Final answer:
The involuntary sphincter at the top of the urinary bladder is known as the internal urinary sphincter, composed of smooth muscle that relaxes to allow urine to enter the urethra.
Step-by-step explanation:
The sphincter that is the involuntary, superior sphincter surrounding the neck of the urinary bladder is the internal urinary sphincter. This sphincter is made up of smooth muscle at the juncture of the bladder and urethra and is one of the anatomical sphincters which can restrict flow when contracted. Its function is physiological, as it consists of circular smooth muscle that is indistinguishable from adjacent muscle but possesses differential innervations. The internal urinary sphincter relaxes as the bladder fills, allowing urine to enter the urethra, which is then ultimately expelled when the external urethral sphincter, a striated muscle under voluntary control, also relaxes.