Lower Urinary Tract : Normal Anatomy & Physiology

The lower urinary tract (LUT) is comprised of the urinary bladder, the urethra and the prostate in men. The function of the LUT is the storage of urine and its voluntary expulsion from the body. These mutually exclusive states are mediated by complex neural networks (Sekhon & Ballaro, 2022). This neural system coordinates the activity of the urinary bladder and an outlet including bladder neck, urethra, and striated muscles of the pelvic floor. During the storage phase, the outlet is closed and the bladder is quiescent to maintain a low intravesical pressure and continence, and during the voiding phase, the outlet relaxes and the bladder contracts to promote efficient release of urine (Wada, N. 2022).

Urinary Bladder

The urinary bladder is a temporary storage reservoir for urine. It is located posterior to the symphysis pubis, and below the parietal peritoneum. The size and shape of the urinary bladder varies with the amount of urine it contains and with the pressure it receives from surrounding organs.

The inner lining of the urinary bladder is a mucous membrane of transitional epithelium that is continuous with that in the ureters. When the bladder is empty, the mucosa has numerous folds called rugae. The rugae and transitional epithelium allow the bladder to expand as it fills.

The second layer in the walls is the submucosa, which supports the mucous membrane. It is composed of connective tissue with elastic fibers.

The next layer is the muscularis, which is composed of smooth muscle. The smooth muscle fibers are interwoven in all directions and, collectively, these are called the detrusor muscle. Contraction of this muscle expels urine from the bladder. On the superior surface, the outer layer of the bladder wall is parietal peritoneum. In all other regions, the outer layer is fibrous connective tissue.

The trigone is between the three openings in the floor of the urinary bladder. Two of the openings are from the ureters and form the base of the trigone. Small flaps of mucosa cover these openings and act as valves that allow urine to enter the bladder but prevent it from backing up from the bladder into the ureters. The third opening, at the apex of the trigone, is the opening into the urethra. A band of the detrusor muscle encircles this opening to form the internal urethral sphincter (Urinary bladder, n.d.).

Bladder function

The innervation of the detrusor is different from that of the bladder neck.

• The detrusor is rich in Beta-adrenergic receptors that are stimulated by norepinephrine the neurotransmitter of the Sympathetic component of the autonomic nervous system (ANS). Beta stimulation, via fibers of the hypogastric nerve, suppress contraction of the detrusor.
• Conversely, acetylcholine is the neurotransmitter responsible for Parasympathetic stimulation, by the fibers in the pelvic nerve, that cause the detrusor to contract.
  • Sympathetic stimulation is predominant during bladder filling.
  • Parasympathetic stimulation causes bladder emptying.
• Two sphincters control the bladder outlet. The internal sphincter is composed of smooth muscle like the detrusor and extends into the bladder neck. Like the detrusor, the internal sphincter is controlled by the ANS and is normally closed. The primary receptors in the bladder neck are alpha-adrenergic. Sympathetic stimulation of these alpha receptors, via fibers in the hypogastric nerve, contributes to urinary continence.
• The external sphincter is histologically different from the detrusor and internal sphincter. It is striated muscle. Like skeletal muscle, it's under voluntary control. It receives its innervation from the pudendal nerve, arising from the ventral horns of the sacral cord.
  
  During micturition, supraspinal centers block stimulation by the hypogastric and pudendal nerves. This relaxes the internal and external sphincters and removes the Sympathetic inhibition of the Parasympathetic receptors. The result is unobstructed passage of urine when the detrusor contracts.
• The normal adult bladder can hold about 500 mL of urine. After emptying, the bladder may still retain about 50 mL residual volume. At about 150 mL of volume, stretch receptors in the detrusor begin signaling the CNS via afferent nerves; at 400 mL we are "seeking" an appropriate toilet.

Micturition by parasympathetic reflex (involuntary emptying)

Involuntary parasympathetic reflex voiding occurs in infants and children until the age of 3-5 years, after which it becomes voluntary (Fowler, 2008). It also can occur after a supra-sacral spinal cord injury that blocks impulses to or from the Pons, resulting in the reflex or involuntary contraction of the bladder.

Parasympathetic innervation originates in the sacrum and serves to promote urination. Parasympathetic nerves activate the detrusor muscle causing it to contract and to relax the internal sphincter (Rahman, 2022).

1. The bladder fills and expands
   1. Expansion distends the detrusor muscle causing stretch receptors to generate action potentials that are transmitted to the sacral spinal cord through afferent fibers of the pelvic nerves.
   2. The sacral micturition center (SMC) responds to afferent impulses received from the pelvic nerve by generating action potentials in post-ganglionics parasympathetic fibers of the pelvic nerve.
      
      Parasympathetic post-ganglionic action potentials travel to the synaptic terminals and cause the release of the neurotransmsitter acetylcholine (ACh) near the muscarinic receptors of the detrusor muscle. It is believed that voltage-gated Ca ⁺⁺ channels of the detrusor muscle cells open when ACh binds to muscarinic receptors. When the Ca ⁺⁺ channels open, Ca ⁺⁺ ions flood into the detrusor muscle cell causing muscle contraction (Griffin, 2018).

Urine storage mode sustained by sympathetic dominance (adult voluntary voiding)

Sympathetic innervation originates in the L1,L2 segments of the spinal cord and serves to inhibit urination. Sympathetic nerves inhibit the detrusor muscle and activate the internal sphincter to contract (Rahman, 2018).

1. Stretch receptors in the detrusor send afferent action potentials via the pelvic nerve to the sacral spinal cord. 
   1. The spinal cord conducts signals up to the pontine micturition center (PCM) where "urge" is perceived and relayed to the the cerebral cortex. 
   2. The adult cerebral cortex exercises voluntary control over the release of urine by signaling the pons to provoke or suppress the micturition reflex.
2. The PCM causes the neurotransmitter, norepinephrine to be released near receptors on muscles of the detrusor. Norepinephrine causes an outflow of potassium from detrusor muscles cell, keeping them below the depolarization threshold, which suppresses contraction.
3. Further expansion increases the frequency and amplitude of action potentials to a stimulation threshold triggering parasympathetic dominance which releases acetylcholine from terminals of the pelvic nerve causing and influx of calcium in the detrusor smooth muscle, whic then contracts.