Answer to Question 1
A psychological addiction occurs when a person relies on a chemical substance, or any behavior that he or she becomes dependent upon, to meet his or her psychological needs (such as filling a void or to reduce anxiety/stress). A physiological addiction is a state of physical need for a drug, with development of withdrawal syndrome following abrupt cessation of the use of the substance.
Examples include social networking, compulsive spending, gambling, Internet addiction, alcohol, food, and so on.
Answer to Question 2
Each skeletal muscle is composed of many muscle fibers (cells), each of which has the ability to contract or relax. A muscle fiber contracts when it receives a nerve impulse from the central nervous system (brain and spinal cord). After contracting, a muscle fiber relaxes until it receives another nerve impulse.