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A pure motor stroke primarily affects the posterior limb of the internal capsule. This area contains corticospinal fibers that are crucial for voluntary motor control. When a stroke occurs here, it disrupts the pathways responsible for transmitting motor signals from the brain to various parts of the body, resulting in motor deficits.

The posterior limb's damage leads to contralateral weakness or paralysis, typically affecting the face, arm, and leg in a proportion that reflects the distribution of the motor cortex. In essence, the specific nature of a pure motor stroke—characterized by the inability to perform voluntary movements without significant sensory or visual deficits—is a hallmark of injury to the posterior limb of the internal capsule, as it primarily deals with motor information.

While the anterior limb of the internal capsule, basal ganglia, and cerebellum are essential components of the brain that are involved in motor function, they are more associated with other conditions or mixed deficits rather than isolation leading to a pure motor stroke. For instance, the basal ganglia are involved in the regulation of movements and can lead to more complex movement disorders rather than pure motor weakness.