The smooth muscle of the bladder wall forms the detrusor muscle. Toward the opening of the urethra, the muscle fibers form the involuntary internal urethral sphincter, a ringlike arrangement of muscle around the opening of the urethra. The smooth muscle bundles of the detrusor muscle are less regularly arranged than that of the tubular portions of the excretory passages, and thus the muscle and collagen bundles are randomly mixed (Plate 79, page 738). Contraction of the detrusor muscle of the bladder compresses the entire organ and forces the urine into the urethra.
The bladder is innervated by both sympathetic and parasympathetic divisions of the autonomic nervous system:  Sympathetic fbers form a plexus in the adventitia of the bladder wall. These fibers probably innervate blood vessels in the wall.  Parasympathetic fbers originate from S2 to S4 seg ments of the spinal cord and travel with pelvic splanchnic nerves into the bladder. They end in terminal ganglia in the muscle bundles and the adventitia and are the efferent fibers of the micturition refex.  Sensory fbers from the bladder to the sacral portion of the spinal cord are the afferent fibers of the micturition reflex.
The urethra is the fibromuscular tube that conveys urine from the urinary bladder to the exterior through the external urethral orifce. The size, structure, and functions of the urethra differ in males and females.
In the male, the urethra serves as the terminal duct for both the urinary and genital systems. It is about 20 cm long and has three distinct segments:  Prostatic urethra extends for 3 to 4 cm from the neck of the bladder through the prostate gland (see page 785). It is lined with transitional epithelium (urothelium). The ejaculatory ducts of the genital system enter the posterior wall of this segment, and many small prostatic ducts also empty into this segment.
 Membranous urethra extends for about 1 cm from the apex of the prostate gland to the bulb of the penis. It passes through the deep perineal pouch of the pelvic floor as it enters the perineum. Skeletal muscle of the deep perineal pouch surrounding the membranous urethra forms the external (voluntary) sphincter of the urethra. Transitional epithelium ends in the membranous urethra. This segment is lined with a stratified or pseudostratified columnar epithelium that resembles the epithelium of the genital duct system more than it resembles the epithelium of the more proximal portions of the urinary duct system.
 Penile (spongy) urethra extends for about 15 cm through the length of the penis and opens on the body surface at the glans penis. The penile urethra is surrounded by the corpus spongiosum as it passes through the length of the penis. It is lined with pseudostratified columnar epithelium except at its distal end, where it is lined with stratified squamous epithelium continuous with that of the skin of the penis. Ducts of the bulbourethral glands (Cowper’s glands) and of the mucus-secreting urethral glands (glands of Littré) empty into the penile urethra.