Case Study for Chapter

Episodic Ataxia

A 3-year-old child was brought to the pediatrician because of visible muscle twitching. The parents described the twitches as looking like worms crawling under the skin. The child also periodically complained that her legs hurt, and the mother reported she could feel that the child's leg muscles were somewhat rigid at these times. Occasionally, the child would exhibit a loss of motor coordination (ataxia) that lasted 20 to 30 minutes; these episodes sometimes followed exertion or startle. Neurological function seemed normal between these episodes; the parents reported that the child's motor development seemed similar to that of their older child. The neurological examination confirms the parents' perception. Electromyographic analysis of the child's leg muscles indicate no abnormality in muscle membrane responses and a muscle biopsy is histologically normal. Spinal anesthesia eliminated the muscle twitching. The child's mother indicates that one of the child's sisters also had frequent muscle twitches as a child, but did not have episodes of ataxia. Questions

1. What is the likely source of the abnormal muscle activity?

2. What information in the presentation supports your answer to question 1?

3. Spontaneous muscle twitches indicate hyperexcitability of nerve or muscle. If this hyperexcitability is a result of an abnormality in action potential repolarization, what channels associated with the nerve action potential might lead to this condition?

Answers to Case Study Questions for Chapter 3

1. The abnormal muscle activity derives from the motor neurons.

2. Spontaneous muscle twitching could be a result of a defect in the muscle, the motor neurons that control the muscle, the neuromuscular junction (synapse), or the central nervous system elements that control spinal motor neurons. The description of muscle twitches that look like worms crawling under the skin indicates that individual motor units are firing randomly and spontaneously. (A motor unit is one motor neuron and all of the muscle fibers it innervates.) The muscle biopsy and electromyographic studies indicate it is not the muscle. Spinal anesthesia eliminates the muscle twitching indicating that the defect is at the level of the motor neurons.

3. The nerve action potential may fail to repolarize properly if there is a defect in the inactivation of voltage-gated sodium channels or in the activation of voltage-gated potassium channels. Genetic analysis in this individual, whose diagnosis is episodic ataxia with myokymia, would indicate a mutation in the potassium channel.

References

Adelman JP, Bond CT, Pessia M, Maylie J. Episodic ataxia results from voltage-dependent potassium channels with altered functions. Neuron 1995;15:1449-1454. Browne DL, Gancher ST, Nutt JG, et al. Episodic ataxia/myokymia syndrome is associated with point mutations in the human potassium channel gene, KCNA1. Nat Genet 1994;8:136-140.

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