Bladder Anatomy

The Anatomy of the Bladder

• The body of the bladder temporarily stores urine
• The neck connects the bladder to the urethra
• The trigone consists of ureteric orifices and an internal urethral orifice at the angles of a triangle
• The detrusor muscle is made from a meshwork of muscle fibres in roughly 3 layers
• Fibres are arranged in inner longitudinal, middle circular, and outer longitudinal layers
• This arrangement provides bladder strength regardless of the direction it's stretched
• The detrusor muscle receives spinal nerve supply bilaterally, and is supplied by the autonomic nervous system, so is not under voluntary control
• The internal urethral sphincter is a continuation of the detrusor muscle, made of smooth muscle
• The internal urethral sphincter is a physiological sphincter at the bladder neck
• The internal urethral sphincter is a physiological sphincter with no muscle thickening - action is due to structure
• The internal urethral sphincter is the primary muscle of continence
• The external urethral sphincter is an anatomical sphincter of localised circular muscle thickening for action
• The external urethral sphincter derived from pelvic floor muscles
• The external urethral sphincter is skeletal muscle, under somatic, voluntary control
• The external urethral sphincter contracts to constrict the urethra and "hold in” urine

Innervation of the Bladder

• The nervous system provides sensations of bladder filling and pain
• The nervous system allows the bladder to relax and accommodate increasing volumes of urine
• The nervous system initiates and maintains voiding so the bladder empties completely, with minimal residual volume
• The nervous system provides integrated regulation of the smooth and skeletal muscle sphincters of the urethra.
• Lower urinary tract activity is divided into two phases: filling and voiding
• During filling, the bladder relaxes and accommodates increasing urine volumes, while urethral sphincters increase tone to maintain continence
• During voiding, the urethral sphincters relax, and the bladder contracts.
• The extent of bladder contraction is greater in men
• In men, the prostatic urethra functions to prevent retrograde ejaculation into the bladder.
• Pelvic nerves perform parasympathetic functions (S2-S4)
• Ach M3 receptors cause contraction
• Hypogastric nerves perform sympathetic functions (T10-L2)
• NA B3 receptors cause relaxation
• Hypogastric nerves (T10-L2): NA a₁ receptors cause contraction of the internal urethral sphincter
• The contraction of the internal urethral sphincter is a sympathetic function
• Pudendal nerves perform somatic functions (S2-S4)
• Spinal motor outflow comes from Onof's Nucleus of the ventral horn of the cord
• Ach Nicotinic receptors cause contraction of the external urethral sphincter

The Normal Voiding Reflex

• The threshold for feeling a full bladder is ~400ml
• When the bladder is full, an urge to urinate arises
• Brain and Spinal micturition centres control normal voiding reflex
• Parasympathetic neurons are involved in micturition
• Increased parasympathetic stimulation to the bladder via the Pelvic nerve causes the Detrusor to contract, and increase intravesicular pressure
• The Cerebral Cortex makes an executive decision to urinate, reducing somatic stimulation to the External Urethral Sphincter
• Detrusor contraction coupled with the relaxation of the External Urethral Sphincter results in the bladder emptying through the urethra.
• Ureters, urinary bladder, and internal and external urethral sphincters work together to pass and store urine in the urinary bladder
• Bladder walls have many folds, and distend when filling with urine, so intravesicular pressure hardly changes as the bladder fills.
• At around 400ml of bladder filling, afferent nerves from the bladder wall signal the need to void (pain/temperature sensation), with possible stretch receptors
• Active Brain and Spinal Continence Centres and Sympathetic Neurons increase sympathetic stimulation to the bladder via the hypogastric nerve
• That stimulation causes the Detrusor to relax, and the Internal Urethral Sphincter to contract
• The Cerebral Cortex decides not to urinate, and increases somatic stimulation to the External Urethral Sphincter
• Increasing stimulation causes the External Urethral Sphincter contracts, constricting the urethra
• The relaxation of the Detrusor, and the simultaneous contraction of the Internal and External Urethral sphincters reduces intravesicular pressure, constricting the urethra and preventing micturition.