonvisual sense of the position and movements of the body.)

• The cuneate (CUE-nee-ate) fasciculus joins the gracile fasciculus at the T6 level. It occupies the lateral portion of the dorsal column and forces the gracile fasciculus medially. It carries the same type of sensory signals, originating from level T6 and up (from the upper limb and chest). Its fibers end in the cuneate nucleus on the ipsilateral side of the medulla oblongata. In the medulla, second-order fibers of the gracile and cuneate systems decussate and form the medial lemniscus (lem-NIS-cus), a tract of nerve fibers that leads the rest of the way up the brainstem to the thalamus. Third-order fibers go from the thalamus to the cerebral cortex. Because of decussation, the signals carried by the gracile and cuneate fasciculi ultimately go to the contralateral cerebral hemisphere.

• The spinothalamic (SPY-no-tha-LAM-ic) tract and some smaller tracts form the anterolateral system, which passes up the anterior and lateral columns of the spinal cord. The spinothalamic tract carries signals for pain, temperature, pressure, tickle, itch, and light or crude touch. Light touch is the sensation produced by stroking hairless skin with a feather or cotton wisp, without indenting the skin; crude touch is touch whose location one can only vaguely identify. In this pathway, first-order neurons end in the dorsal horn of the spinal cord near the point of entry. Second-order neurons decussate to the opposite side of the spinal cord and there form the ascending spinothalamic tract. These fibers lead all the way to the thalamus. Third-order neurons continue from there to the cerebral cortex.

• The dorsal and ventral spinocerebellar (SPY-noSERR-eh-BEL-ur) tracts travel through the lateral column and carry proprioceptive signals from the limbs and trunk to the cerebellum, a large motor control area at the rear of the brain. The first-order neurons of this system originate in the muscles and tendons and end in the dorsal horn of the spinal cord. Second-order neurons send their fibers up the spinocerebellar tracts and end in the cerebellum. Fibers of the dorsal tract travel up the ipsilateral side of the spinal cord. Those of the ventral tract cross over and travel up the contralateral side but then cross back in the brainstem to enter the ipsilateral cerebellum. Both tracts provide the cerebellum with feedback needed to coordinate muscle action,

Descending Tracts

Descending tracts carry motor signals down the brainstem and spinal cord. A descending motor pathway typically involves two neurons called the upper and lower motor neuron. The upper motor neuron begins with a soma in the cerebral cortex or brainstem and has an axon that terminates on a lower motor neuron in the brainstem or spinal cord. The axon of the lower motor neuron then leads the rest of the way to the muscle or other target organ. The names of most descending tracts consist of a word root denoting the point of origin in the brain, followed by the suffix -spinal. The major descending tracts are described here

• The corticospinal (COR-tih-co-SPY-nul) tracts carry motor signals from the cerebral cortex for precise, finely coordinated limb movements. The fibers of this system form ridges called pyramids on the ventral surface of the medulla oblongata, so these tracts were once called pyramidal tracts. Most corticospinal fibers decussate in the lower medulla and form the lateral corticospinal tract on the contralateral side of the spinal cord. A few fibers remain uncrossed and form the ventral corticospinal tract on the ipsilateral side (fig. 13.6). Fibers of the ventral tract decussate lower in the spinal cord, however, so even they control contralateral muscles.

• The tectospinal (TEC-toe-SPY-nul) tract begins in a midbrain region called the tectum and crosses to the contralateral side of the brainstem. In the lower medulla, it branches into lateral and medial tectospinal tracts of the upper spinal cord. These are involved in reflex movements of the head, especially in response to visual and auditory stimuli.

• The lateral and medial reticulospinal (reh-TIC-you-loSPY-nul) tracts originate in the reticular formation of the brainstem. They control muscles of the upper and lower limbs, especially to maintain posture and balance. They also contain descending analgesic pathways that reduce the transmission of pain signals to the brain

• The vestibulospinal (vess-TIB-you-lo-SPY-nul) tract begins in a brainstem vestibular nucleus that receives impulses for balance from the inner ear. The tract passes down the ventral column of the spinal cord and controls limb muscles that maintain balance and posture.

Rubrospinal tracts are prominent in other mammals, where they aid in muscle coordination. Although often pictured in illustrations of human anatomy, they are almost nonexistent in humans and have little functional importance.

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Think About It

You are blindfolded and either a tennis ball or an iron ball is placed in your right hand. What spinal tract(s) would carry the signals that enable you to discriminate between these two objects?

Clinical Application Poliomyelitis and Amyotrophic Lateral Sclerosis

Poliomyelitis and amyotrophic lateral sclerosis (ALS) are two diseases that involve destruction of motor neurons. In both diseases, the skeletal muscles atrophy from lack of innervation. Poliomyelitis is caused by the poliovirus, which destroys motor neurons in the brainstem and ventral horn of the spinal cord. Signs of polio include muscle pain, weakness, and loss of some reflexes, followed by paralysis, muscular atrophy, and sometimes respiratory arrest. The virus spreads by fecal contamination of water. Historically, polio afflicted mainly children, who sometimes contracted the virus in the summer by swimming in contaminated pools. The polio vaccine has nearly eliminated new cases. ALS is also known as Lou Gehrig disease after the baseball player who contracted it. It is marked not only by the degeneration of motor neurons and atrophy of the muscles, but also sclerosis of the lateral regions of the spinal cord—hence its name. In most cases of ALS, neurons are destroyed by an inability of astrocytes to reabsorb glutamate from the tissue fluid, allowing this neurotransmitter to accumulate to a toxic level. The early signs of ALS include muscular weakness and difficulty in speaking, swallowing, and using the hands. Sensory and intellectual functions remain unaffected, as evidenced by the accomplishments of astrophysicist and best-selling author Stephen Hawking, who was stricken with ALS while he was in college. Despite near-total paralysis, he remains highly productive and communicates with the aid of a speech synthesizer and computer. Tragically, many people are quick to assume that those who have lost most of their ability to communicate their ideas and feelings have no ideas and feelings to communicate. To a victim, this may be more unbearable than the loss of motor function itself.

The Spinal Nerves

Objectives

When you have completed this section, you should be able to
• describe the attachment of a spinal nerve to the spinal cord;
• trace the branches of a spinal nerve distal to its attachment;
• name the five plexuses of spinal nerves and describe their general anatomy;
• name some major nerves that arise from each plexus; and
• explain the relationship of dermatomes to the spinal nerves.

General Anatomy of Nerves and Ganglia

The spinal cord communicates with the rest of the body by way of the spinal nerves. Before we discuss those specific nerves, however, it is necessary to be familiar with the structure of nerves and ganglia in general. A nerve is a cordlike organ composed of numerous nerve fibers (axons) bound together by connective tissue. If we compare a nerve fiber to a wire carrying an electrical current in one direction, a nerve would be comparable to an electrical cable composed of thousands of wires carrying currents in opposite directions. A nerve contains anywhere from a few nerve fibers to more than a million. Nerves usually have a pearly white color and resemble frayed string as they divide into smaller and smaller branches.

Nerve fibers of the peripheral nervous system are ensheathed in Schwann cells, which form a neurilemma and often a myelin sheath around the axon. External to the neurilemma, each fiber is surrounded by a basal lamina and then a thin sleeve of loose connective tissue called the endoneurium. In most nerves, the nerve fibers are gathered in bundles called fascicles, each wrapped in a sheath called the perineuriumThe perineurium is composed of one to six layers of overlapping, squamous, epithelium-like cells. Several fascicles are then bundled together and wrapped in an outer epineurium to compose the nerve as a whole. The epineurium is composed of dense irregular fibrous connective tissue and protects the nerve from stretching and injury. Nerves have a high metabolic rate and need a plentiful blood supply. Blood vessels penetrate as far as the perineurium, and oxygen and nutrients diffuse through the extracellular fluid from there to the nerve fibers.

Think About It

How does the structure of a nerve compare to that of a skeletal muscle? Which of the descriptive terms for nerves have similar counterparts in muscle histology?

Peripheral nerve fibers are of two kinds: sensory (afferent) fibers carry signals from sensory receptors to the CNS, and motor (efferent) fibers carry signals from the CNS to muscles and glands. Both sensory and motor fibers can also be described as somatic or visceral and as general or special depending on the organs they innervate (table 13.2). A mixed nerve consists of both sensory and motor fibers and thus transmits signals in two directions, although any one nerve fiber within the nerve transmits signals one way only. Most nerves are mixed. Purely sensory nerves, composed entirely of sensory axons, are less common; they include the olfactory and optic nerves discussed in chapter 14. Nerves that carry only motor fibers are called motor nerves. Many nerves often described as motor are actually mixed because they carry sensory signals of proprioception from the muscle back to the CNS. If a nerve resembles a thread, a ganglion resembles a knot in the thread. A ganglion is a cluster of cell bodies (somas) outside the CNS. It is enveloped in an epineurium continuous with that of the nerve. Among the somas are bundles of nerve fibers leading into and out of the ganglion. Figure 13.9 shows a type of ganglion called the dorsal root ganglion associated with the spinal nerves.

Spinal Nerves

There are 31 pairs of spinal nerves: 8 cervical (C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), and 1 coccygeal (Co) . The first cervical nerve emerges between the skull and atlas, and the others emerge through intervertebral foramina, including the anterior and posterior foramina of the sacrum.

Proximal Branches

Each spinal nerve has two points of attachment to the spinal cord . Dorsally, a branch of the spinal nerve called the dorsal root divides into six to eight nerve rootlets that enter the spinal cord (fig. 13.12). A little distal to the rootlets is a swelling called the dorsal root ganglion, which contains the somas of afferent neurons. Ventrally, another row of six to eight rootlets leave the spinal cord and converge to form the ventral root.

The dorsal and ventral roots merge, penetrate the dural sac, enter the intervertebral foramen, and there form the spinal nerve proper. Spinal nerves are mixed nerves, with a two-way traffic of afferent (sensory) and efferent (motor) signals. Afferent signals approach the cord by way of the dorsal root and enter the dorsal horn of the gray matter. Efferent signals begin at the somas of motor neurons in the ventral horn and leave the spinal cord via the ventral root. Some viruses invade the central nervous system by way of these roots (see insight 13.3). The dorsal and ventral roots are shortest in the cervical region and become longer inferiorly. The roots that arise from segments L2 to Co of the cord form the cauda equina.

Distal Branches

Distal to the vertebrae, the branches of a spinal nerve are more comple (fig. 13.13). Immediately after emerging from the intervertebral foramen, the nerve divides into a dorsal ramus, a ventral ramus, and a small meningeal branch. The meningeal branch (see fig. 13.11) reenters the vertebral canal and innervates the meninges, vertebrae, and spinal ligaments. The dorsal ramus innervates the muscles and joints in that region of the spine and the skin of the back. The ventral ramus innervates the ventral and lateral skin and muscles of the trunk and gives rise to nerves of the limbs

Think About It

Do you think the meningeal branch is sensory, motor, or mixed? Explain your reasoning.

The ventral ramus differs from one region of the trunk to another. In the thoracic region, it forms an intercostal nerve that travels along the inferior margin of a rib and innervates the skin and intercostal muscles (thus contributing to breathing), as well as the internal oblique, external oblique, and transversus abdominis muscles. All other ventral rami form the nerve plexuses described next.

Clinical Application Shingles

Chickenpox (varicella), a common disease of early childhood, is caused by the varicella-zoster virus. It produces an itchy rash that usually clears up without complications. The virus, however, remains for life in the dorsal root ganglia. The immune system normally keeps it in check. If the immune system is compromised, however, the virus can travel down the sensory nerves by fast axonal transport and cause shingles (herpes zoster). This is characterized by a painful trail of skin discoloration and fluid-filled vesicles along the path of the nerve. These signs usually appear in the chest and waist, often on just one side of the body. Shingles usually occurs after the age of 50. While it can be very painful and may last 6 months or longer, it eventually heals spontaneously and requires no special treatment other than aspirin and steroidal ointment to relieve pain and inflammation.