Vertebrae Meninges

Spinal cord injuries commonly result from fractures of vertebrae C5 to C6, but never from fractures of L3 to L5. Explain both observations.

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.

2menin = membrane

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Cervical enlargement

Lumbar-enlargement

Medullary -cone

Cauda equina -

Coccygeal -ligament n

Cervical spinal nerves

Dura mater-

and arachnoid mater

Thoracic spinal nerves

Lumbar spinal nerves

Sacral spinal nerves

Figure 1S.1 The Spinal Cord, Dorsal Aspect.

The dura mater3 (DOO-ruh MAH-tur) forms a loose-fitting 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 (fig. 13.2a). Anesthetics are sometimes introduced to this space to block pain signals during childbirth or surgery; this procedure is called epidural anesthesia.

The arachnoid4 (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 cere-brospinal fluid (CSF), a clear liquid discussed in chapter 14.

The pia5 (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

4arachn = spider, spider web + oid = resembling spia = tender, soft

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Arachnoid Mater The Spinal Cord

Fat in epidural space Dural sheath Arachnoid mater

Subarachnoid space

Spinal cord Denticulate ligament

Spinal nerve

Pia mater Bone of vertebra

Posterior median sulcus Dorsal horn Gray commissure

Lateral column

Lateral horn Ventral horn Anterior median fissure

Shaped Column Dorsal Ventral Horn

Spinal nerve

Ventral root of spinal nerve

Ventral column

Figure 13.2 Cross Section of the Thoracic Spinal Cord. (a) Relationship to the vertebra, meninges, and spinal nerve. (b) Anatomy of the spinal cord itself.

Lateral horn Ventral horn Anterior median fissure

Central canal Dorsal column

Dorsal root of spinal nerve

Dorsal root ganglion

Spinal nerve

Ventral root of spinal nerve

Ventral column

Figure 13.2 Cross Section of the Thoracic Spinal Cord. (a) Relationship to the vertebra, meninges, and spinal nerve. (b) Anatomy of the spinal cord itself.

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.

Insight 13.1 Clinical Application

Spina Bifida

About one baby in 1,000 is born with spina bifida (SPY-nuh BIF-ih-duh), a congenital defect resulting from the failure of one or more vertebrae to form a complete vertebral arch for enclosure of the spinal cord. This is especially common in the lumbosacral region. One form, spina bifida occulta,6 involves only one to a few vertebrae and causes no functional problems. Its only external sign is a dimple or hairy pigmented spot. Spina bifida cystica7 is more serious. A sac protrudes from the spine and may contain meninges, cerebrospinal fluid, and parts of the spinal cord and nerve roots (fig. 13.3). In extreme cases, inferior spinal cord function is absent, causing lack of bowel control and paralysis of the lower limbs and urinary bladder. The last of these conditions can lead to chronic urinary infections and renal failure. Pregnant women can significantly reduce the risk of spina bifida by taking supplemental folic acid (a B vitamin) during early pregnancy. Good sources of folic acid include green leafy vegetables, black beans, lentils, and enriched bread and pasta.

6bifid = divided, forked + occult = hidden

7 cyst = sac, bladder

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Anatomy And Physiology Spina Bifida

Figure 13.3 Spina Bifida Cystica.

Cross-Sectional Anatomy

Figure 13.2a shows the relationship of the spinal cord to a vertebra and spinal nerve, and figure 13.2fc shows the cord itself in more detail. 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 ependy-mal 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 histo-logical 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 fasciculi9 (fah-SIC-you-lye).

8funicul = little rope, cord afascicul = little bundle

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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 decussation10 (DEE-cuh-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 contralateral11 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.12

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 brain-stem; 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 first-through 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 gracile13 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

10decuss = to cross, form an X 11 contra = opposite

12ipsi = the same + later = side '3gracil = thin, slender

10decuss = to cross, form an X 11 contra = opposite

12ipsi = the same + later = side '3gracil = thin, slender

Table 13.1 Major Spinal Tracts

Tract

Column

Decussation

Functions

Ascending (sensory) Tracts

Gracile fasciculus

Cuneate fasciculus Spinothalamic Dorsal spinocerebellar Ventral spinocerebellar

Dorsal Dorsal

Lateral and ventral

Lateral

In medulla In spinal cord None

In spinal cord

Limb and trunk position and movement, deep touch, visceral pain, vibration, below level T6

Same as gracile fasciculus, from level T6 up Light touch, tickle, itch, temperature, pain, and pressure Feedback from muscles (proprioception) Same as dorsal spinocerebellar

Descending (motor) Tracts

Lateral corticospinal Ventral corticospinal Tectospinal Lateral reticulospinal Medial reticulospinal Vestibulospinal

Lateral Ventral

Lateral and ventral Lateral Ventral Ventral

In medulla None

In midbrain None None None

Fine control of limbs Fine control of limbs

Reflexive head-turning in response to visual and auditory stimuli Balance and posture; regulation of awareness of pain Same as lateral reticulospinal Balance and posture

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Anatomy Spinal Cord Cuneate

Figure 13.4 Tracts of the Spinal Cord. All of the illustrated tracts occur on both sides of the cord, but only the ascending sensory tracts are shown on the left (red), and only the descending motor tracts on the right (green).

If you were told that this cross section is either at level T4 or T10, how could you determine which is correct?

Figure 13.4 Tracts of the Spinal Cord. All of the illustrated tracts occur on both sides of the cord, but only the ascending sensory tracts are shown on the left (red), and only the descending motor tracts on the right (green).

If you were told that this cross section is either at level T4 or T10, how could you determine which is correct?

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 cuneate15 (CUE-nee-ate) fasciculus (fig. 13.5a) 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 lemniscus16 (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 (fig. 13.5b) and some smaller tracts form the anterolateral system, which passes up the anterior

14proprio = one's own + cept = receive, sense

15cune = wedge

16lemniscus = ribbon 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-no-SERR-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, as discussed in chapter 14.

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Midbrain

Gracile nucleus

Medulla

First-order neuron

Spinal cord

Midbrain

Gracile nucleus

Medulla

First-order neuron

Spinal cord

Dorsal Medial Lemniscal Pathway

Third-order neuron

Thalamus

Medial lemniscus

Gracile fasciculus Cuneate fasciculus

Second-order neuron

Cuneate nucleus

Medial lemniscus

■ Receptors for body movement, limb positions, fine touch discrimination, and pressure

Somesthetic cortex

) (postcentral gyrus)

Third-order neuron

Thalamus

Medial lemniscus

Second-order neuron

Cuneate nucleus

Medial lemniscus

Gracile fasciculus Cuneate fasciculus

■ Receptors for body movement, limb positions, fine touch discrimination, and pressure

Midbrain

Medulla

Spinal cord

First-order neuron

Spinothalamic Tract

Somesthetic cortex (postcentral gyrus)

Third-order neuron

Thalamus

Anterolateral system

Second-order neuron

-Spinothalamic tract

Receptors for pain, heat, and cold

Somesthetic cortex (postcentral gyrus)

Third-order neuron

Thalamus

Midbrain

Medulla

Second-order neuron

Spinal cord

First-order neuron

-Spinothalamic tract

Anterolateral system

Receptors for pain, heat, and cold

Figure 13.5 Some Ascending Pathways of the CNS. The spinal cord, medulla, and midbrain are shown in cross section and the cerebrum and thalamus (top) in frontal section. Nerve signals enter the spinal cord at the bottom of the figure and carry somatosensory information up to the cerebral cortex. (a) The cuneate fasciculus and medial lemniscus; (b) the spinothalamic tract.

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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-lo-SPY-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 (see chapter 16).

• 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.

Chapter 13 The Spinal Cord, Spinal Nerves, and Somatic Reflexes 489

Corticospinal Tract Mcgraw Hill
Figure 13.6 Two Descending Pathways of the CNS. The lateral and ventral corticospinal tracts, which carry signals for voluntary muscle contraction. Nerve signals originate in the cerebral cortex at the top of the figure and carry motor commands down the spinal cord.

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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?

Insight 13.2 Clinical Application

Poliomyelitis and Amyotrophic Lateral Sclerosis

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.

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Essentials of Human Physiology

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Responses

  • stephanie
    What is the posterior gray column in the spinal cord?
    6 years ago
  • LAVINIA
    What is the anterior corticospinal tract damage symptoms?
    5 years ago
  • willow
    What is vestibulospinal pathway?
    5 years ago

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