S2-S3 roots There are two sphincters of the urethraa. Internal urethral sphincter (also k/a sphincter vesicae)- it is involuntary in nature- made up of smooth muscle- supplied by sympathetic nerves, from lower thoracic & upper lumbar segments of spinal cordb. External urethral sphincter (also k/a sphincter vesicae)- it is voluntary in nature- made up of striated muscle fibres- supplied by the perineal branch of the pudendal nerve (derived from spinal nerves S2,S3 & S4) - it controls the membranous urethra and is responsible for the voluntary holding of urine.