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spastic paralysis in which muscle tone increases, with very little atrophy of the muscles. However, uncoordinated reflex activity (hyper-reflexia) usually occurs, when the flexor and extensor muscles of affected limbs alternately spasm.

Injury to motor neurons or their fibers in the horns of the spinal cord results in lower motor neuron syndrome. It is characterized by flaccid paralysis, a total loss of muscle tone and reflex activity, and the muscles atrophy.

Several new treatments are on the horizon for spinal cord injuries. They work in three ways:

1. Limiting damage during the acute phase. When 17-year-old Chinese gymnast Sang Lan misjudged a practice vault at the Goodwill Games in New York City in the summer of 1998, fracturing two cervical vertebrae, prompt administration of an experimental drug called GM1 ganglioside limited damage. This carbohydrate, normally found on neuron cell membranes, blocks the actions of amino acids that function as excitatory neurotransmitters, which cuts the deadly calcium ion influx into cells. It also blocks apoptosis (programmed cell death) and stimulates synthesis of nerve growth factor.

2. Restoring or compensating for function. A new drug called 4-aminopyridine blocks potassium channels on neurons. This boosts electrical transmission and compensates for the myelin-

stripping effects of the injury. Being developed for patients injured at least 18 months previously, this drug can restore some sexual, bowel, and bladder function.

3. Regeneration. Neurobiologists have known since 1981 that regeneration of damaged spinal cord cells should be possible. Experiments then showed that spinal axons can grow in the PNS, but not in the CNS. In 1988, researchers discovered a protein in the spinal cord that inhibits regeneration. Blocking this protein may allow some regeneration. In rats with damaged spinal cords, a neural cellular adhesion molecule blocks the inhibitor, restoring some walking ability. Using another approach, investigators have effectively patched severed rat spinal cords with implants of peripheral nervous tissue from the chest area and added growth factors. These animals too regained partial walking ability.

Clinical trials are underway to see if implants of neural stem cells can regenerate spinal cord neurons. One day, neural stem cells taken from an injured person's brain might be expanded in culture and used to "patch" a severed spinal cord. ■

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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