Apraxia is a neurological condition, where a person is unable to do a task intentionally, even though they do not have any muscle weakness or incoordination. Apraxia is caused due to damage in the areas of the brain that are responsible for motor planning and motor execution. Because of this, people with Apraxia find it difficult to perform a task when instructed.  

This article outlines the anatomy and physiology of the voice, vocal health and how to use your voice effectively to exude confidence and clarity.

Voice is generally characterized by pitch, volume and quality. Generally, an average voice often has an appropriate pitch, good quality and an adequate amount of volume. Rapid changes in these often result in an unstable voice. 

I was an actor for many years (and, in fact, I still am from time to time). This means I have many old friends who are still working in the industry and rely on their voices to make a living.