Is composed of four layers, including the urothelium and the detrusor muscle
Has an impermeable urothelium to urine, its components and toxic substances
Is prone to infection because the storage and emptying process is complicated
Has no afferent neurologic input involved in its function
Is made of a thick detrusor muscle made of interlacing striated muscle bundles
The trigone:
Is an important functional unit which facilitates voiding
Corresponds to a deep layer of detrusor muscle close to the dome of the bladder
Is named after its triangular shape which extends from the bladder neck to the ureteric orifices
Is embryologically identical to the rest of the bladder wall
Is located between the bladder body and the bladder base
The urethra:
Is short in the female and made of a spongy tissue located between the mucosa and the outer muscular layers
The sphincteric region is located in the same area in men and women
Men have two sphincteric regions located below the level of the prostate
The extrinsic sphincteric unit is an involuntary system which is part of the urogenital diaphragm
The sphincter mechanism is an extra-abdominal structure whose location is essential to maintain continence.
Sympathetic nerves:
Arise from T12 to L3 spinal segments
Activate a dense plexus of beta 1 adrenergic excitatory receptors in the bladder base
Through the hypogastric plexus, these nerves supply the bladder and proximal urethra
Are responsible for a contraction of the urethral sphincter at the same time the bladder smooth muscle contracts to start micturition
Convey the release of norepinephrine from the pelvic floor muscles to aid in relaxation before
The somatic nervous system:
Is mediated by the hypogastric nerve
Causes contraction of the external striated urethal sphincter
Stimulates release of nicotinic cholinergic receptors in the bladder wall
Uniquely innervates all striated muscles of the pelvic floor
Is influenced by the pudendal nerve which originates from the dorsal horn of sacral roots S2-S4
Bladder filling:
Is dependent on the excitatory behavior of the parasympathetic system
Is coordinated by Onuf’s nucleus
Is mediated by a dense network of M3 muscarinic receptors sensitive to norepinephrine
Is related to inhibition of the detrusor muscle and concomitant relaxation of the smooth and striated muscle sphincters
Is controlled by postganglionic parasympathetic fibers from S2-S4 travelling down through the pelvic nerves to innervate the muscle fibers of the detrusor
Voiding:
Is dependent on parasympathetic innervation via norepinephrine receptors
Is a coordinated event requiring relaxation of the urethral sphincteric unit a few milliseconds before the onset of the detrusor contraction
Is primarily controlled by the S1-S5 sacral segments
Originates from an uncoordinated event secondarily regulated by the central nervous systems and a complex network of spinal interneuron reflexes.
Can only occur in privacy because of the voluntary contraction of the pelvic floor muscles.
The pontine micturition center:
Is located in the frontal lobe of the right brain
Is under the influence of the hypothalamus more so than other structures like the cerebellum or the basal ganglia
Is the integrating center for all afferent bladder signals and has no relationship with the efferent motor activity
Is less important than the periaqueductal gray matter in controlling the voiding process
Is the control tower that coordinates and integrates bladder and sphincteric activities
