Normal motor function depends on the transmission of signals from the brain to the brainstem or spinal cord by upper motor neurons, and from there to skeletal muscle by lower motor neurons (Figure 9-1). A lesion that involves this pathway anywhere along its length may impair motor function. Anatomic structures involved in the regulation or execution of motor activity include the pyramidal and extrapyramidal systems, cerebellum, and lower motor neurons in the cranial nerve nuclei of the brainstem and anterior horns of the spinal cord.
Anatomic basis of the upper motor neuron and lower motor neuron concepts.
The pyramidal system (Figure 9-2) consists of upper motor neuron fibers that descend from the cerebral cortex through the internal capsule, traverse the medullary pyramid, and then mostly decussate, to descend in the lateral corticospinal tract on the opposite side, where they synapse on interneurons and lower motor neurons in the spinal cord.
Upper motor neuron pathways. Tracts at bottom left are shown outside the cord for clarity only. (Used with permission from McPhee SJ, Hammer GD: Pathophysiology of Disease: An Introduction to Clinical Medicine. 6th ed. New York, NY: McGraw-Hill; 2009.)
All other descending influences on lower motor neurons belong to the extrapyramidal system and originate primarily in the basal ganglia and cerebellum. Disorders of the basal ganglia (see Chapter 11, Movement Disorders) and cerebellum (see Chapter 8, Disorders of Equilibrium) are considered separately.
The motor fibers in the cranial and peripheral nerves arise from the lower motor neurons (Figure 9-3). Dysfunction at any point in the peripheral nervous system (anterior horn cell, nerve root, limb plexus, peripheral nerve, or neuromuscular junction) can impair motor function, as can disease of the muscles.
Anatomic components of the motor unit.
Patients with motor deficits generally complain of weakness, heaviness, stiffness, clumsiness, impaired muscular control, or difficulty in executing movements. The term weakness is sometimes used in a nonspecific way to denote fatigue or loss of energy, drive, or enthusiasm, and its meaning must therefore always be clarified. The word is properly used to mean loss of muscle power, and it is in this sense that it is employed here.
HISTORY OF PRESENT ILLNESS
An abrupt onset suggests a vascular disturbance, such as a stroke, or certain toxic or metabolic disturbances, whereas subacute onset over days to weeks is commonly associated with a neoplastic, infective, or inflammatory process (Table 9-1). Weakness ...