Finally, the complexity of motor activity is almost beyond imagination. Reference was made earlier to the reciprocal innervation involved in an act as simple as making a fist. Scratching one’s shoulder has been estimated to recruit about 75 muscles. But what must be involved in playing a piano concerto? Over a century ago Hughlings Jackson commented that “There are, we shall say, over thirty muscles in the hand; these are represented in the nervous centers in thousands of different combinations, that is, as very many movements; it is just as many chords, musical expressions, and tunes can be made out of a few notes.” The execution of these complex movements, many of them learned and habitual, is made possible by the cooperative activities of not just the motor and sensory cortices but integrally of the basal ganglia, cerebellum, and reticular formation of the brainstem. All are continuously integrated and controlled by feedback mechanisms from the sensory and spinal motor neurons. These points, already touched upon in this chapter, are elaborated in the following three chapters.
A historical perspective that outlines the development of these concepts is given by Faglioni and Basso and an authoritative review of the subject of apraxia can be found in the chapter by Heilman and Gonzalez-Rothi.
The diagnostic considerations in cases of paralysis can be simplified by using the following subdivision, based on the location and distribution of the muscle weakness: 1.
Monoplegia refers to weakness or paralysis of all the muscles of one leg or arm. This term is not applied to paralysis of isolated muscles or groups of muscles supplied by a single nerve or motor root.
2.
Hemiplegia, the commonest form of paralysis, involves the arm, the leg, and sometimes the face on one side of the body. With rare exceptions, mentioned further on, hemiplegia is attributable to a lesion of the corticospinal system on the side opposite to the paralysis.
3.
Paraplegia indicates weakness or paralysis of both legs. It is most often the result of diseases of the thoracic spinal cord, cauda equina, or peripheral nerves, and rarely, both medial frontal cortices.
4.
Quadriplegia (tetraplegia) denotes weakness or paralysis of all four extremities. It may result from disease of the peripheral nerves, muscles, or myoneural junctions; gray matter of the spinal cord; or the upper motor neurons bilaterally in the cervical cord, brainstem, or cerebrum. Diplegia is a special form of quadriplegia in which the legs are affected more than the arms. Triplegia occurs most often as a transitional condition in the development of, or partial recovery from, tetraplegia.
5.
Isolated paralysis of one or more muscle groups due to disease of muscle, anterior horn cells, or nerve roots.
6.
Nonparalytic disorders of movement (e.g., apraxia, ataxia, rigidity).
7.
Hysterical paralysis.
The examination of patients who complain of weakness of one limb often discloses an asymptomatic weakness of another, and the condition is actually a hemiparesis or paraparesis. Or, instead of weakness of most of the muscles in a limb, only isolated groups are found to be affected. Ataxia, sensory disturbances, or reluctance to move the limb because of pain should not be misinterpreted as weakness. Parkinsonism may give rise to the same error, as can other causes of rigidity or bradykinesia or a mechanical limitation resulting from arthritis and bursitis. The presence or absence of atrophy of muscles in a monoplegic limb is of particular diagnostic help, as indicated below.