14: Diseases of the Nerve and Motor Unit

Introduction

• Disorders of the Peripheral Nerve, Neuromuscular Junction, and Muscle Can Be Distinguished Clinically

• A Variety of Diseases Target Motor Neurons and Peripheral Nerves

  • Motor Neuron Diseases Do Not Affect Sensory Neurons

  • Diseases of Peripheral Nerves Affect Conduction of the Action Potential

  • The Molecular Bases of Some Inherited Peripheral Neuropathies Have Been Defined

• Diseases of the Neuromuscular Junction Have Multiple Causes

  • Myasthenia Gravis Is the Best Studied Example of a Neuromuscular Junction Disease

  • Treatment of Myasthenia Targets the Physiological Effects and Autoimmune Pathogenesis of the Disease

  • There Are Two Distinct Congenital Forms of Myasthenia Gravis

  • Lambert-Eaton Syndrome and Botulism Are Two Other Disorders of Neuromuscular Transmission

• Diseases of Skeletal Muscle Can Be Inherited or Acquired

  • Dermatomyositis Exemplifies Acquired Myopathy

  • Muscular Dystrophies Are the Most Common Inherited Myopathies

  • Some Inherited Diseases of Skeletal Muscle Arise from Genetic Defects in Voltage-Gated Ion Channels

  • Periodic Paralysis Is Associated with Altered Muscle Excitability and Abnormal Levels of Serum Potassium

• An Overall View

• Postscript: Diagnosis of Motor Unit Disorders Is Aided by Laboratory Criteria

The major consequence of the elaborate information processing that takes place in the brain is the contraction of skeletal muscles. Indeed, animals are distinguishable from plants by their ability to make precise, goal-directed movements of their body parts. As we shall see in Chapter 16, the problem of deciding when and how to move is, to a large degree, the driving force behind the evolution of the nervous system.

In all but the most primitive animals, specialized muscle cells generate movement. There are three general types of muscles: Smooth muscle is used primarily for internal actions such as peristalsis and control of blood flow; cardiac muscle is used exclusively for pumping blood; and skeletal muscle is used primarily for moving bones. In this chapter we examine a variety of neurological disorders in mammals that affect movement by altering action potential conduction in a motor nerve, synaptic transmission from nerve to muscle, or muscle contraction itself.

In 1925 Charles Sherrington introduced the term motor unit to designate the basic unit of motor function—a motor neuron and the group of muscle fibers it innervates (see Chapter 34). The number of muscle fibers innervated by a single motor neuron varies widely throughout the body, depending on the dexterity of the movements controlled and the mass of the body part to be moved. Thus motor units with fewer than 100 muscle fibers finely control eye movements, whereas in the leg a single motor unit contains up to 1,000 muscle fibers. In each case all the muscles innervated by a motor unit are of the same type. Moreover, motor units are recruited in a fixed order for both voluntary and reflex movements. The smallest motor units ...