Poliomyelitis and Amyotrophic Lateral Sclerosis

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Poliomyelitis17 and amyotrophic lateral sclerosis18 (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

Stephen Hawking Before Als
Figure 13.7 Stephen Hawking (1942- ), Lucasian Professor of Mathematics at Cambridge University.

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.

17 polio = gray matter + myel = spinal cord + itis = inflammation

18 a = without + myo = muscle + troph = nourishment

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. Name the four major regions and two enlargements of the spinal cord.
  2. Describe the distal (inferior) end of the spinal cord and the contents of the vertebral canal from level L2 to S5.
  3. Sketch a cross section of the spinal cord showing the dorsal and ventral horns. Where are the gray and white matter? Where are the columns and tracts?
  4. Give an anatomical explanation as to why a stroke in the right cerebral hemisphere can paralyze the limbs on the left side of the body.

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 (fig. 13.8). 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.

Saladin: Anatomy & I 13. The Spinal Cord, Spinal I Text I © The McGraw-Hill

Physiology: The Unity of Nerves, and Somatic Companies, 2003

Form and Function, Third Reflexes Edition

Chapter 13 The Spinal Cord, Spinal Nerves, and Somatic Reflexes 491

Ventral root Dorsal root Dorsal root ganglion

Spinal nerve

Spinal cord (a)

Epineurium

Perineurium—

Endoneurium^ around individual axon

  • Blood vessels
  • Fascicle

"Unmyelinated axon

-Schwann cell of myelinated axon -Axon

Physiology Polio

Figure 13.8 Anatomy of a Nerve. (a) A spinal nerve and its association with the spinal cord. (b) Cross section of a nerve (SEM). Myelinated nerve fibers appear as white rings and unmyelinated fibers as solid gray. Credit for b: Richard E. Kessel and Randy H. Kardon, Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, 1979, W. H. Freeman and Company.

Figure 13.8 Anatomy of a Nerve. (a) A spinal nerve and its association with the spinal cord. (b) Cross section of a nerve (SEM). Myelinated nerve fibers appear as white rings and unmyelinated fibers as solid gray. Credit for b: Richard E. Kessel and Randy H. Kardon, Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, 1979, W. H. Freeman and Company.

Nerve fibers of the peripheral nervous system are ensheathed in Schwann cells, which form a neurilemma and often a myelin sheath around the axon (see chapter 12). External to the neurilemma, each fiber is surrounded by a basal lamina and then a thin sleeve of loose connective tis sue called the endoneurium. In most nerves, the nerve fibers are gathered in bundles called fascicles, each wrapped in a sheath called the perineurium. The per-ineurium is composed of one to six layers of overlapping, squamous, epithelium-like cells. Several fascicles are then

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Physiology: The Unity of Nerves, and Somatic Companies, 2003

Form and Function, Third Reflexes Edition

492 Part Three Integration and Control 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 ganglion19 resembles a knot in the thread. A ganglion is a cluster of cell

19gangli = knot

Table 13.2 The Classification of Nerve Fibers 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) (fig. 13.10). 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 (fig. 13.11). 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. Ven-trally, 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 complex (fig. 13.13). Immediately after emerging from the intervertebral foramen, the nerve divides into a dorsal ramus,20 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

Class Description

Afferent fibers Carry sensory signals from receptors to the CNS Efferent fibers Carry motor signals from the CNS to effectors Somatic fibers Innervate skin, skeletal muscles, bones, and joints Visceral fibers Innervate blood vessels, glands, and viscera General fibers Innervate widespread organs such as muscles, skin, glands, viscera, and blood vessels Special fibers Innervate more localized organs in the head, including the eyes, ears, olfactory and taste receptors, and muscles of chewing, swallowing, and facial expression

20ramus = branch

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Physiology: The Unity of Nerves, and Somatic Companies, 2003

Form and Function, Third Reflexes Edition

Chapter 13 The Spinal Cord, Spinal Nerves, and Somatic Reflexes 493

Spinal cord Ventral root

Connective tissue

Spinal cord Ventral root

Physiology Poliomyelitis

Blood vessels

Spinal nerve

Connective tissue

Fibers of motor-(efferent) neurons

Figure 13.9 Anatomy of a Ganglion. The dorsal root ganglion contains the somas of unipolar sensory neurons conducting signals to the spinal cord. To the left of it is the ventral root of the spinal nerve, which conducts motor signals away from the spinal cord. (The ventral root is not part of the ganglion.)

Where are the somas of the motor neurons located?

Fibers of motor-(efferent) neurons

Dorsal root

Epineurium of dorsal root

Epineurium of ganglion

Fibers of somatosensory (afferent) neurons

Somas of somatosensory (afferent) neurons

Dorsal root ganglion

Direction of signal transmission

Blood vessels

Spinal nerve

Figure 13.9 Anatomy of a Ganglion. The dorsal root ganglion contains the somas of unipolar sensory neurons conducting signals to the spinal cord. To the left of it is the ventral root of the spinal nerve, which conducts motor signals away from the spinal cord. (The ventral root is not part of the ganglion.)

Where are the somas of the motor neurons located?

Insight 13.3 Clinical Application 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.

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.

Saladin: Anatomy & I 13. The Spinal Cord, Spinal I Text I © The McGraw-Hill

Physiology: The Unity of Nerves, and Somatic Companies, 2003

Form and Function, Third Reflexes Edition

494 Part Three Integration and Control

Cervical plexus (C1-C5)-Brachial plexus (C5-T1) -

C1 C2 C3

Intercostal (thoracic) nerves rve

Sciatic-nerve

Intercostal (thoracic) nerves rve

- -

—^

^m:

ft

y?

1

Ca

C7 Ca

T1

T2

T3

? "1

T4

TS

Ta

T7

Ta

T9

H

T1G

T11

sai

. T12

¥

Thoracic nerves (12 pairs)

L4 LS

Atlas (first cervical vertebra) Cervical nerves (8 pairs)

Cervical enlargement ^_st thoracic vertebra k aci

Thoracic nerves (12 pairs)

Lumbar enlargement

1st lumbar vertebra Medullary cone

Lumbar nerves (5 pairs)

-Cauda equina

- Ilium

  • Sacral nerves (5 pairs)
  • Coccygeal nerves (1 pair)

Insight 13.4 Clinical Application

Figure 13.10 The Spinal Nerve Roots and Plexuses, Dorsal View.

Nerve Plexuses

Except in the thoracic region, the ventral rami branch and anastomose (merge) repeatedly to form five weblike nerve plexuses: the small cervical plexus deep in the neck, the brachial plexus near the shoulder, the lumbar plexus of the lower back, the sacral plexus immediately inferior to this, and finally the tiny coccygeal plexus adjacent to the lower sacrum and coccyx. A general view of these plexuses is shown in figure 13.10; they are illustrated and described in tables 13.3 through 13.6. The muscle actions controlled by these nerves are described in the muscle tables in chapter 10.

Spinal Nerve Injuries

The radial and sciatic nerves are especially vulnerable to injury. The radial nerve, which passes through the axilla, may be compressed against the humerus by improperly adjusted crutches, causing crutch paralysis. A similar injury often resulted from the now-discredited practice of trying to correct a dislocated shoulder by putting a foot in a person's armpit and pulling on the arm. One consequence of radial nerve injury is wrist drop—the fingers, hand, and wrist are chronically flexed because the extensor muscles supplied by the radial nerve are paralyzed.

Because of its position and length, the sciatic nerve of the hip and thigh is the most vulnerable nerve in the body. Trauma to this nerve produces sciatica, a sharp pain that travels from the gluteal region along the posterior side of the thigh and leg as far as the ankle. Ninety percent of cases result from a herniated intervertebral disc or osteoarthritis of the lower spine, but sciatica can also be caused by pressure from a pregnant uterus, dislocation of the hip, injections in the wrong area of the buttock, or sitting for a long time on the edge of a hard chair. Men sometimes suffer sciatica from the habit of sitting on a wallet carried in the hip pocket.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Chapter 13 The Spinal Cord, Spinal Nerves, and Somatic Reflexes 495

Posterior

Dorsal root

Dorsal root ganglion

Dorsal ramus

Ventral ramus Ventral root

Body of vertebra

Posterior

Dorsal root

Dorsal root ganglion

Dorsal ramus

Ventral ramus Ventral root

Body of vertebra

Meningeal Branch Spinal Nerve

Sympathetic ganglion

Spine of vertebra Deep muscles of back Spinal cord

Spinal nerve Meningeal branch

Communicating rami

Sympathetic ganglion

Anterior

Figure 13.11 Branches of a Spinal Nerve in Relation to the Spinal Cord and Vertebra (cross section).

Posterior median sulcus

Gracile fasciculus

Cuneate fasciculus Lateral column

Segment C5

Cross section

Arachnoid mater

Dura mater

Posterior median sulcus

Gracile fasciculus

Cuneate fasciculus Lateral column

Segment C5

Cross section

Arachnoid mater

Dura mater

Spinal Cord Arachnoid Mater

Figure 13.12 The Point of Entry of Two Spinal Nerves into the Spinal Cord. Dorsal view with vertebrae cut away. Note that each dorsal root divides into several rootlets that enter the spinal cord. A segment of the spinal cord is the portion receiving all the rootlets of one spinal nerve. In the labeled rootlets of spinal nerve C5, are the nerve fibers afferent or efferent? How do you know?

Figure 13.12 The Point of Entry of Two Spinal Nerves into the Spinal Cord. Dorsal view with vertebrae cut away. Note that each dorsal root divides into several rootlets that enter the spinal cord. A segment of the spinal cord is the portion receiving all the rootlets of one spinal nerve. In the labeled rootlets of spinal nerve C5, are the nerve fibers afferent or efferent? How do you know?

Saladin: Anatomy & I 13. The Spinal Cord, Spinal I Text I © The McGraw-Hill

Physiology: The Unity of Nerves, and Somatic Companies, 2003

Form and Function, Third Reflexes Edition

496 Part Three Integration and Control

Physiology Polio
Rami The Spinal Nerves
Figure 13.13 Rami of the Spinal Nerves. (a) Anterolateral view of the spinal nerves and their subdivisions in relation to the spinal cord and vertebrae. (b) Cross section of the thorax showing innervation of muscles of the chest and back.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Table 13.3 The Cervical Plexus

The cervical plexus (fig. 13.14) receives fibers from the ventral rami of nerves C1 to C5 and gives rise to the nerves listed, in order from superior to inferior. The most important of these are the phrenic2 nerves, which travel down each side of the mediastinum, innervate the diaphragm, and play an essential role in breathing. In addition to the major nerves listed here, there are several motor branches that innervate the geniohyoid, thyrohyoid, scalene, levator scapulae, trapezius, and sternocleidomastoid muscles.

Lesser Occipital Nerve

Composition: Somatosensory

Innervation: Skin of lateral scalp and dorsal part of external ear Great Auricular Nerve

Ansa Cervicalis

Composition: Motor

Innervation: Omohyoid, sternohyoid, and sternothyroid muscles Supraclavicular Nerve

Composition: Somatosensory Innervation: Skin of and around external ear

Transverse Cervical Nerve

Composition: Somatosensory

Innervation: Skin of ventral and lateral aspect of neck

Composition: Somatosensory

Innervation: Skin of lower ventral and lateral neck, shoulder, and ventral chest Phrenic (FREN-ic) Nerve

Composition: Motor Innervation: Diaphragm

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Responses

  • linda
    What is the dorsal rami amyotrophic lateral sclerosis?
    6 years ago
  • Sandra Marchesi
    Which of the descriptive terms for nerves have similar counterparts in muscle histology?
    6 years ago
  • MEERI
    Does polio virus prefer ventral or dorsal root ganglia?
    6 years ago
  • adolfo
    Which statement is true of both poliomyelitis and amyotrophic lateral sclerosis (ALS)?
    11 months ago
  • serena piccio
    Which statement is true of both poliomyelitis and amyotrophic lateral sclerosis (ALS?
    10 months ago

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