PHYSICAL EXAMINATION

LABORATORY STUDIES

DIAGNOSIS

PATHOPHYSIOLOGY OF KEY SYMPTOMS

Outflow from the upper motor neurons of the cerebral motor cortex travels by the corticospinal tracts to synapse with an alpha (α)-motor neuron. Each muscle fiber is innervated by a single α-motor neuron; however, a single α-motor neuron can innervate one or many muscle fibers. The motor unit consists of the α-motor neuron and all of the muscle fibers that it innervates. The strength of muscle contraction depends in part on the number of motor units activated. Smaller motor units are activated first, followed by larger ones.

Complex activities such as walking require the coordinated, sequential activation of multiple motor units. The sequence of activation is determined by neurons in the cerebellum, basal ganglia, and premotor cortex that synapse with the upper motor neurons. This results in the appropriate sequential activation of the muscles involved in walking and in balance, producing a smooth gait.

Action potential transmission depends on the diameter of the axon and on the presence of myelination. Myelination restricts the regions of the axon capable of producing an action potential to the nodes of Ranvier, where the myelin sheath is absent (Fig. 3-1). The wave of depolarization that initiates in the axon hillock travels by local conduction in the region of the axon beneath the myelin and is regenerated as an action potential at the nodes. This form of transmission is called “saltatory” conduction, because the action potential appears to jump from node to node along the length of the axon. Myelinated axons have much faster conduction velocities than do axons that lack myelin.

FIGURE 3–1 Saltatory conduction along a myelinated axon. Flow of electrical current from node to node is illustrated by the arrows.