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A hemi-lesion of the spinal cord (severed on only one side) affecting the corticospinal and spinothalamic tracts can cause Brown-Sequard syndrome. Because ascending tracts cross over at different levels, the injured side of the body becomes paralyzed and loses touch sensation. The other side of the body retains movement but loses sensations of pain and temperature.

Table 11.3 summarizes the nerve tracts of the spinal cord. Clinical Application 11.3 describes injuries to the spinal cord.

H Describe the structure of the spinal cord.

What are ascending and descending tracts?

What is the consequence of fibers crossing over?

Name the major tracts of the spinal cord, and list the kinds of impulses each conducts.

Spinal Cord Injuries

Christopher Reeve's life changed forever in a split second on May 27, 1995. Reeve, best known for his portrayal of Superman in four films, was one of 300 equestrians competing on that bright Saturday in Culpeper County, Virginia. He and his horse, Buck, were poised to clear the third of fifteen hurdles in a 2-mile event. The horse's front legs went over the hurdle, and Reeve's back arched as he propelled himself forward. But Buck stopped, his back legs never clearing the fence. Reeve hurled forward, striking his head on the fence. He landed on the grass — unconscious, not moving or breathing.

Reeve had broken the first and second cervical vertebrae, between the neck and the brain stem. Someone performed mouth-to-mouth resuscitation until paramedics inserted a breathing tube and then stabilized him on a board. At a nearby hospital, Reeve received methylprednisolone, a drug that diminishes the extremely damaging swelling that occurs as the immune system responds to the injury. If given within eight hours of the accident, this drug can save a fifth of the damaged neurons. Reeve was then flown to a larger medical center, where he was sedated and fluid was suctioned from his lungs.

The first few days following a spinal cord injury are devastating. At first, the vertebrae are compressed and may break, which sets off action potentials in neurons, many of which soon die. The massive neuron death releases calcium ions, which activate tissue-degrading enzymes. Then white blood cells arrive and produce inflammation that can destroy healthy as well as damaged neurons. Axons tear, myelin coatings are stripped off, and vital connections between nerves and muscles are cut. The tissue cannot regenerate.

Thousands of people sustain spinal cord injuries each year. The consequences depend on the extent of damage the cord sustains and where the damage occurs.

Normal spinal reflexes depend on two-way communication between the spinal cord and the brain. Injuring nerve pathways depresses the cord's reflex activities in sites below the injury. At the same time, sensations and muscular tone in the parts the affected fibers innervate lessen. This condition, spinal shock, may last for days or weeks, although normal reflex activity may eventually return. However, if nerve fibers are severed, some of the cord's functions may be permanently lost.

Less severe injuries to the spinal cord, as from a blow to the head, whiplash, or rupture of an intervertebral disk, compress or distort the cord. Pain, weakness, and muscular atrophy in the regions the damaged nerve fibers supply may occur.

The most common cause of severe direct injury to the spinal cord is vehicular accidents (fig. 11C and table 11A.). Regardless of the cause, if nerve fibers in ascending tracts are cut, sensations arising from receptors below the level of the injury are lost. Damage to descending tracts results in loss of motor functions. For example, if the right lateral corticospinal tract is severed in the neck near the first cervical vertebra, control of the voluntary muscles in the right upper and lower limbs is lost, paralyzing them (hemiplegia). Problems of this type in fibers of the descending tracts produce upper motor neuron syndrome, characterized by

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, begins with slight stiffening and weakening of the upper and lower limbs, loss of finger dexterity, wasting hand muscles, severe muscle cramps, and difficulty swallowing. Muscle function declines throughout the body, and usually the person dies within five years from respiratory muscle paralysis. Some people, however, live many years with ALS, such as noted astronomer and author Stephen Hawking.

In ALS, motor neurons degenerate within the spinal cord, brain stem, and cerebral cortex. Fibrous tissue replaces them. By studying some of the 10% of ALS patients who inherit the disorder, researchers traced a cause to an abnormal form of an enzyme, superoxide dismutase, which normally dismantles oxygen free radicals, which are toxic by-products of metabolism.

Spinal cord Dens

Atlas

Axis

Spinal cord Dens

Atlas

Axis

ure 11C

Figure

A dislocation of the atlas may cause a compression injury to the spinal cord.

Causes of Spinal Cord Injuries

Cause

Percentage of cases

Motor vehicle

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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