Muscle tone - Basic physiology

Muscle tone is traditionally defined as 'the tension in the relaxed muscle' or 'the resistance, felt by the examiner during passive stretching of a joint when the muscles are at rest'.
What is muscle tone and what causes it?
Muscle tone is regulated by signals that travel from the brain to the nerves and tell the muscle to contract. Hypertonia happens when the regions of the brain or spinal cord that control these signals are damaged.