… to move things is all that mankind can do, for such the sole executant is muscle, whether in whispering a syllable or in felling a forest.
Charles Sherrington, 1924
A MAJOR TASK OF THE ELABORATE information processing that takes place in the brain is the contraction of skeletal muscles. The challenge of deciding when and how to move is, to a large degree, the driving force behind the evolution of the nervous system (Chapter 30).
In all but the most primitive animals, movement is generated by specialized muscle cells. 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 either 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 (Chapter 31). The number of muscle fibers innervated by a single motor neuron varies widely throughout the body depending on the dexterity of the movements being controlled and the mass of the body part to be moved. Thus, eye movements are finely controlled by motor units with fewer than 100 muscle fibers, 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 are the first to be recruited, joined later by larger units as muscle force increases.
The motor unit is a common target of disease. The distinguishing features of diseases of the motor unit vary depending on which functional component is primarily affected: (1) the cell body of the motor or sensory neuron, (2) the corresponding axons, (3) the neuromuscular junction (the synapse between the motor axon and muscle), or (4) the muscle fibers innervated by the motor neuron. Accordingly, disorders of the motor unit have traditionally been grouped into motor neuron diseases, peripheral neuropathies, disorders of the neuromuscular junction, and primary muscle diseases (myopathies) (Figure 57–1).
The four types of motor unit disorders. Motor unit disorders are categorized according to the part of the motor unit that is affected. Motor neuron diseases affect the cell body of the neuron, while peripheral neuropathies target the axon. Diseases of the neuromuscular junction affect the functioning of the synapse, and myopathies affect muscle fibers.