Final answer:
The external sphincter is the type of muscle involved in urination. It is voluntary and skeletal, and it relaxes and opens the external urethral orifice to allow urine to leave the body.
Step-by-step explanation:
The type of muscle involved in urination is the external sphincter. This muscle is voluntary and skeletal, meaning it is under conscious control. It is responsible for relaxing and opening the external urethral orifice to allow urine to leave the body.