[TA] the complete collar of smooth muscle cells of the neck of the urinary bladder that extends distally to surround the preprostatic portion of the male urethra. There is no comparable structure in the neck of the female bladder; the internal urethral sphincter may exist to prevent reflux of semen into the bladder. SYN: musculus sphincter urethrae internus*, preprostatic sphincter*, supracollicular sphincter*, anulus urethralis, muscular sphincter supracollicularis, musculus sphincter vesicae, preprostate urethral sphincter, proximal urethral sphincter, sphincter muscle of urinary bladder, sphincter vesicae.